Questions & Answers
ICSE - Grade - 10
Subject: Physics
Chapter - 05 - Refraction through Lens
Types of Questions
MCQ
- What type of lens is thicker at the center than at the edges?
A. Concave lens
B. Convex lens
C. Plane mirror
D. Prism
Answer: B. Convex lens
- Which lens diverges parallel rays of light?
A. Convex lens
B. Plane lens
C. Concave lens
D. Cylindrical lens
Answer: C. Concave lens
- The principal axis of a lens is:
A. Any line through the lens
B. A line perpendicular to the optical center
C. A line joining the two foci
D. A line passing through the optical center and perpendicular to the lens surfaces
Answer: D. A line passing through the optical center and perpendicular to the lens surfaces
- Which point does a ray passing through the optical center follow?
A. Gets refracted
B. Passes undeviated
C. Diverges
D. Forms virtual image
Answer: B. Passes undeviated
- Focal length of a concave lens is:
A. Positive
B. Zero
C. Infinite
D. Negative
Answer: D. Negative
- A convex lens always forms a real and inverted image except when the object is:
A. At F
B. Beyond 2F
C. Between F and O
D. At 2F
Answer: C. Between F and O
- The lens formula is:
A. 1/f = u – v
B. f = uv/(u+v)
C. 1/f = 1/v – 1/u
D. f = v – u
Answer: C. 1/f = 1/v – 1/u
- The power of a lens is measured in:
A. Newtons
B. Diopters
C. Hertz
D. Joules
Answer: B. Diopters
- A concave lens always forms an image that is:
A. Real, inverted, and magnified
B. Virtual, erect, and magnified
C. Virtual, erect, and diminished
D. Real, erect, and diminished
Answer: C. Virtual, erect, and diminished
- When an object is placed at 2F in a convex lens, the image is formed at:
A. F
B. Beyond 2F
C. At 2F
D. At O
Answer: C. At 2F
- Which type of lens is used in magnifying glass?
A. Concave lens
B. Cylindrical lens
C. Convex lens
D. Plano-concave lens
Answer: C. Convex lens
- A convex lens converges parallel rays to a point called:
A. Center
B. Pole
C. Focus
D. Axis
Answer: C. Focus
- When object lies at F in a convex lens, the image is formed at:
A. Focus
B. Infinity
C. Centre of curvature
D. Optical center
Answer: B. Infinity
- The unit of focal length when calculating power is:
A. mm
B. m
C. cm
D. Diopters
Answer: C. cm
- Power of a convex lens is:
A. Zero
B. Positive
C. Negative
D. Imaginary
Answer: B. Positive
- The relation between magnification (m), image and object distances is:
A. m = v × u
B. m = v/u
C. m = u/v
D. m = h/h’
Answer: B. m = v/u
- In a concave lens, image is formed:
A. On the opposite side
B. At focus
C. At infinity
D. On the same side as the object
Answer: D. On the same side as the object
- The sign of object distance in lens formula is usually:
A. Positive
B. Negative
C. Zero
D. Undefined
Answer: B. Negative
- Which method is NOT used to determine the focal length of a convex lens?
A. Pin and plane mirror method
B. Distant object method
C. One pin method
D. Virtual object method
Answer: D. Virtual object method
- When object is between F and O of a convex lens, the image is:
A. Real, erect
B. Real, inverted
C. Virtual, erect
D. Virtual, inverted
Answer: C. Virtual, erect
- A real image is always:
A. Erect
B. Virtual
C. Inverted
D. Enlarged
Answer: C. Inverted
- For a convex lens, if object distance is 10 cm and image distance is 20 cm, the focal length is:
A. 10 cm
B. 15 cm
C. 5 cm
D. 6.67 cm
Answer: B. 15 cm
- Which of the following has a negative power?
A. Convex lens of 50 cm focal length
B. Concave lens of 25 cm focal length
C. Concave lens of 40 cm focal length
D. Both B and C
Answer: D. Both B and C
- Which of the following rays passes undeviated through the lens?
A. Through principal focus
B. Parallel to principal axis
C. Through optical center
D. Through curvature center
Answer: C. Through optical center
- A converging lens forms a real, inverted image 2x the size of the object. The object must be placed:
A. Between O and F
B. At F
C. Between F and 2F
D. Beyond 2F
Answer: C. Between F and 2F
- The image formed by a concave lens is always located:
A. At focus
B. On the opposite side
C. Between optical center and focus
D. At infinity
Answer: C. Between optical center and focus
- What is the magnification of an image that is the same size as the object?
A. Zero
B. 1
C. –1
D. 2
Answer: B. 1
- A convex lens can form:
A. Only real images
B. Only virtual images
C. Both real and virtual images
D. No image
Answer: C. Both real and virtual images
- A concave lens is used in:
A. Projectors
B. Cameras
C. Magnifying glass
D. Spectacles for myopia
Answer: D. Spectacles for myopia
- In lens terminology, F stands for:
A. Focus
B. Force
C. Focal surface
D. Factor
Answer: A. Focus
- The lens with the least power is:
A. Lens of f = 50 cm
B. Lens of f = 10 cm
C. Lens of f = 100 cm
D. Lens of f = 25 cm
Answer: C. Lens of f = 100 cm
- Virtual images can be obtained by a convex lens when the object is placed:
A. At infinity
B. Between F and optical center
C. At 2F
D. Beyond 2F
Answer: B. Between F and optical center
- When is the magnification negative?
A. When image is erect
B. When image is virtual
C. When image is real and inverted
D. When object is virtual
Answer: C. When image is real and inverted
- Which lens forms a real image on screen?
A. Plane mirror
B. Convex lens
C. Concave lens
D. None of these
Answer: B. Convex lens
- A real image is always formed:
A. Behind the lens
B. On the same side as object
C. On the opposite side to the object
D. Inside the lens
Answer: C. On the opposite side to the object
- What is the power of a lens with f = 50 cm?
A. +2 D
B. +5 D
C. +0.5 D
D. +3 D
Answer: A. +2 D
- What happens to focal length if lens is made thicker?
A. Increases
B. Decreases
C. Remains same
D. Becomes negative
Answer: B. Decreases
- Which optical device uses both convex and concave lenses?
A. Projector
B. Telescope
C. Spectacles
D. Microscope
Answer: B. Telescope
- What type of lens is used in a peephole of a door?
A. Convex
B. Concave
C. Cylindrical
D. Plano-concave
Answer: B. Concave
- Ray passing through the focus and then the lens emerges as:
A. Parallel to principal axis
B. Through the optical center
C. Divergent
D. Refracted downwards
Answer: A. Parallel to principal axis
- What is the sign of v in convex lens when image is real and formed on other side?
A. Positive
B. Negative
C. Zero
D. Depends on uAnswer: A. Positive
- Image formed by convex lens for distant object is at:
A. 2F
B. At F
C. At O
D. Between F and 2F
Answer: B. At F
- A negative magnification means:
A. Virtual and erect
B. Real and inverted
C. Real and erect
D. Virtual and inverted
Answer: B. Real and inverted
- A magnifying glass must be held:
A. At 2F
B. Beyond 2F
C. At F
D. Within focal length
Answer: D. Within focal length
- Which of these rays is not a principal ray in ray diagram construction?
A. Through focus
B. Parallel to principal axis
C. Through optical center
D. Through curvature center
Answer: D. Through curvature center
- Optical bench is used to measure:
A. Power of lens
B. Density
C. Focal length
D. Refraction index
Answer: C. Focal length
- Lens is made of:
A. Metal
B. Wood
C. Transparent material
D. Opaque plastic
Answer: C. Transparent material
- Which lens has greater bending of rays?
A. Thick lens
B. Thin lens
C. Both same
D. None
Answer: A. Thick lens
- Which device cannot use concave lens?
A. Myopia spectacles
B. Microscope
C. Door viewer
D. Laser pointer
Answer: B. Microscope
- The image formed in a magnifying glass is:
A. Real, inverted
B. Virtual, erect
C. Real, erect
D. Virtual, inverted
Answer: B. Virtual, erect
Fill in the Blanks
- A lens is a transparent medium bound by two surfaces, at least one of which is _______.
Answer: curved - A convex lens is _______ at the center than at the edges.
Answer: thicker - A concave lens is _______ at the edges than at the center.
Answer: thicker - A convex lens is also called a _______ lens.
Answer: converging - A concave lens is also called a _______ lens.
Answer: diverging - The central point of a lens is known as the _______.
Answer: optical center - The straight line that passes through the optical center is called the _______.
Answer: principal axis - The point where parallel rays converge in a convex lens is called the _______.
Answer: focus - The distance between the optical center and the focus is known as the _______.
Answer: focal length - A convex lens acts like two prisms placed _______ to _______.
Answer: base, base - A concave lens acts like two prisms placed _______ to _______.
Answer: apex, apex - Rays parallel to the principal axis in a convex lens pass through the _______ after refraction.
Answer: focus - Rays parallel to the principal axis in a concave lens appear to diverge from the _______.
Answer: focus - A ray passing through the optical center of a lens passes _______.
Answer: undeviated - The image formed by a convex lens when the object is at 2F is located at _______.
Answer: 2F - The image formed by a convex lens when the object is between F and 2F is _______ and _______.
Answer: real, magnified - The image formed by a convex lens when the object is at focus is formed at _______.
Answer: infinity - A concave lens always forms an image that is _______ and _______.
Answer: virtual, diminished - When the object is between F and optical center in a convex lens, the image is _______ and _______.
Answer: virtual, magnified - The lens formula is _______.
Answer: 1/f = 1/v – 1/u - In the lens formula, ‘u’ denotes the _______ distance.
Answer: object - In the lens formula, ‘v’ denotes the _______ distance.
Answer: image - In the lens formula, ‘f’ represents the _______ of the lens.
Answer: focal length - All distances in the lens formula are measured from the _______.
Answer: optical center - The sign convention considers distances against the incident light as _______.
Answer: negative - The unit of power of a lens is the _______.
Answer: diopter - The power of a convex lens is taken as _______.
Answer: positive - The power of a concave lens is taken as _______.
Answer: negative - The formula for power of a lens in cm is _______.
Answer: P = 100/f - The magnification formula is _______.
Answer: m = h’/h = v/u - If the magnification is negative, the image is _______ and _______.
Answer: real, inverted - If the magnification is positive, the image is _______ and _______.
Answer: virtual, erect - A magnifying glass is a device that uses a _______ lens.
Answer: convex - A magnifying glass produces a _______ image.
Answer: virtual - A magnifying glass must be held _______ the focal length of the lens.
Answer: within - The image formed in a magnifying glass is _______ and _______.
Answer: erect, magnified - A ray passing through the focus of a convex lens emerges _______ to the principal axis.
Answer: parallel - A ray parallel to the principal axis passes through the _______ after refraction in a convex lens.
Answer: focus - The lens that always forms diminished images is the _______ lens.
Answer: concave - A convex lens can form both _______ and _______ images.
Answer: real, virtual - A concave lens forms images on the _______ side as the object.
Answer: same - The power of a lens is the reciprocal of its _______ in meters.
Answer: focal length - A real image can be obtained on a _______.
Answer: screen - A virtual image cannot be obtained on a _______.
Answer: screen - The focal length of a lens depends on the _______ and curvature of the lens surfaces.
Answer: refractive index - The optical center lies at the _______ of a symmetrical lens.
Answer: geometrical center - The experimental method that uses the Sun to find focal length is the _______ method.
Answer: distant object - A thick lens has a _______ focal length than a thin lens.
Answer: shorter - In an optical bench method, a _______ is used to locate the sharp image.
Answer: screen - The image formed by a concave lens is always located between the _______ and _______.
Answer: optical center, focus
Name the Following
- Name the point on the lens through which a ray passes without deviation.
Answer: Optical center - Name the line that passes through the optical center and is perpendicular to the lens surfaces.
Answer: Principal axis - Name the lens that is thicker at the center than at the edges.
Answer: Convex lens - Name the lens that is thicker at the edges than at the center.
Answer: Concave lens - Name the lens that converges parallel rays of light.
Answer: Convex lens - Name the lens that diverges parallel rays of light.
Answer: Concave lens - Name the unit in which power of a lens is measured.
Answer: Diopter - Name the lens used in a magnifying glass.
Answer: Convex lens - Name the type of image that cannot be obtained on a screen.
Answer: Virtual image - Name the type of image that can be obtained on a screen.
Answer: Real image - Name the formula used to relate object distance, image distance, and focal length.
Answer: Lens formula - Name the image formed by a concave lens.
Answer: Virtual, erect, and diminished - Name the lens used to correct myopia.
Answer: Concave lens - Name the lens used to correct hypermetropia.
Answer: Convex lens - Name the method used to find focal length using the Sun’s image.
Answer: Distant object method - Name the lens with positive focal length.
Answer: Convex lens - Name the lens with negative focal length.
Answer: Concave lens - Name the quantity represented by the ratio v/u.
Answer: Magnification - Name the standard unit of focal length used in power formula.
Answer: Centimeter - Name the lens that forms real and inverted images depending on object position.
Answer: Convex lens - Name the rays used to draw ray diagrams.
Answer: Principal rays or construction rays - Name the phenomenon due to which a lens forms images.
Answer: Refraction - Name the two types of spherical lenses.
Answer: Convex lens and Concave lens - Name the part of lens construction that explains converging or diverging behavior.
Answer: Action as a set of prisms - Name the surface in a lens where light changes direction.
Answer: Refracting surface - Name the condition under which a convex lens forms a virtual image.
Answer: Object placed between focus and optical center - Name the side where the image is formed by a concave lens.
Answer: Same side as the object - Name the quantity whose reciprocal gives power of a lens.
Answer: Focal length (in meters) - Name the physical quantity that remains unchanged for a ray through the optical center.
Answer: Direction of light - Name the lens that always gives a magnification less than 1.
Answer: Concave lens - Name the lens that can have magnification less than, equal to, or greater than 1.
Answer: Convex lens - Name the standard expression of power of a lens in SI units.
Answer: P = 100/f - Name the point on principal axis where rays converge in a convex lens.
Answer: Focus - Name the nature of image when object is at infinity for a convex lens.
Answer: Real, inverted, highly diminished - Name the nature of image when object is at focus of a convex lens.
Answer: Formed at infinity - Name the image formed when object is placed at 2F of a convex lens.
Answer: Real, inverted, same size - Name the image formed by a convex lens when object is between F and 2F.
Answer: Real, inverted, magnified - Name the term for the bending of light as it passes through a lens.
Answer: Refraction - Name the lens which always forms diminished images.
Answer: Concave lens - Name the property of a lens that allows it to bring light to a point.
Answer: Converging power - Name the lens used in cameras.
Answer: Convex lens - Name the type of lens used in peepholes of doors.
Answer: Concave lens - Name the method of determining focal length using a pin and screen.
Answer: One pin method - Name the method involving a plane mirror to find focal length.
Answer: Plane mirror method - Name the angle between the incident ray and refracted ray.
Answer: Angle of refraction - Name the axis used as reference for all measurements in lenses.
Answer: Principal axis - Name the property that helps to identify convex and concave lenses physically.
Answer: Thickness at center and edges - Name the instrument that uses a convex lens for low magnification.
Answer: Simple microscope - Name the term used when a ray changes its speed and direction in another medium.
Answer: Refraction - Name the nature of lens required to diverge light rays.
Answer: Concave lens
Answer in One Word
- What is the central point of a lens called?
Optical - What type of lens converges light rays?
Convex - What type of lens diverges light rays?
Concave - What is the point where rays converge in a convex lens?
Focus - What is the unit of power of a lens?
Diopter - What is the path along which light travels through the center of a lens?
Axis - What is the SI unit of focal length in power formula?
Centimeter - What kind of image cannot be formed on a screen?
Virtual - What kind of image can be formed on a screen?
Real - What phenomenon helps lenses form images?
Refraction - What type of lens has a positive focal length?
Convex - What type of lens has a negative focal length?
Concave - What is the formula used to calculate focal length of a lens?
Lens - What is the distance between optical center and focus called?
Focal - What do you call the straight line that passes through optical center?
Axis - What is the nature of image formed by a concave lens?
Diminished - What is the nature of image formed by convex lens when object is at 2F?
Equal - What is the nature of image formed by convex lens when object is between F and O?
Magnified - What is the formula symbol for magnification?
m - What type of lens is used in magnifying glass?
Convex - What kind of lens is used to correct myopia?
Concave - What kind of lens is used to correct hypermetropia?
Convex - What kind of lens is used in door viewers?
Concave - What is the name of the lens used in cameras?
Convex - What is the result of a ray passing through optical center?
Undeviated - What type of image is formed when object is at infinity in convex lens?
Point - What kind of rays are used to construct ray diagrams?
Principal - What happens to focal length if curvature increases?
Decreases - What is the symbol for object distance in lens formula?
u - What is the symbol for image distance in lens formula?
v - What is the symbol for focal length in lens formula?
f - What kind of image has a negative magnification?
Real - What kind of image has a positive magnification?
Virtual - What is the numerical ratio v/u called?
Magnification - What method uses the Sun to find focal length?
Distant - What method uses pin and screen to find focal length?
Pin - What is the sign of focal length of a concave lens?
Negative - What is the sign of focal length of a convex lens?
Positive - What happens to rays passing through the focus in a convex lens?
Parallel - What happens to parallel rays in a concave lens?
Diverge - What part of the lens affects the bending of rays?
Curvature - What is the unit of lens power in SI system?
Diopter - What kind of image is formed when object is at F in a convex lens?
Infinite - What is the direction of a ray through optical center?
Straight - What kind of lens has both surfaces curved outward?
Convex - What kind of lens has both surfaces curved inward?
Concave - What is the term for real and inverted images’ orientation?
Inverted - What kind of image is always formed by concave lens?
Virtual - What is the power of a lens with 50 cm focal length?
2 - What is the unit used to express lens curvature effect?
Diopter
ICSE - Grade 10 - Physics
All Chapters
- Chapter 1 – Force
- Chapter 2 – Work, Energy and Power
- Chapter 3 – Machines
- Chapter 4 – Refraction of Light at Plane Surfaces
- Chapter 5 – Refraction through Lens
- Chapter 6 – Spectrum
- Chapter 7 – Sound
- Chapter 8 – Current Electricity
- Chapter 9 – Electrical Power and Household Circuits
- Chapter 10 – Electro-magnetism
- Chapter 11 – Calorimetry
- Chapter 12 – Radioactivity
ICSE - Grade 10 - Chemistry
All Chapters
- Chapter 1 The Language of Chemistry
- Chapter 2 Chemical Changes and Reactions
- Chapter 3 Water
- Chapter 4 Atomic Structure and Chemical Bonding
- Chapter 5 The periodic table
- Chapter 6 Study of the first Element Hydrogen
- Chapter 7 Study of Gas laws
- Chapter 8 Atmospheric Pollution
ICSE - Grade 10 - Mathematics
All Chapters
- Chapter 1 Rational and Irrational Numbers
- Chapter 2 Compound Interest [Without Using Formula]
- Chapter 3 Compound Interest [Using Formula]
- Chapter 4 Expansions
- Chapter 5 Factorisation
- Chapter 6 Simultaneous Equations
- Chapter 7 Indices
- Chapter 8 Logarithms
- Chapter 9 Triangles
- Chapter 10 Isosceles Triangles
- Chapter 11 Inequalities
- Chapter 12 Midpoint and Its Converse
- Chapter 13 Pythagoras Theorem
- Chapter 14 Rectilinear Figures
- Chapter 15 Construction of Polygons
- Chapter 16 Area Theorems
- Chapter 17 Circle
- Chapter 18 Statistics
- Chapter 19 Mean and Median
- Chapter 20 Area and Perimeter of Plane Figures
- Chapter 21 Solids
- Chapter 22 Trigonometrical Ratios
- Chapter 23 Trigonometrical Ratios of Standard Angles
- Chapter 24 Solutions of Right Triangles
- Chapter 25 Complementary Angles
- Chapter 26 Coordinate Geometry
- Chapter 27 Graphical Solution
- Chapter 28 Distance Formula
ICSE - Grade 10 - Biology
All Chapters
- Chapter 1 Introducing Biology
- Chapter 2 Cell: The Unit Of Life
- Chapter 3 Tissues: Plant And Animal Tissue
- Chapter 4 The Flower
- Chapter 5 Pollination and Fertilization
- Chapter 6 Seeds: Structure and Germination
- Chapter 7 Respiration in Plants
- Chapter 8 Five Kingdom Classification
- Chapter 9 Economic Importance of Bacteria and Fungi
- Chapter 10 Nutrition
- Chapter 11 Digestive system
- Chapter 12 Skeleton: Movement and Locomotion
- Chapter 13 Skin: The Jack of all trades
- Chapter 14 The Respiratory System
- Chapter 15 Hygiene: [A key to Healthy Life]
- Chapter 16 Diseases: Cause and Control
- Chapter 17 Aids to Health
- Chapter 18 Health Organizations
- Chapter 19 Waste Generation and Management
ICSE - Grade 10 - History
All Chapters
- Chapter 1 – The Harappan Civilisation
- Chapter 2 – The Vedic Period
- Chapter 3 – Jainism and Buddhism
- Chapter 4 – The Mauryan Empire
- History — Chapter 5
The Sangam Age - Chapter 6 – The Age of the Guptas
- Chapter 7 – Medieval India — (A) The Cholas
- Chapter 8 – Medieval India — (B) The Delhi Sultanate
- Chapter 9 – Medieval India — (C) The Mughal Empire
- Chapter 10 – Medieval India — (D) Composite Culture
- Chapter 11 – The Modern Age in Europe — (A) Renaissance
- Chapter 12 – The Modern Age in Europe — (B) Reformation
- Chapter 13 – The Modern Age in Europe — (C) Industrial Revolution
ICSE - Grade 10 - Civics
All Chapters
- Chapter 1: Our Constitution
- Chapter 2: Salient Features of the Constitution — I
- Chapter 3: Salient Features of the
- Constitution — II
- Chapter 4: Elections
- Chapter 5: Local Self-Government — Rural
- Chapter 6: Local Self-Government — Urban
ICSE - Grade 10 - Geography
All Chapters
- Ch 1 – Earth as a Planet
Ch 2 – Geographic Grid: Latitudes and Longitudes
Ch 3 – Rotation and Revolution
Ch 4 – Earth’s Structure
Ch 5 – Landforms of the Earth
Ch 6 – Rocks
Ch 7 – Volcanoes
Ch 8 – Earthquakes
Ch 9 – Weathering
Ch 10 – Denudation
Ch 11 – Hydrosphere
Ch 12 – Composition and Structure of the Atmosphere
Ch 13 – Insolation
Ch 14 – Atmospheric Pressure and Winds
Ch 15 – Humidity
Ch 16 – Pollution
Ch 17 – Sources of Pollution
Ch 18 – Effects of Pollution
Ch 19 – Preventive Measures
Ch 20 – Natural Regions of the World
Find the Odd Man Out
- Convex lens, Concave lens, Plane mirror, EquiConvex lens
Answer: Plane mirror
Explanation: Only plane mirror does not refract light like lenses do. - Optical center, Focus, Principal axis, Magnetic field
Answer: Magnetic field
Explanation: Magnetic field is unrelated to optics and lenses. - Real, Virtual, Inverted, Transparent
Answer: Transparent
Explanation: Transparent is a material property, not an image characteristic. - Diopter, Focal length, Lens formula, Newton
Answer: Newton
Explanation: Newton is a unit of force, not related to lens measurements. - Real, Virtual, Parallel, Inverted
Answer: Parallel
Explanation: Parallel is a ray direction, not a type of image. - Convex, Concave, Plano-convex, Rectangular
Answer: Rectangular
Explanation: Rectangular is not a lens type. - Diminished, Enlarged, Erect, Concave
Answer: Concave
Explanation: Concave is a lens type, others are image characteristics. - v, u, f, h
Answer: h
Explanation: h (height) is not a distance used in lens formula. - Converging, Diverging, Refraction, Dispersion
Answer: Dispersion
Explanation: Dispersion is separation of light, not bending through lenses. - Distant object method, One pin method, Mirror method, Prism formula
Answer: Prism formula
Explanation: Prism formula is not used for finding focal length of a lens. - Focus, Pole, Principal axis, Optical center
Answer: Pole
Explanation: Pole is a mirror term, not used in lens terminology. - Myopia, Hypermetropia, Cataract, Presbyopia
Answer: Cataract
Explanation: Cataract is a biological issue, others are refractive defects. - Virtual, Inverted, Erect, Upright
Answer: Inverted
Explanation: Inverted is opposite of the others. - Convex lens, Concave lens, Prism, Biconcave lens
Answer: Prism
Explanation: Prism is not a lens. - Focus, Object, Image, Medium
Answer: Medium
Explanation: Medium is not a positional reference in lens diagrams. - Projector, Microscope, Telescope, Myopia lens
Answer: Myopia lens
Explanation: It corrects vision, others are instruments for viewing. - Virtual, Upright, Real, Enlarged
Answer: Real
Explanation: Real is the only non-erect image type here. - 2F, F, O, M
Answer: M
Explanation: M is not a lens-related point. - Lens, Mirror, Glass slab, Screen
Answer: Screen
Explanation: Screen does not affect light; it only receives images. - m = v/u, m = h’/h, P = F/m, m = -v/u
Answer: P = F/m
Explanation: P = F/m is from mechanics, not optics. - Hypermetropia, Myopia, Diopter, Presbyopia
Answer: Diopter
Explanation: Diopter is a unit, others are vision defects. - Convex lens, Microscope, Concave mirror, Peephole
Answer: Microscope
Explanation: Microscope is a device, others are optical components. - Sun, Distant object, Candle flame, Focus
Answer: Focus
Explanation: Focus is not an object. - Refraction, Reflection, Dispersion, Conduction
Answer: Conduction
Explanation: Conduction is not an optical phenomenon. - Parallel rays, Focus, Diverging rays, Sound wave
Answer: Sound wave
Explanation: Sound wave is unrelated to light. - Focal length, Power, Diopter, Joule
Answer: Joule
Explanation: Joule is a unit of energy, not optical measurement. - Real, Virtual, Erect, Light
Answer: Light
Explanation: Light is not a type of image. - O, F, 2F, 3F
Answer: 3F
Explanation: 3F is not a standard point in lens diagrams. - Eye lens, Contact lens, Mirror, Spectacle lens
Answer: Mirror
Explanation: Mirror reflects; others are refractive lenses. - Upright, Erect, Virtual, Negative
Answer: Negative
Explanation: Negative is a sign convention, not image type. - Lens formula, Ray diagram, Newton’s law, Power formula
Answer: Newton’s law
Explanation: Newton’s law belongs to mechanics, not optics. - Projector, Spectacles, Telescope, Periscope
Answer: Periscope
Explanation: Periscope uses mirrors, not lenses. - Base to base, Apex to apex, Focus to focus, Prism setup
Answer: Focus to focus
Explanation: Not a valid lens construction model. - Camera, Eye, Door viewer, Calculator
Answer: Calculator
Explanation: Calculator doesn’t use lenses. - Power, Resistance, Focal length, Magnification
Answer: Resistance
Explanation: Resistance is from electricity, not optics. - Convex lens, Concave lens, Concavo-convex lens, Cuboid
Answer: Cuboid
Explanation: Cuboid is not a lens. - Erect, Enlarged, Inverted, Amplified
Answer: Amplified
Explanation: Amplified refers to sound, not image. - 1/f = 1/v – 1/u, m = v/u, F = ma, P = 100/f
Answer: F = ma
Explanation: F = ma is not related to lenses. - Inverted, Virtual, Erect, Dimensional
Answer: Dimensional
Explanation: Dimensional is not an image orientation. - Telescope, Spectacle, Eye, Magnet
Answer: Magnet
Explanation: Magnet is not an optical device. - Power, Magnification, Resistance, Focal length
Answer: Resistance
Explanation: Resistance belongs to electricity. - Biconvex, Equiconcave, Plano-convex, Metallic
Answer: Metallic
Explanation: Metallic is not a lens type. - Prism, Lens, Plane mirror, Ray
Answer: Ray
Explanation: Ray is not a physical object. - Eye lens, Torch lens, Magnifying glass, Battery
Answer: Battery
Explanation: Battery is not an optical component. - P = 100/f, P = 1/f, m = h’/h, m = v/u
Answer: P = 1/f
Explanation: P = 1/f is correct in meters, not in centimeters. - Curved, Transparent, Flat, Refractive
Answer: Flat
Explanation: Flat surfaces don’t refract light like lenses. - Candle, Pin, Paper, Optical center
Answer: Optical center
Explanation: Optical center is not a physical object to be placed. - Focal length, Diopter, u, Ampere
Answer: Ampere
Explanation: Ampere is a unit of current, unrelated to optics. - Light ray, Image, Object, Conduction
Answer: Conduction
Explanation: Conduction is not a lens-related term. - Real, Virtual, Axis, Inverted
Answer: Axis
Explanation: Axis is a reference line, not a type of image.
Match the Pair
Set 1
Column A
- Convex lens
- Concave lens
- Focal length
- Power of a lens
- Diopter
Column B
A. Measured in meters
B. Unit of lens power
C. Diverges light rays
D. Converges light rays
E. Reciprocal of focal length (in meters)
Correct Answers
1 – D
2 – C
3 – A
4 – E
5 – B
Set 2
Column A
- Optical center
- Principal axis
- Focus
- Ray passing through optical center
- Real image
Column B
A. Central point of lens
B. Lies on principal axis, rays meet or appear to meet
C. Straight line through optical center
D. Always inverted and can be caught on a screen
E. Passes undeviated
Correct Answers
1 – A
2 – C
3 – B
4 – E
5 – D
Set 3
Column A
- Virtual image
- Object at 2F (convex lens)
- Object at F (convex lens)
- Object beyond 2F (convex lens)
- Object between F and optical center
Column B
A. Real, inverted, diminished
B. Formed at infinity
C. Image on same side, erect and magnified
D. Same size, real and inverted
E. Cannot be caught on screen
Correct Answers
1 – E
2 – D
3 – B
4 – A
5 – C
Set 4
Column A
- Concave lens image
- Ray parallel to principal axis (convex lens)
- Ray through focus (convex lens)
- Ray through optical center
- Action of convex lens
Column B
A. Converging like base-to-base prisms
B. Undeviated
C. Always virtual, erect, and diminished
D. Emerges parallel to principal axis
E. Passes through focus
Correct Answers
1 – C
2 – E
3 – D
4 – B
5 – A
Set 5
Column A
- Lens formula
- Power formula
- Magnification formula
- u (in lens formula)
- v (in lens formula)
Column B
A. Object distance
B. 1/f = 1/v – 1/u
C. Image distance
D. m = h’/h = v/u
E. P = 100/f
Correct Answers
1 – B
2 – E
3 – D
4 – A
5 – C
Set 6
Column A
- Plano-convex lens
- Biconcave lens
- Thick lens
- Transparent medium
- Image at infinity
Column B
A. Curved on one side, flat on other
B. Object placed at focus
C. Light can pass through
D. Curved inward on both sides
E. More bending, shorter focal length
Correct Answers
1 – A
2 – D
3 – E
4 – C
5 – B
Set 7
Column A
- Spectacles for myopia
- Spectacles for hypermetropia
- Microscope
- Door viewer
- Camera lens
Column B
A. Convex lens
B. Convex lens (magnifying system)
C. Concave lens
D. Convex lens (image formation)
E. Concave lens
Correct Answers
1 – C
2 – A
3 – B
4 – E
5 – D
Set 8
Column A
- Experiment using sunlight
- One pin method
- Plane mirror method
- Sharpest image on screen
- Used to determine focal length
Column B
A. Uses pin and screen
B. Uses pin and plane mirror
C. Uses distant object
D. Image with maximum contrast
E. All three methods above
Correct Answers
1 – C
2 – A
3 – B
4 – D
5 – E
Set 9
Column A
- Converging lens
- Diverging lens
- Magnified virtual image
- Diminished virtual image
- Always erect
Column B
A. Formed by concave lens
B. Convex lens inside focal length
C. Convex lens
D. Concave lens
E. Characteristic of virtual image
Correct Answers
1 – C
2 – D
3 – B
4 – A
5 – E
Set 10
Column A
- Diopter
- Focal length = 50 cm
- Power = 4 D
- Negative power
- Magnification = 1
Column B
A. Virtual image with same size
B. SI unit of lens power
C. f = 25 cm
D. Concave lens
E. f = 0.5 m
Correct Answers
1 – B
2 – E
3 – C
4 – D
5 – A
Short Answer Questions
- What is a lens?
A lens is a transparent medium bounded by two surfaces, at least one of which is curved. - What are the two main types of lenses?
Convex lens and concave lens. - What is the function of a convex lens?
A convex lens converges parallel rays of light. - What is the function of a concave lens?
A concave lens diverges parallel rays of light. - What is meant by the optical center of a lens?
The optical center is the central point of a lens through which a ray passes undeviated. - Define principal axis.
It is the straight line that passes through the optical center and is perpendicular to the lens surfaces. - What is focus (F) of a lens?
It is the point where light rays parallel to the principal axis either converge (convex) or appear to diverge from (concave). - What is focal length?
The focal length is the distance between the optical center and the focus. - State the sign of focal length for a convex lens.
The focal length of a convex lens is positive. - State the sign of focal length for a concave lens.
The focal length of a concave lens is negative. - What is the lens formula?
1/f = 1/v – 1/u - What is ‘u’ in the lens formula?
It is the object distance from the optical center. - What is ‘v’ in the lens formula?
It is the image distance from the optical center. - What is ‘f’ in the lens formula?
It is the focal length of the lens. - What is the unit of focal length in SI system?
The SI unit of focal length is meter. - What is magnification?
Magnification is the ratio of the height of the image to the height of the object. - What is the formula for magnification?
m = h’/h = v/u - What does a negative magnification signify?
Negative magnification indicates a real and inverted image. - What does a positive magnification indicate?
Positive magnification indicates a virtual and erect image. - What is the unit of power of a lens?
Diopter (D) - State the formula for power of a lens.
P = 100/f (where f is in cm) - What type of lens has positive power?
Convex lens - What type of lens has negative power?
Concave lens - Which type of lens is used in magnifying glasses?
Convex lens - What type of image is formed by a magnifying glass?
Virtual, erect, and magnified - What kind of image is formed by a concave lens?
Virtual, erect, and diminished - What kind of image is formed when object is at 2F in a convex lens?
Real, inverted, and same size - What happens when the object is at F in a convex lens?
The image is formed at infinity. - What happens when the object is beyond 2F in a convex lens?
The image is real, inverted, and diminished. - What happens when the object is between F and 2F in a convex lens?
The image is real, inverted, and magnified. - What happens when the object is between F and O in a convex lens?
The image is virtual, erect, and magnified. - What is the nature of image formed by concave lens irrespective of object position?
Always virtual, erect, and diminished. - What happens to a ray passing through the optical center?
It passes through undeviated. - What happens to a ray parallel to principal axis in convex lens?
It passes through the focus after refraction. - What happens to a ray that passes through focus in convex lens?
It emerges parallel to the principal axis. - How does a concave lens behave like a prism system?
It acts like two prisms placed apex to apex, diverging rays. - How does a convex lens behave like a prism system?
It acts like two prisms placed base to base, converging rays. - How is the focal length of a convex lens determined experimentally?
By distant object method, pin method, or plane mirror method. - What does the distant object method use to determine focal length?
It uses sunlight or far objects and a screen to form sharp image. - Which lens is used to correct myopia?
Concave lens - Which lens is used to correct hypermetropia?
Convex lens - What kind of lens is used in peepholes of doors?
Concave lens - What kind of lens is used in cameras?
Convex lens - What is the power of a lens with focal length 50 cm?
+2 D - What does a power of –4 D indicate?
A concave lens with 25 cm focal length. - What is the image distance if object is at 2F in convex lens?
At 2F on the other side - Where is image formed when object is at infinity in convex lens?
At focus - What is the magnification if image and object heights are equal?
1 - What is the magnification if image is twice the height of object?
2 - What is the magnification if the image is real and inverted and half the size?
–0.5
Puzzles
- I am a lens that never forms real images. Who am I?
Concave lens - I am the point where parallel rays converge. I lie on the principal axis. Who am I?
Focus (F) - I’m used in spectacles to correct myopia. What type of lens am I?
Concave lens - My power is +4D. Am I converging or diverging?
Converging - I always produce erect, diminished, virtual images regardless of object position. What am I?
Concave lens - I’m used in magnifying glass and sometimes form virtual images. Who am I?
Convex lens - My image is on the same side as the object. I diverge rays. Identify me.
Concave lens - When object is at 2F, I form an image at 2F. What kind of lens am I?
Convex lens - I form real, inverted, and enlarged images when object is between F and 2F. Who am I?
Convex lens - I always form diminished images, never magnified. Identify me.
Concave lens - You see me in a door’s peephole. Who am I?
Concave lens - What lens has a negative focal length and spreads rays apart?
Concave lens - I’m a lens with power –3D. What is my focal length in cm?
–33.3 cm - I can project images on a screen if placed correctly. What lens am I?
Convex lens - My power is zero. What shape am I?
Plane glass (no lens effect) - I am the point through which light passes undeviated. What am I?
Optical center - I bend light but do not meet rays. My image is virtual. What lens am I?
Concave lens - I act like two prisms placed base-to-base. Who am I?
Convex lens - I act like two prisms placed apex-to-apex. Who am I?
Concave lens - What happens to parallel rays in a convex lens?
They converge at the focus - What happens to rays parallel to the principal axis in a concave lens?
They diverge as if from the focus - When an object is at infinity, where does a convex lens form the image?
At focus - What’s the magnification of a convex lens when image and object are same size?
–1 - Which lens is thicker at the center than edges?
Convex lens - I always form images between optical center and focus. Who am I?
Concave lens - If f = –25 cm, what is the power of the lens?
–4 D - Which type of image can never be caught on screen?
Virtual image - What type of lens is used in a projector?
Convex lens - My image is highly enlarged and formed at infinity. Where is the object placed?
At focus - I bend light more when I’m more curved. Who am I?
Convex lens - I can form both virtual and real images. Who am I?
Convex lens - My magnification is always less than 1 but positive. What lens am I?
Concave lens - The image formed by me is always diminished and virtual. Who am I?
Concave lens - What sign does a convex lens have for its focal length?
Positive - What sign does a concave lens have for its focal length?
Negative - If magnification is +1.5, what type of image is formed?
Virtual and erect - If magnification is –0.5, what type of image is formed?
Real and inverted - I form a same-size image at 2F when object is at 2F. Who am I?
Convex lens - I never form real, inverted images. What am I?
Concave lens - I diverge light rays so they appear to come from one point. Who am I?
Concave lens - A ray through the optical center does not bend. Why?
Because it passes symmetrically - When object is between F and O in a convex lens, where is the image?
On the same side as the object - What is the image nature when object is at F in a convex lens?
Highly enlarged and at infinity - In which lens is image always formed closer to the lens than the object?
Concave lens - I can produce an image that is real, inverted, and diminished. Who am I?
Convex lens - What is the formula for power of a lens?
P = 100/f (in cm) - What is the SI unit of lens power?
Diopter - The ray emerges parallel to principal axis if incident ray passes through what point?
Focus (in convex lens) - What type of lens combination gives zero power?
Convex and concave lenses of equal and opposite power - You need to make a real, diminished image on a screen. What should you use and how?
Use a convex lens with object placed beyond 2F
Difference Between:
- Difference between Convex Lens and Concave Lens
Convex Lens: Thicker at the center and converges parallel rays.
Concave Lens: Thinner at the center and diverges parallel rays.
- Difference between Real Image and Virtual Image
Real Image: Formed by actual intersection of rays; can be captured on a screen.
Virtual Image: Formed by apparent divergence; cannot be captured on a screen.
- Difference between Focal Length of Convex and Concave Lens
Convex Lens: Has a positive focal length.
Concave Lens: Has a negative focal length.
- Difference between Use of Convex and Concave Lenses
Convex Lens: Used in magnifiers, cameras, projectors.
Concave Lens: Used in spectacles for myopia, peepholes.
- Difference between Image Formed at 2F and F in Convex Lens
At 2F: Image is real, inverted, and same size.
At F: Image is real, inverted, and formed at infinity.
- Difference between Lens and Mirror
Lens: Refraction takes place; light passes through.
Mirror: Reflection takes place; light bounces back.
- Difference between Optical Center and Principal Focus
Optical Center: Central point through which rays pass undeviated.
Principal Focus: Point where parallel rays converge or appear to diverge.
- Difference between Magnification > 1 and < 1
> 1: Image is magnified (larger than object).
< 1: Image is diminished (smaller than object).
- Difference between Positive and Negative Power of Lens
Positive Power: Indicates a convex lens.
Negative Power: Indicates a concave lens.
- Difference between Principal Axis and Axis of Lens
Principal Axis: Line passing through optical center and both focal points.
Axis of Lens: General alignment of the lens setup, not always principal.
- Difference between Refraction through Lens and Prism
Lens: Has two spherical surfaces; bends light to form image.
Prism: Has flat surfaces; deviates light without image formation.
- Difference between Equiconvex and Planoconvex Lens
Equiconvex: Both sides equally curved outward.
Planoconvex: One side flat, other side curved outward.
- Difference between Virtual Image by Convex and Concave Lens
Convex Lens: Virtual image is magnified, erect.
Concave Lens: Virtual image is diminished, erect.
- Difference between Rays Passing Through Optical Center and Not
Through Optical Center: No deviation.
Not Through Optical Center: Undergoes refraction and bends.
- Difference between Image at F and 2F in Convex Lens
At F: Image at infinity, highly enlarged.
At 2F: Image at 2F, same size.
- Difference between Diminished Image by Concave and Convex Lens
Concave Lens: Always diminished regardless of position.
Convex Lens: Diminished only when object is beyond 2F.
- Difference between Power of Lens and Focal Length
Power: Reciprocal of focal length; measured in Diopters.
Focal Length: Distance from optical center to focus; measured in cm or m.
- Difference between Object at Infinity and Object at F (Convex Lens)
At Infinity: Image at focus, highly diminished.
At F: Image at infinity, highly enlarged.
- Difference between Converging and Diverging Rays
Converging Rays: Move toward each other to meet at a point.
Diverging Rays: Move apart, appear to come from a point.
- Difference between Concave Lens in Myopia vs Convex Lens in Hypermetropia
Concave Lens (Myopia): Diverges rays, reducing focal length.
Convex Lens (Hypermetropia): Converges rays, increasing effective focusing.
Assertion and Reason
- (A) Both A and R are true, and R is the correct explanation of A
- (B) Both A and R are true, but R is not the correct explanation of A
- (C) A is true, but R is false
- (D) A is false, but R is true
- (E) Both A and R are false
Assertion (A): A convex lens always forms virtual images.
Reason (R): Convex lenses diverge light rays.
Answer: D
Assertion (A): A concave lens has a negative focal length.
Reason (R): It diverges light rays.
Answer: A
Assertion (A): Real images can be obtained on a screen.
Reason (R): Real images are formed by actual intersection of light rays.
Answer: A
Assertion (A): A ray passing through the optical center of a lens bends towards the principal axis.
Reason (R): The optical center of a lens is a point of minimum deviation.
Answer: C
Assertion (A): Convex lenses are used in magnifying glasses.
Reason (R): Convex lenses can produce virtual, magnified images.
Answer: A
Assertion (A): A concave lens always forms magnified images.
Reason (R): Concave lenses always produce virtual and diminished images.
Answer: D
Assertion (A): The power of a concave lens is negative.
Reason (R): The focal length of a concave lens is negative.
Answer: A
Assertion (A): In a convex lens, the image of an object placed between F and 2F is magnified.
Reason (R): As the object comes closer to F, the image distance increases.
Answer: A
Assertion (A): A real image is always erect.
Reason (R): Real images are formed by diverging rays.
Answer: E
Assertion (A): Light changes direction when it passes through a lens.
Reason (R): Refraction occurs due to change in speed of light in different media.
Answer: A
Assertion (A): All lenses bend light at the same angle.
Reason (R): The curvature and refractive index of lenses are always identical.
Answer: E
Assertion (A): Power of a lens depends on its focal length.
Reason (R): Power is the reciprocal of focal length in meters.
Answer: A
Assertion (A): Concave lenses can produce real images.
Reason (R): Diverging lenses always form real images.
Answer: E
Assertion (A): The image formed by a concave lens is always on the same side as the object.
Reason (R): A concave lens always produces a virtual image.
Answer: A
Assertion (A): The focal length of a convex lens is measured from its pole.
Reason (R): The pole is the center point of a spherical lens.
Answer: E
Assertion (A): The formula 1/f = 1/v – 1/u is applicable only to convex lenses.
Reason (R): Concave lenses do not obey the lens formula.
Answer: E
Assertion (A): Magnification of a real image is always negative.
Reason (R): Real images are always inverted.
Answer: A
Assertion (A): A ray passing through the focus of a convex lens emerges parallel to the principal axis.
Reason (R): This is a principal ray used in ray diagrams.
Answer: A
Assertion (A): Rays passing through the optical center are refracted twice.
Reason (R): Light always bends at every interface.
Answer: C
Assertion (A): All virtual images formed by lenses are inverted.
Reason (R): Virtual images are produced by converging light rays.
Answer: E
Assertion (A): The sign convention for lenses is the same as that for mirrors.
Reason (R): Both follow the Cartesian coordinate system.
Answer: A
Assertion (A): A concave lens forms a virtual, erect, and diminished image.
Reason (R): The image lies between the focus and the optical center.
Answer: A
Assertion (A): An image at infinity formed by a convex lens is highly magnified.
Reason (R): Object is placed at the focus in this case.
Answer: A
Assertion (A): A lens with more curvature has more power.
Reason (R): Focal length decreases with increase in curvature.
Answer: A
Assertion (A): The focal length of a lens is always positive.
Reason (R): Both concave and convex lenses are converging.
Answer: E
Assertion (A): A concave lens always produces an image smaller than the object.
Reason (R): It diverges rays away from the principal axis.
Answer: A
Assertion (A): A convex lens can never form a virtual image.
Reason (R): Converging lenses only produce real images.
Answer: E
Assertion (A): A pin is used in determining focal length in the laboratory.
Reason (R): It helps in locating the position of image.
Answer: A
Assertion (A): The lens used in spectacles for reading is always concave.
Reason (R): Reading glasses are used to correct myopia.
Answer: D
Assertion (A): A lens with focal length 25 cm has more power than one with 50 cm.
Reason (R): Power is inversely proportional to focal length.
Answer: A
Assertion (A): Rays diverging from a point source can never meet after passing through a concave lens.
Reason (R): Concave lenses diverge all incident rays.
Answer: A
Assertion (A): The image formed by a convex lens can be both real and virtual.
Reason (R): The nature of image depends on the object distance.
Answer: A
Assertion (A): If v = 20 cm and u = –10 cm, the image is virtual.
Reason (R): v is positive, so image is on the same side as the object.
Answer: C
Assertion (A): Convex lenses are used in cameras to form sharp images.
Reason (R): They can form real, inverted images on screen.
Answer: A
Assertion (A): Focal length of a biconvex lens is positive.
Reason (R): Biconvex lens is a converging lens.
Answer: A
Assertion (A): A lens always forms an image at the optical center.
Reason (R): The optical center is the only fixed point of a lens.
Answer: E
Assertion (A): Convex lenses bend light rays inward.
Reason (R): They act as converging mediums.
Answer: A
Assertion (A): In the lens formula, object distance is always taken as positive.
Reason (R): Object is always placed in front of the lens.
Answer: C
Assertion (A): A concave lens forms images that are always inverted.
Reason (R): Concave lenses produce real images.
Answer: E
Assertion (A): A real image is always formed on the opposite side of a convex lens.
Reason (R): Real images require actual convergence of rays.
Answer: A
Assertion (A): Virtual images are always magnified.
Reason (R): They are formed only by concave lenses.
Answer: E
Assertion (A): The image distance (v) can be negative in lenses.
Reason (R): Negative v indicates image on the same side as the object.
Answer: A
Assertion (A): In lenses, distances are measured from the focus.
Reason (R): The optical center is not relevant for measurements.
Answer: D
Assertion (A): Lens formula can be applied to both convex and concave lenses.
Reason (R): It is derived from the geometry of ray diagrams.
Answer: A
Assertion (A): Concave lenses are used in door viewers.
Reason (R): They provide a wider field of view.
Answer: A
Assertion (A): Virtual images are formed by actual convergence of light.
Reason (R): They can be captured on a screen.
Answer: E
Assertion (A): Focal length and power are directly proportional.
Reason (R): A greater focal length implies greater power.
Answer: E
Assertion (A): A convex lens acts like two prisms joined at their bases.
Reason (R): This makes it converge light rays.
Answer: A
Assertion (A): The term “magnification” refers to size of the object.
Reason (R): It is the ratio of image distance to object distance.
Answer: C
Assertion (A): Convex lenses are thicker at the center.
Reason (R): This allows them to bend light inward.
Answer: A
True or False
- A convex lens is also known as a converging lens.
True - A concave lens always forms a real image.
False - The optical center is the midpoint of a lens.
True - A ray passing through the optical center bends sharply.
False - A concave lens diverges parallel rays of light.
True - Real images can be obtained on a screen.
True - Virtual images can be projected on a screen.
False - The focal length of a concave lens is negative.
True - Convex lenses always produce diminished images.
False - The unit of lens power is Diopter.
True - The power of a convex lens is always negative.
False - A concave lens forms images that are always inverted.
False - The lens formula is 1/f = 1/v – 1/u.
True - Object distance (u) is always taken as positive in lens formula.
False - A ray through the focus of a convex lens emerges parallel to the principal axis.
True - All lenses are opaque.
False - A convex lens can form both real and virtual images.
True - Concave lenses can be used in magnifying glasses.
False - A magnifying glass uses a convex lens.
True - The image formed by a magnifying glass is always real and magnified.
False - Rays that pass through the optical center of a lens go undeviated.
True - In concave lenses, the image is always formed on the opposite side of the object.
False - A convex lens forms a real, inverted, and same-sized image when the object is at 2F.
True - When the object is between F and O in a convex lens, the image is real and inverted.
False - Concave lens always produces a virtual, erect, and diminished image.
True - The principal axis is a straight line passing through the pole of a mirror.
False - A ray parallel to the principal axis passes through the focus after refraction by a convex lens.
True - The magnification produced by a concave lens is always positive.
True - The power of a lens increases with increase in its focal length.
False - A diverging lens has negative power.
True - A convex lens has two focal points.
True - A real image formed by a convex lens is always erect.
False - A convex lens can be used to project images on a screen.
True - The more curved the lens, the longer its focal length.
False - In optical instruments, concave lenses are used for enlarging images.
False - If focal length is 20 cm, power is 5D.
True - The power formula is P = 100/f where f is in centimeters.
True - A concave lens is thicker at the center than at the edges.
False - Convex lenses are used in telescopes and cameras.
True - In concave lenses, images are always formed beyond 2F.
False - The image formed by a convex lens for a distant object is at focus.
True - The position of the object affects the nature of the image formed by a convex lens.
True - The unit Diopter is used for measuring focal length.
False - Plane mirrors and lenses follow the same sign convention.
False - The image formed when the object is at F in a convex lens is at infinity.
True - Concave lenses are used in the correction of hypermetropia.
False - A convex lens always forms magnified images.
False - The image formed by a concave lens is located between the optical center and the focus.
True - All real images are inverted.
True - The focal length of a convex lens is measured from its pole.
False
Long Answer Questions
- Explain the construction and working of a convex lens using ray diagrams.
A convex lens is a transparent medium with both surfaces bulging outward. It converges parallel rays to a point called the focus. In ray diagrams, three principal rays are used: (i) A ray parallel to the principal axis passes through the focus after refraction, (ii) A ray through the optical center passes undeviated, and (iii) A ray passing through the focus emerges parallel to the principal axis. These help locate the image depending on the object’s position.
- Describe the image formation in a convex lens when the object is at infinity.
When the object is placed at infinity, the rays incident on the convex lens are parallel. After refraction, these rays converge at the principal focus of the lens. The image formed is real, inverted, highly diminished, and located at the focus (F).
- Describe the image formation when the object is placed at 2F in a convex lens.
When an object is placed at 2F in a convex lens, the image is formed on the other side of the lens at 2F itself. The image is real, inverted, and same in size as the object. This is a symmetrical position in lens image formation.
- Explain how a convex lens forms a magnified image.
When the object is placed between F and 2F, the image is formed beyond 2F on the other side of the convex lens. This image is real, inverted, and magnified. If the object is placed between F and O, the image is virtual, erect, and magnified, formed on the same side of the lens.
- Describe the nature and position of the image formed by a convex lens when the object is placed at the focus.
When the object is placed at focus (F), the refracted rays become parallel and never meet. Therefore, the image is formed at infinity, and it is real, inverted, and highly enlarged.
- Explain the image formation by a concave lens using ray diagrams.
A concave lens always forms a virtual, erect, and diminished image regardless of the object’s position. Principal rays used in ray diagrams include: (i) A ray parallel to the principal axis appears to diverge from the focus, (ii) A ray passing through the optical center passes undeviated, and (iii) A ray directed toward the focus emerges parallel to the principal axis.
- What is meant by principal axis, optical center, and focus of a lens?
The principal axis is the straight line passing through the optical center and perpendicular to the lens surfaces. The optical center is the geometric center of the lens where light passes undeviated. The focus is the point where parallel rays converge (convex) or appear to diverge (concave) after refraction.
- State and explain the lens formula.
The lens formula is given as: 1/f = 1/v – 1/u, where f is the focal length, v is the image distance, and u is the object distance (measured from the optical center). It relates the distances for image and object with the lens focal length and is applicable for both convex and concave lenses under the sign convention.
- What is magnification? Derive its formula.
Magnification is the ratio of the height of the image to the height of the object, i.e., m = h’/h. Also, from ray diagrams and geometry, m = v/u, where v is image distance and u is object distance. If m > 1, the image is magnified; if m < 1, it is diminished.
- Explain how a convex lens behaves like a combination of prisms.
A convex lens acts like two prisms joined base to base. These prisms bend light rays toward the base, resulting in the convergence of light rays toward the principal axis. Hence, a convex lens focuses parallel rays at the focal point.
- Explain how a concave lens behaves like a combination of prisms.
A concave lens behaves like two prisms placed apex to apex. These prisms bend incoming parallel rays outward, making them appear to diverge from a common point called the focus. Thus, concave lenses always diverge light rays.
- Describe how sign convention is applied to lenses.
Distances are measured from the optical center. Distances measured in the direction of incident light are positive, and those opposite are negative. Heights above the principal axis are positive, while below are negative. This convention is used in lens and magnification formulas.
- Derive the formula for power of a lens and state its unit.
The power P of a lens is the reciprocal of its focal length in meters: P = 1/f. The unit of power is Diopter (D). If f is expressed in centimeters, then P = 100/f (cm). Convex lenses have positive power, concave lenses have negative power.
- How can focal length of a convex lens be determined using the distant object method?
Place the convex lens in front of a white screen and face it toward the Sun or a distant object. Move the lens until a sharp, clear image is formed on the screen. Measure the distance between the lens and the screen; this distance is approximately the focal length.
- Describe the experimental setup using a plane mirror to determine focal length of a convex lens.
Place a convex lens vertically on a stand. Keep a plane mirror behind the lens. Place a pin in front and adjust the pin’s position such that its image coincides with it. The distance between the pin and the optical center of the lens gives the focal length.
- How is the one pin method used to determine focal length of a convex lens?
Place the lens on an optical bench. Place a pin (object) in front of it and move a screen behind the lens until a sharp image is formed. Measure the object distance (u) and image distance (v), then use lens formula to calculate focal length.
- Describe the image characteristics formed by a convex lens at different object positions.
- At infinity: real, inverted, highly diminished (at F)
- Beyond 2F: real, inverted, diminished (between F and 2F)
- At 2F: real, inverted, same size (at 2F)
- Between F and 2F: real, inverted, magnified (beyond 2F)
- At F: image at infinity
- Between F and O: virtual, erect, magnified (same side)
- What are the characteristics of an image formed by a concave lens?
A concave lens always forms a virtual, erect, and diminished image regardless of object position. The image appears on the same side as the object and is closer to the optical center than the object.
- List three applications of convex lenses.
Convex lenses are used in cameras for focusing light, in magnifying glasses to enlarge images, and in spectacles for correcting hypermetropia (farsightedness).
- List three applications of concave lenses.
Concave lenses are used in spectacles to correct myopia (nearsightedness), in peepholes of doors to provide a wide field of view, and in laser instruments to expand beams.
- Explain why a convex lens can form both real and virtual images.
A convex lens can converge light rays; when the object is outside the focal length, it forms real, inverted images. If the object is within the focal length, it cannot converge rays, forming a virtual, erect, magnified image on the same side.
- Explain why a concave lens forms only virtual images.
A concave lens diverges all incident parallel rays, so the refracted rays never meet. Hence, a virtual image is formed by extending diverging rays backward. This image is always erect and diminished.
- Why is the image formed by a concave lens always diminished?
Due to divergence, the rays spread out more after refraction. The extensions of these rays appear to come from a point closer to the lens, producing a smaller image compared to the object.
- Why does a convex lens converge rays?
Due to its outward bulging shape, the convex lens bends rays toward the principal axis due to refraction. The curvature causes the light to focus at a point known as the focus.
- Explain why the focal length of a thick lens is shorter.
A thicker lens has a higher curvature, causing greater bending of light rays. This results in the rays converging sooner, thus decreasing the focal length.
- Why does a concave lens have negative focal length?
Because a concave lens diverges rays, the focus lies on the same side as the object, making the focal length negative by sign convention.
- How does a convex lens magnify objects in a magnifying glass?
When the object is placed within the focal length of the convex lens, it forms a virtual, erect, and enlarged image on the same side, thus magnifying the object.
- How is a concave lens helpful in correcting myopia?
A concave lens diverges light rays before they enter the eye, so they can be focused correctly on the retina instead of in front of it.
- How does a convex lens help in correcting hypermetropia?
A convex lens converges light rays before they enter the eye, allowing the eye to focus them properly on the retina rather than behind it.
- Why can’t concave lenses produce real images?
Because concave lenses diverge rays, they do not bring them to an actual meeting point, hence cannot produce real images.
- What are the three principal rays used in ray diagrams for lenses?
(i) Ray parallel to principal axis (passes through or appears to diverge from focus),
(ii) Ray through optical center (undeviated),
(iii) Ray through focus (emerges parallel to principal axis).
- Explain how to find image position using a ray diagram.
Draw any two of the three principal rays from the object. Their intersection (real or virtual) gives the position of the image.
- What are the sign conventions used in lens formula?
Distances measured in the direction of incident light are positive, opposite are negative; heights above principal axis are positive, below are negative.
- How can the power of a lens be increased?
By decreasing the focal length or increasing the curvature and refractive index of the lens.
- Describe the characteristics of an image formed by a convex lens when object is between F and O.
The image is virtual, erect, magnified, and formed on the same side of the lens as the object.
- State and explain the nature of images formed by a concave lens with a ray diagram.
Concave lenses form virtual, erect, and diminished images by making parallel rays appear to diverge from a point called the focus on the same side as the object.
- When is the image formed at 2F in a convex lens?
When the object is also placed at 2F, the image is real, inverted, and of the same size, formed on the opposite side at 2F.
- What is the role of lens thickness in image formation?
A thicker lens bends rays more due to increased curvature, affecting the focal length and image position accordingly.
- How does curvature of the lens affect its focal length?
Greater curvature causes stronger bending of light rays, resulting in a shorter focal length.
- Why is a screen required to obtain real images?
Because real images are formed by actual convergence of rays and can only be seen when intercepted on a screen.
- What is the use of a convex lens in a projector?
It forms real, enlarged, inverted images of objects that can be projected on a screen.
- What is the function of a concave lens in a door viewer?
It diverges incoming light rays to give a wide-angle view from a small peephole.
- Why is a convex lens used in cameras?
It focuses light rays from objects onto a film or sensor, forming a sharp, real image.
- How does a biconvex lens differ from a plano-convex lens?
A biconvex lens has two outward curved surfaces, whereas a plano-convex lens has one flat and one curved surface; both converge light but with different focal properties.
- What is meant by the term ‘equiconvex lens’?
A lens where both convex surfaces have equal radii of curvature, giving symmetric convergence of light.
- How is the focal length of a convex lens related to its power?
They are inversely related; P = 100/f (when f is in cm), so shorter focal length means higher power.
- Describe the behavior of light in an equiconcave lens.
An equiconcave lens diverges light rays, making them appear to come from a virtual focus on the same side as the object.
- What happens when two convex lenses are used together?
Their combined power is the sum of individual powers, resulting in stronger convergence and a shorter combined focal length.
- How does increasing the refractive index of lens material affect its focusing ability?
A higher refractive index increases light bending, resulting in a shorter focal length and greater converging power.
- Summarize all image types a convex lens can form based on object position.
Convex lenses can form:
- Real, inverted, diminished (beyond 2F)
- Real, inverted, same size (at 2F)
- Real, inverted, magnified (between F and 2F)
- Virtual, erect, magnified (between F and O)
Give Reasons
- Give reason: A convex lens is called a converging lens.
Because it bends parallel rays of light toward the principal axis, making them meet at a single point called the focus.
- Give reason: A concave lens is called a diverging lens.
Because it spreads out parallel rays of light, making them appear to diverge from a virtual focus.
- Give reason: A convex lens can produce a real image.
Because it converges rays of light to a point on the opposite side of the lens, allowing an actual intersection of rays.
- Give reason: Concave lenses always produce virtual images.
Because the refracted rays diverge and never actually meet, so the image is formed by back projection of diverging rays.
- Give reason: The image formed by a convex lens can be inverted.
Because when the object is beyond the focal point, the refracted rays actually meet on the opposite side, forming a real and inverted image.
- Give reason: A magnifying glass uses a convex lens.
Because when an object is placed within the focal length, it forms a virtual, erect, and magnified image.
- Give reason: Concave lenses cannot form magnified images.
Because the diverging rays always produce images smaller than the object, regardless of object position.
- Give reason: Real images are always inverted.
Because they are formed by actual convergence of rays which cross each other, resulting in inversion.
- Give reason: Virtual images cannot be obtained on a screen.
Because virtual images are formed by extensions of diverging rays and do not actually exist in space.
- Give reason: The focal length of a convex lens is taken as positive.
Because the image is formed on the side opposite to the incident light, in the direction of light travel.
- Give reason: The focal length of a concave lens is negative.
Because the image is formed on the same side as the object, opposite to the direction of light.
- Give reason: Power of a convex lens is positive.
Because it has a positive focal length, and power is inversely proportional to focal length.
- Give reason: A concave lens has negative power.
Because its focal length is negative, and power = 100/f (in cm).
- Give reason: A convex lens forms an image at infinity when the object is at focus.
Because the refracted rays become parallel and never meet on the other side.
- Give reason: A lens has two focal points.
Because light can pass through it from either side, requiring two symmetrical points on both sides.
- Give reason: A ray passing through the optical center emerges undeviated.
Because it passes symmetrically through the lens without bending, having equal angle of incidence and emergence.
- Give reason: A convex lens can form a virtual image.
Because if the object is within its focal length, rays diverge after refraction and appear to come from a virtual point.
- Give reason: Concave lens is used in myopia correction.
Because it diverges light rays, helping the eye focus them correctly on the retina.
- Give reason: Convex lens is used in hypermetropia correction.
Because it converges light rays so that they focus correctly on the retina instead of behind it.
- Give reason: The power of a lens is inversely proportional to its focal length.
Because power (P) is defined as P = 100/f, where f is the focal length in cm.
- Give reason: A thick lens has a shorter focal length.
Because it bends light more strongly due to higher curvature.
- Give reason: A concave lens always forms an erect image.
Because the rays appear to diverge from a point without actually crossing, maintaining the upright orientation.
- Give reason: Real images can be caught on a screen.
Because they are formed by the actual intersection of light rays.
- Give reason: A concave lens does not produce a real image.
Because it diverges rays and they never intersect in real space.
- Give reason: Convex lenses are used in cameras.
Because they can form real, inverted, and focused images on a screen or film.
- Give reason: Magnification is negative for real images.
Because real images are inverted, indicating a negative height ratio.
- Give reason: The image formed by a concave lens is always smaller.
Because diverging rays result in an image that is closer to the optical center and therefore diminished.
- Give reason: A concave lens is thinner at the center.
Because this shape causes light to bend outward and diverge.
- Give reason: The lens formula applies to both convex and concave lenses.
Because it is derived using sign conventions that adjust for the nature of the lens.
- Give reason: A lens behaves like a combination of prisms.
Because the shape of the lens causes refraction similar to light bending in prism sets.
- Give reason: Equiconvex lenses form real images of distant objects.
Because parallel rays converge at the focus after passing through both curved surfaces.
- Give reason: Plane mirrors cannot form real images.
Because the reflected rays do not actually converge but only appear to diverge from behind the mirror.
- Give reason: Concave lenses are used in peepholes.
Because they provide a wide field of view by diverging incoming rays.
- Give reason: Lens power is expressed in diopters.
Because it represents the refractive ability of the lens in terms of inverse focal length in meters.
- Give reason: Rays directed toward the focus in a concave lens emerge parallel.
Because the lens bends these rays outward such that they follow the direction parallel to the principal axis.
- Give reason: A convex lens forms an image on the opposite side.
Because it converges rays and forms real images where the rays meet.
- Give reason: Virtual images formed by convex lenses are erect.
Because the rays diverge after refraction and appear to come from an upright point.
- Give reason: The image formed by a convex lens can be diminished.
Because when the object is beyond 2F, the image is formed between F and 2F, resulting in a smaller image.
- Give reason: Light bends when passing through a lens.
Because of the change in speed as it enters a medium of different refractive index.
- Give reason: Magnification more than 1 indicates enlargement.
Because it means the image height is greater than the object height.
- Give reason: Magnification equal to 1 indicates same size image.
Because the image height equals the object height.
- Give reason: Inverted images are considered negative in sign convention.
Because they lie below the principal axis and hence are assigned a negative value.
- Give reason: A convex lens used in projectors must be placed beyond F.
Because it needs to form real and enlarged images on the screen.
- Give reason: Focal length must be in meters for calculating power.
Because the SI unit of power (Diopter) is based on focal length in meters.
- Give reason: A lens forms an image only due to refraction.
Because lenses bend light by refraction at both curved surfaces.
- Give reason: A ray through the optical center suffers no deviation.
Because it enters and exits symmetrically without change in medium angle.
- Give reason: All incident rays don’t pass through the focus.
Because only rays parallel to the principal axis do so after refraction.
- Give reason: A convex lens forms virtual image only when the object is close.
Because the rays diverge when object is within the focal length, forming a virtual image.
- Give reason: Image in convex lens is formed at different locations depending on object position.
Because the nature of refraction varies with object distance, altering where rays meet.
- Give reason: Concave lens images are always on the same side as the object.
Because diverged rays appear to come from a point on the same side as the source.
Arrange the Words
Case Studies
Case Study 1:
Riya placed a candle 30 cm in front of a convex lens of focal length 15 cm.
Q1. What type of image is formed?
Q2. Where is the image formed?
Q3. What is the nature (real/virtual) and orientation of the image?
Answers:
- Real and inverted
- 30 cm on the other side of the lens
- Real and inverted
Case Study 2:
A student places an object at the focus of a convex lens.
Q1. Where will the image be formed?
Q2. What is the nature of the image?
Q3. What will be the size of the image?
Answers:
- At infinity
- Real and inverted
- Highly enlarged
Case Study 3:
An optician gives a patient a lens with focal length –50 cm.
Q1. What type of lens is it?
Q2. What is its power?
Q3. Is it used for myopia or hypermetropia?
Answers:
- Concave lens
- –2D
- Myopia
Case Study 4:
A student uses a concave lens to view an object placed 20 cm in front.
Q1. What is the nature of the image?
Q2. Is the image real or virtual?
Q3. Is the image magnified or diminished?
Answers:
- Erect and diminished
- Virtual
- Diminished
Case Study 5:
An object is placed at 2F of a convex lens.
Q1. Where is the image formed?
Q2. What is the size of the image?
Q3. What is the orientation of the image?
Answers:
- At 2F on the other side
- Same size
- Inverted
Case Study 6:
A convex lens of power +5D is used in a device.
Q1. What is the focal length of the lens?
Q2. Is the lens converging or diverging?
Q3. Suggest one application of this lens.
Answers:
- 20 cm
- Converging
- Magnifying glass
Case Study 7:
Rahul wants to make a real image on a screen using a convex lens.
Q1. Where should he place the object?
Q2. What is the condition to form a real image?
Q3. Will the image be erect or inverted?
Answers:
- Beyond focus
- Object must be beyond F
- Inverted
Case Study 8:
A concave lens forms an image 15 cm from its optical center, while the object is placed 30 cm in front.
Q1. What is the magnification?
Q2. Is the image on the same side or opposite side of the object?
Q3. What is the sign of magnification?
Answers:
- 0.5
- Same side
- Positive
Case Study 9:
An image formed by a lens is real, inverted, and smaller than the object.
Q1. What type of lens is used?
Q2. Where is the object placed relative to the lens?
Q3. What is the magnification?
Answers:
- Convex lens
- Beyond 2F
- Less than 1 (negative)
Case Study 10:
A person holds a convex lens close to an object and sees an enlarged, erect image.
Q1. Where is the object placed?
Q2. What is the nature of the image?
Q3. Is the image real or virtual?
Answers:
- Between F and O
- Enlarged and erect
- Virtual
Case Study 11:
In a lab, a student finds that light rays after passing through a lens converge at a point.
Q1. Which lens is used?
Q2. What is that point called?
Q3. What is the sign of its focal length?
Answers:
- Convex lens
- Focus
- Positive
Case Study 12:
A bi-convex lens has equal radii of curvature on both sides.
Q1. What is it called?
Q2. What is its function?
Q3. Can it form a virtual image?
Answers:
- Equiconvex lens
- Converge light rays
- Yes, when object is within focus
Case Study 13:
An object is placed 10 cm in front of a convex lens of focal length 20 cm.
Q1. Where is the image formed?
Q2. What is the nature of the image?
Q3. What is the magnification?
Answers:
- On the same side
- Virtual, erect
- Greater than 1
Case Study 14:
A concave lens has a focal length of –25 cm.
Q1. What is its power?
Q2. Is it converging or diverging?
Q3. What will happen to parallel rays falling on it?
Answers:
- –4D
- Diverging
- They will diverge as if coming from the focus
Case Study 15:
A projector uses a convex lens to form images.
Q1. What type of image is formed?
Q2. What is the orientation?
Q3. Is the image larger or smaller than the object?
Answers:
- Real
- Inverted
- Enlarged
Case Study 16:
A magnifying glass has a lens of focal length 10 cm.
Q1. What type of lens is used?
Q2. Where should the object be placed?
Q3. What is the nature of the image?
Answers:
- Convex lens
- Within 10 cm (within focus)
- Virtual, erect, magnified
Case Study 17:
An object placed 20 cm from a concave lens forms an image 10 cm away.
Q1. What is the magnification?
Q2. Is the image real or virtual?
Q3. Is the image erect or inverted?
Answers:
- 0.5
- Virtual
- Erect
Case Study 18:
A convex lens forms an image of the same size as the object.
Q1. Where is the object placed?
Q2. Where is the image formed?
Q3. What is the magnification?
Answers:
- At 2F
- At 2F
- –1
Case Study 19:
In a lab, a student notices that parallel rays appear to diverge after passing through a lens.
Q1. What lens is being used?
Q2. What is the behavior of rays?
Q3. Where do the rays appear to come from?
Answers:
- Concave lens
- Diverge
- Virtual focus
Case Study 20:
An object is moved from beyond 2F to F in front of a convex lens.
Q1. What happens to the image size?
Q2. What happens to the image distance?
Q3. What is the nature of the image?
Answers:
- Image size increases
- Image distance increases
- Real and inverted
Numericals
- A convex lens has a focal length of 20 cm. An object is placed 30 cm in front of it. Find the image distance.
Solution: Given: u = -30 cm, f = +20 cm Using lens formula: 1/f = 1/v – 1/u => 1/20 = 1/v + 1/30 => 1/v = 1/20 – 1/30 = (3 – 2)/60 = 1/60 => v = 60 cm
Answer: v = 60 cm (Real, Inverted)
- An object is placed 25 cm from a concave lens of focal length 15 cm. Find the image distance.
Solution: u = -25 cm, f = -15 cm 1/f = 1/v – 1/u => -1/15 = 1/v + 1/25 => 1/v = -1/15 – 1/25 = (-5 – 3)/75 = -8/75 => v = -9.38 cm
Answer: v = -9.38 cm (Virtual, Erect)
- A convex lens forms an image 40 cm away when the object is 30 cm from it. Find focal length.
Solution: u = -30 cm, v = +40 cm 1/f = 1/v – 1/u = 1/40 + 1/30 = (3 + 4)/120 = 7/120 => f = 17.14 cm
Answer: f = 17.14 cm
- Find the power of a convex lens whose focal length is 50 cm.
Solution: P = 100/f = 100/50 = 2 D
Answer: +2 D
- Find the power of a concave lens with focal length -25 cm.
Solution: P = 100/f = 100/(-25) = -4 D
Answer: -4 D
- An object is 15 cm from a convex lens of focal length 10 cm. Find the image distance.
Solution: u = -15 cm, f = +10 cm 1/f = 1/v – 1/u => 1/10 = 1/v + 1/15 => 1/v = 1/10 – 1/15 = (3 – 2)/30 = 1/30 => v = 30 cm
Answer: v = 30 cm (Real, Inverted)
- An object is placed at 2F in a convex lens of focal length 12 cm. Find image distance.
Solution: 2F = 2 x 12 = 24 cm, u = -24 cm, f = +12 cm 1/f = 1/v – 1/u => 1/12 = 1/v + 1/24 => 1/v = 1/12 – 1/24 = (2 – 1)/24 = 1/24 => v = 24 cm
Answer: v = 24 cm (Real, Inverted, Same Size)
- An object is placed at infinity in front of a convex lens of 20 cm focal length. Where is the image formed?
Solution: For object at infinity, image is at focus.
Answer: v = 20 cm (Real, Inverted, Point Image)
- An object is placed 10 cm in front of a concave lens of focal length 20 cm. Find image position.
Solution: u = -10 cm, f = -20 cm 1/f = 1/v – 1/u => -1/20 = 1/v + 1/10 => 1/v = -1/20 – 1/10 = (-1 – 2)/20 = -3/20 => v = -6.67 cm
Answer: v = -6.67 cm (Virtual, Erect)
- Find the focal length of a lens of power +3D.
Solution: f = 100/P = 100/3 = 33.33 cm
Answer: f = 33.33 cm
- A convex lens forms an image at 45 cm from the lens when the object is placed at 15 cm. Find the focal length of the lens.
Solution: u = –15 cm, v = +45 cm 1/f = 1/v – 1/u = 1/45 + 1/15 = (1 + 3)/45 = 4/45 => f = 11.25 cm
Answer: 11.25 cm
- An object is placed 20 cm in front of a lens. The image is formed at 40 cm on the opposite side. Calculate the focal length.
Solution: u = –20 cm, v = +40 cm 1/f = 1/v – 1/u = 1/40 + 1/20 = (1 + 2)/40 = 3/40 => f = 13.33 cm
Answer: 13.33 cm
- A concave lens forms an image 15 cm away when the object is placed at 30 cm. Find the focal length.
Solution: v = –15 cm, u = –30 cm 1/f = 1/v – 1/u = –1/15 + 1/30 = (–2 + 1)/30 = –1/30 => f = –30 cm
Answer: –30 cm
- A lens has a focal length of –16.7 cm. Calculate its power.
Solution: P = 100/f = 100/(–16.7) ≈ –6 D
Answer: –6 D
- A lens of power +4 D is used. What is its focal length in meters and centimeters?
Solution: f = 100/4 = 25 cm = 0.25 m
Answer: 25 cm or 0.25 m
- An object is placed 25 cm from a convex lens and an image is formed 25 cm on the other side. Find the focal length.
Solution: u = –25 cm, v = +25 cm 1/f = 1/v – 1/u = 1/25 + 1/25 = 2/25 => f = 12.5 cm
Answer: 12.5 cm
- A concave lens forms an image 10 cm away when the object is placed at 20 cm. Calculate the focal length.
Solution: v = –10 cm, u = –20 cm 1/f = 1/v – 1/u = –1/10 + 1/20 = –1/20 => f = –20 cm
Answer: –20 cm
- A convex lens has a power of +2.5 D. What is its focal length?
Solution: f = 100/2.5 = 40 cm
Answer: 40 cm
- The focal length of a lens is –33.3 cm. Calculate its power.
Solution: P = 100/f = 100/(–33.3) ≈ –3 D
Answer: –3 D
- An object is placed 45 cm in front of a convex lens. The image is formed at 90 cm. Find the focal length.
Solution: u = –45 cm, v = +90 cm 1/f = 1/v – 1/u = 1/90 + 1/45 = (1 + 2)/90 = 3/90 = 1/30 => f = 30 cm
Answer: 30 cm
- An object is placed 50 cm in front of a convex lens. Image is formed at 25 cm. Find focal length.
Solution: u = -50 cm, v = +25 cm 1/f = 1/v – 1/u = 1/25 + 1/50 = (2 + 1)/50 = 3/50 => f = 16.67 cm
Answer: 16.67 cm
- If f = +10 cm, find power.
Solution: P = 100/f = 100/10 = 10 D
Answer: +10 D
- A 5 cm tall object produces a 15 cm tall image. Find magnification.
Solution: M = h_i / h_o = 15 / 5 = 3
Answer: 3
- A lens produces image height 4 cm and object height 8 cm. Find nature of image.
Solution: M = h_i / h_o = 4 / 8 = 0.5 (positive, so virtual & erect)
Answer: Virtual, Erect
- A convex lens has f = 25 cm. Object is placed at 50 cm. Find image distance.
Solution: u = -50 cm, f = +25 cm 1/f = 1/v – 1/u => 1/25 = 1/v + 1/50 => 1/v = 1/25 – 1/50 = (2 – 1)/50 = 1/50 => v = 50 cm
Answer: 50 cm
- Find v if u = –20 cm and f = +10 cm (convex lens).
Solution: 1/f = 1/v – 1/u => 1/10 = 1/v + 1/20 => 1/v = 1/10 – 1/20 = (2 – 1)/20 = 1/20 => v = 20 cm
Answer: 20 cm
- An object 5 cm from a concave lens produces image 3.33 cm away. Find focal length.
Solution: u = -5 cm, v = -3.33 cm 1/f = 1/v – 1/u = -1/3.33 + 1/5 = (–0.3 + 0.2) = –0.1 => f = –10 cm
Answer: –10 cm
- What is the magnification if image = –15 cm, object = –30 cm?
Solution: M = v/u = –15 / –30 = 0.5
Answer: 0.5
- An object is at F of convex lens. Where is the image?
Solution: When object is at focus, image is formed at infinity.
Answer: At infinity
- Object = –40 cm, f = +20 cm. Find image distance.
Solution: 1/f = 1/v – 1/u => 1/20 = 1/v + 1/40 => 1/v = 1/20 – 1/40 = (2 – 1)/40 = 1/40 => v = 40 cm
Answer: 40 cm
- Focal length = –10 cm. What is power?
Solution: P = 100/f = 100/(–10) = –10 D
Answer: –10 D
- If u = –15 cm, f = –30 cm (concave lens), find v.
Solution: 1/f = 1/v – 1/u => –1/30 = 1/v + 1/15 => 1/v = –1/30 – 1/15 = –3/30 = –1/10 => v = –10 cm
Answer: –10 cm
- f = +50 cm. What is power?
Solution: P = 100/f = 100/50 = 2 D
Answer: +2 D
- An object 30 cm from lens forms an image at 60 cm. Find focal length.
Solution: u = -30 cm, v = +60 cm 1/f = 1/v – 1/u = 1/60 + 1/30 = (1 + 2)/60 = 3/60 = 1/20 => f = 20 cm
Answer: 20 cm
- A convex lens forms image same size as object. Where is object placed?
Solution: At 2F (i.e., twice the focal length).
Answer: At 2F
- If image is 2 times taller and inverted, what is magnification?
Solution: Inverted => negative magnification, M = –2
Answer: –2
- Object 24 cm from lens. Image formed at 8 cm. Find focal length.
Solution: u = –24 cm, v = +8 cm 1/f = 1/v – 1/u = 1/8 + 1/24 = (3 + 1)/24 = 4/24 = 1/6 => f = 6 cm
Answer: 6 cm
- A convex lens with f = 10 cm. Where to place object to get virtual image?
Solution: Object must be placed within focal length (u < f).
Answer: Less than 10 cm
- Object = –25 cm, f = +20 cm. Find image distance.
Solution: 1/f = 1/v – 1/u => 1/20 = 1/v + 1/25 => 1/v = 1/20 – 1/25 = (5 – 4)/100 = 1/100 => v = 100 cm
Answer: 100 cm
- Object = –30 cm, f = +15 cm. Find v.
Solution: 1/f = 1/v – 1/u => 1/15 = 1/v + 1/30 => 1/v = 1/15 – 1/30 = (2 – 1)/30 = 1/30 => v = 30 cm
Answer: 30 cm
- f = +25 cm, u = –50 cm. Find v.
Solution: 1/f = 1/v – 1/u => 1/25 = 1/v + 1/50 => 1/v = 1/25 – 1/50 = (2 – 1)/50 = 1/50 => v = 50 cm
Answer: 50 cm
- f = –20 cm, u = –40 cm. Find v.
Solution: 1/f = 1/v – 1/u => –1/20 = 1/v + 1/40 => 1/v = –1/20 – 1/40 = –3/40 => v = –13.33 cm
Answer: –13.33 cm
- A lens has focal length 10 cm. Find power.
Solution: P = 100/f = 100/10 = 10 D
Answer: 10 D
- f = –12.5 cm. What is P?
Solution: P = 100/f = 100/(–12.5) = –8 D
Answer: –8 D
- u = –60 cm, f = +20 cm. Find v.
Solution: 1/f = 1/v – 1/u => 1/20 = 1/v + 1/60 => 1/v = 1/20 – 1/60 = (3 – 1)/60 = 2/60 = 1/30 => v = 30 cm
Answer: 30 cm
- An object 10 cm tall forms image 15 cm tall. What is M?
Solution: M = h_i / h_o = 15 / 10 = 1.5
Answer: 1.5
- Image = 10 cm, Object = –20 cm. Find f if lens is convex.
Solution: u = –20 cm, v = 10 cm 1/f = 1/v – 1/u = 1/10 + 1/20 = (2 + 1)/20 = 3/20 => f = 6.67 cm
Answer: 6.67 cm
- If magnification is –1, and object = –30 cm, find image distance.
Solution: M = v/u => –1 = v/–30 => v = 30 cm
Answer: 30 cm
- A concave lens forms image 3/4th the object height. What is M?
Solution: M = +0.75 (positive, virtual & erect)
Answer: 0.75
- A lens has f = 40 cm. What is power?
Solution: P = 100/f = 100/40 = 2.5 D
Answer: 2.5 D
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