Class 10 Light Reflection and Refraction Important Questions with Answers

THE BUSY BRAINS • BY MANSI SARASWAT

LIGHT – REFLECTION AND REFRACTION

Class 10 Science | Important Questions with Answers
Pre-Board & Board Exam Preparation | NCERT-Based Questions | Numericals | MCQs | Case-Based Questions

Class 10 Science Light – Reflection and Refraction is one of the most important chapters for understanding reflection of light, spherical mirrors, mirror formula, magnification, refraction of light, refractive index, spherical lenses, lens formula and power of a lens.

This comprehensive Class 10 Light Reflection and Refraction Important Questions with Answers resource is designed for CBSE Pre-Board and Board Exam preparation. It covers important NCERT concepts, definitions, laws, ray-diagram concepts, image formation, numerical problems, MCQs and case-based questions.

Students should especially practise concave mirror and convex mirror image formation, New Cartesian Sign Convention, mirror formula, magnification, Snell's law, refractive index, convex lens, concave lens, lens formula and power of lens.

MUST DO

Mirror Formula, Lens Formula, Sign Convention, Ray Diagrams & Image Formation

VERY IMPORTANT

Refraction, Refractive Index, Magnification, Spherical Mirrors & Lenses

PRACTICE

Numericals, MCQs, Case-Based Questions & NCERT Exercise Questions

⭐ Board Exam Focus: Practise numerical questions using the correct sign convention. A small sign error in u, v, f, R can change the complete answer.

1. Reflection of Light – Important Questions

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Q1. What is reflection of light?

Answer: Reflection of light is the phenomenon in which light falling on a reflecting surface is sent back into the same medium.

Q2. State the two laws of reflection of light.

Answer:
  1. The angle of incidence is equal to the angle of reflection.
  2. The incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.
The NCERT chapter states these laws and notes that they apply to spherical reflecting surfaces as well. :chatgpt-content-reference{index="1"}

Q3. Write the characteristics of the image formed by a plane mirror.

Answer:
  • The image is virtual and erect.
  • It is of the same size as the object.
  • It is formed at the same distance behind the mirror as the object is in front.
  • The image is laterally inverted.

Q4. Why does a polished mirror reflect most of the light falling on it?

Answer: A highly polished surface such as a mirror has a smooth reflecting surface and therefore reflects most of the incident light.

2. Spherical Mirrors – Important Questions

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Q5. What is a spherical mirror?

Answer: A spherical mirror is a mirror whose reflecting surface forms a part of the surface of a sphere.

Q6. What is a concave mirror?

Answer: A spherical mirror whose reflecting surface is curved inwards and faces towards the centre of the sphere is called a concave mirror.

Q7. What is a convex mirror?

Answer: A spherical mirror whose reflecting surface is curved outwards is called a convex mirror.
VERY IMPORTANT

Q8. Define pole, centre of curvature, radius of curvature and principal axis.

  • Pole (P): The centre of the reflecting surface of a spherical mirror.
  • Centre of curvature (C): The centre of the sphere of which the reflecting surface of the mirror forms a part.
  • Radius of curvature (R): The radius of the sphere of which the reflecting surface of the spherical mirror forms a part.
  • Principal axis: The straight line passing through the pole and the centre of curvature.

Q9. What is the principal focus of a concave mirror?

Answer: The point on the principal axis of a concave mirror at which rays parallel to the principal axis converge after reflection is called the principal focus of the concave mirror.

Q10. What is the principal focus of a convex mirror?

Answer: The point on the principal axis from which rays parallel to the principal axis appear to diverge after reflection from a convex mirror is called its principal focus.
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Q11. What is the relationship between radius of curvature and focal length of a spherical mirror?

Answer: For spherical mirrors of small aperture:
R = 2f
Therefore, f = R/2 The principal focus lies midway between the pole and centre of curvature.

Q12. What is aperture of a spherical mirror?

Answer: The diameter of the reflecting surface of a spherical mirror is called its aperture.

Q13. State two uses of concave mirrors.

Answer:
  • Used in torches, searchlights and vehicle headlights to obtain powerful parallel beams.
  • Used as shaving mirrors to obtain a larger image of the face.
  • Used by dentists to see enlarged images of teeth.
  • Large concave mirrors are used in solar furnaces to concentrate sunlight.
VERY IMPORTANT

Q14. Why are convex mirrors used as rear-view mirrors in vehicles?

Answer: Convex mirrors are preferred because they:
  • Always form an erect image.
  • Form a diminished image.
  • Provide a wider field of view.
  • Allow the driver to see a larger area behind the vehicle.

3. Image Formation by Spherical Mirrors

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Q15. Write the image formation by a concave mirror for different positions of the object.

Object Position Image Position Size Nature
At infinity At F Highly diminished, point-sized Real and inverted
Beyond C Between F and C Diminished Real and inverted
At C At C Same size Real and inverted
Between C and F Beyond C Enlarged Real and inverted
At F At infinity Image not formed —
Between P and F Behind mirror Enlarged Virtual and erect

Q16. What type of image is formed by a convex mirror?

Answer: A convex mirror always forms a virtual, erect and diminished image behind the mirror.

Q17. Which mirror can form an erect and enlarged image?

Answer: A concave mirror forms an erect and enlarged image when the object is placed between the pole and principal focus.
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PRACTICE

Q18. Why can a convex mirror show the full image of a large object?

Answer: A convex mirror provides a wider field of view and forms diminished images. Therefore, a large object can be viewed in its entirety in a relatively small convex mirror.

Q19. State the important rules used for drawing ray diagrams for spherical mirrors.

  1. A ray parallel to the principal axis is reflected through the principal focus of a concave mirror or appears to diverge from the principal focus of a convex mirror.
  2. A ray passing through the principal focus of a concave mirror is reflected parallel to the principal axis.
  3. A ray passing through the centre of curvature of a concave mirror is reflected back along the same path.
  4. A ray incident at the pole is reflected according to the laws of reflection.

4. Mirror Formula, Magnification & Sign Convention

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Q20. State the New Cartesian Sign Convention for spherical mirrors.

  • The pole of the mirror is taken as the origin.
  • The principal axis is taken as the x-axis.
  • The object is placed to the left of the mirror.
  • Distances measured to the right are positive.
  • Distances measured to the left are negative.
  • Distances measured above the principal axis are positive.
  • Distances measured below the principal axis are negative.
MUST DO

Q21. Write the mirror formula.

1/v + 1/u = 1/f
where:
  • u = object distance
  • v = image distance
  • f = focal length

Q22. Define magnification produced by a spherical mirror.

Answer: Magnification is the ratio of the height of the image to the height of the object.
m = h′/h = −v/u
A negative magnification indicates a real image, while a positive magnification indicates a virtual image.

Q23. What does negative magnification indicate?

Answer: Negative magnification indicates that the image is real and inverted.

Q24. What does positive magnification indicate?

Answer: Positive magnification indicates that the image is virtual and erect.

5. Refraction of Light – Important Questions

MUST DO

Q25. What is refraction of light?

Answer: Refraction of light is the phenomenon in which the direction of propagation of light changes when light travels obliquely from one transparent medium to another.

Q26. Give two everyday examples of refraction of light.

  • A pencil partly immersed in water appears displaced.
  • The bottom of a tank or pond appears raised.
  • Letters viewed through a glass slab appear raised.
  • A lemon kept in water may appear larger when viewed from the side.
VERY IMPORTANT

Q27. What happens when light travels from a rarer medium to a denser medium?

Answer: When light travels obliquely from an optically rarer medium to an optically denser medium, it bends towards the normal.

Q28. What happens when light travels from a denser medium to a rarer medium?

Answer: When light travels obliquely from an optically denser medium to an optically rarer medium, it bends away from the normal.

Q29. Why does refraction occur?

Answer: Refraction occurs due to a change in the speed of light when it enters from one transparent medium into another.

Q30. State the laws of refraction.

  1. The incident ray, refracted ray and the normal to the interface at the point of incidence all lie in the same plane.
  2. For a given colour of light and a given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant.
    sin i / sin r = constant
    This is known as Snell's law.

6. Refractive Index – Important Questions

MUST DO

Q31. Define refractive index.

Answer: The refractive index of a medium is a measure of the extent to which light is refracted by that medium. For absolute refractive index:
n = c/v
where c is the speed of light in vacuum and v is the speed of light in the medium.

Q32. What is the speed of light in vacuum?

Answer:
c = 3 × 108 m/s

Q33. What is optical density?

Answer: Optical density refers to the ability of a medium to refract light. A medium having a larger refractive index is optically denser than a medium having a smaller refractive index.

Q34. Does optical density mean mass density?

Answer: No. Optical density and mass density are different concepts. A medium may be optically denser without having greater mass density.
PRACTICE

Q35. The refractive index of diamond is 2.42. What does this mean?

Answer: It means that the ratio of the speed of light in vacuum to the speed of light in diamond is 2.42.
n = c/v = 2.42

Q36. Write the refractive indices of water and diamond given in the NCERT table.

Answer:
  • Water = 1.33
  • Diamond = 2.42

7. Refraction Through a Rectangular Glass Slab

VERY IMPORTANT

Q37. What happens when a ray of light passes through a rectangular glass slab?

Answer: When light enters the glass slab from air, it bends towards the normal. When it emerges from glass into air, it bends away from the normal. Because the opposite surfaces of the slab are parallel, the emergent ray becomes parallel to the incident ray, although it is shifted sideways.

Q38. Why does the emergent ray from a rectangular glass slab remain parallel to the incident ray?

Answer: The bending at the two parallel surfaces is equal and opposite. Therefore, the emergent ray is parallel to the incident ray, although it suffers a lateral displacement.

8. Spherical Lenses – Important Questions

MUST DO

Q39. What is a lens?

Answer: A lens is a transparent material bounded by two surfaces, of which one or both surfaces are spherical.

Q40. Differentiate between convex and concave lenses.

Convex Lens Concave Lens
Thicker at the middle. Thicker at the edges.
Thinner at the edges. Thinner at the middle.
Converges light rays. Diverges light rays.
Called a converging lens. Called a diverging lens.
Focal length is positive. Focal length is negative.

Q41. What is the optical centre of a lens?

Answer: The central point of a lens is called its optical centre. A ray passing through the optical centre of a lens passes without suffering deviation.

Q42. What is the principal axis of a spherical lens?

Answer: The imaginary straight line passing through the two centres of curvature of a spherical lens is called its principal axis.
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Q43. What is the focal length of a lens?

Answer: The distance between the optical centre and the principal focus of a lens is called its focal length.

Q44. What happens to parallel rays incident on a convex lens?

Answer: Parallel rays incident on a convex lens converge at its principal focus after refraction.

Q45. What happens to parallel rays incident on a concave lens?

Answer: Parallel rays incident on a concave lens diverge after refraction and appear to come from its principal focus.

9. Image Formation by Lenses

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Q46. Write the image formation by a convex lens for different positions of the object.

Object Position Image Position Size Nature
At infinity At F2 Highly diminished, point-sized Real and inverted
Beyond 2F1 Between F2 and 2F2 Diminished Real and inverted
At 2F1 At 2F2 Same size Real and inverted
Between F1 and 2F1 Beyond 2F2 Enlarged Real and inverted
At F1 At infinity Image not formed —
Between F1 and O Same side as object Enlarged Virtual and erect

Q47. What type of image is always formed by a concave lens?

Answer: A concave lens always forms a virtual, erect and diminished image, irrespective of the position of the object.

Q48. State two important rules for drawing ray diagrams of lenses.

  1. A ray parallel to the principal axis passes through the principal focus after refraction through a convex lens. For a concave lens, the ray appears to diverge from the principal focus.
  2. A ray passing through the optical centre of a lens passes without deviation.

10. Lens Formula & Magnification

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Q49. Write the lens formula.

1/v − 1/u = 1/f
The formula relates object distance u, image distance v and focal length f.

Q50. Write the formula for magnification produced by a lens.

m = h′/h = v/u
where h is the height of the object and h′ is the height of the image.

Q51. What is the sign of focal length for convex and concave lenses?

Answer:
  • Convex lens → positive focal length
  • Concave lens → negative focal length

11. Power of a Lens

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Q52. Define the power of a lens.

Answer: The power of a lens is a measure of its ability to converge or diverge light rays.
P = 1/f
Here, f must be expressed in metres.

Q53. What is the SI unit of power of a lens?

Answer: The SI unit of power of a lens is dioptre (D).
1 D = 1 m−1

Q54. What is the sign of power of a convex and concave lens?

  • Convex lens → positive power
  • Concave lens → negative power

Q55. What is the power of two lenses placed in contact?

If lenses are placed in contact, their powers add algebraically:
P = P1 + P2 + P3 + ...

12. Important Numericals with Solutions

PRACTICE

Q56. The radius of curvature of a spherical mirror is 20 cm. Find its focal length.

Given: R = 20 cm

Using:
R = 2f
Therefore: f = R/2
f = 20/2
f = 10 cm

Q57. Find the focal length of a convex mirror whose radius of curvature is 32 cm.

Given: R = +32 cm

f = R/2
f = +32/2
f = +16 cm
PRACTICE

Q58. Light enters glass from air. The refractive index of glass is 1.50. Find the speed of light in glass.

Given: n = 1.50
c = 3 × 108 m/s

Using:
n = c/v
Therefore: v = c/n
v = (3 × 108)/1.50
v = 2 × 108 m/s

Q59. Find the power of a concave lens of focal length 2 m.

For a concave lens: f = −2 m

Using:
P = 1/f
Therefore: P = 1/(−2)
P = −0.5 D
MUST DO

Q60. A convex lens has a focal length of 10 cm. Find its power.

Convert focal length into metres: f = 10 cm = 0.10 m

P = 1/f
P = 1/0.10
P = +10 D

Q61. A convex lens forms a real and inverted image equal in size to the object at a distance of 50 cm. Find the position of the object and the power of the lens.

For a convex lens, an image equal in size to the object is formed when the object is placed at 2F1 and the image is formed at 2F2. Therefore: u = −50 cm v = +50 cm

Using lens formula: 1/v − 1/u = 1/f
1/50 − 1/(−50) = 1/f
2/50 = 1/f
f = 25 cm = 0.25 m

Power: P = 1/0.25
P = +4 D

Answer: Object is placed 50 cm in front of the lens and power is +4 D.

Q62. An object 2 cm high is placed 15 cm in front of a convex lens of focal length 10 cm. Find the image distance and magnification.

Given: h = +2 cm
u = −15 cm
f = +10 cm

Using: 1/v − 1/u = 1/f
1/v − 1/(−15) = 1/10
1/v + 1/15 = 1/10
1/v = 1/10 − 1/15
1/v = 1/30
v = +30 cm

Magnification: m = v/u
m = 30/(−15)
m = −2

The negative sign indicates that the image is real and inverted. The image is twice the size of the object.

Q63. A convex mirror has a radius of curvature of 3 m and an object is placed 5 m in front of it. Find the image distance and magnification.

Given: R = +3 m
u = −5 m
f = R/2 = +1.5 m

Mirror formula: 1/v + 1/u = 1/f
1/v − 1/5 = 1/1.5
Solving gives approximately: v = +1.15 m

Magnification: m = −v/u
m = −(1.15)/(−5)
m ≈ +0.23

The image is virtual, erect and diminished.

13. NCERT-Based Board Questions

MUST DO

Q64. Which material cannot be used to make a lens?

Answer: Clay Clay is opaque and cannot form a transparent lens for transmitting and refracting light as required in the chapter.

Q65. A concave mirror forms a virtual, erect and enlarged image. Where is the object placed?

Answer: The object is placed between the pole P and principal focus F.

Q66. Where should an object be placed in front of a convex lens to obtain a real image of the same size?

Answer: The object should be placed at 2F1.

Q67. A spherical mirror and a thin spherical lens have focal lengths of −15 cm. Identify them.

Answer: A spherical mirror with negative focal length is a concave mirror, and a lens with negative focal length is a concave lens.

14. Important MCQs for Class 10 Board Exam

Q68. Which mirror is used as a rear-view mirror?

(a) Plane mirror
(b) Concave mirror
(c) Convex mirror
(d) None of these
✔ Answer: (c) Convex mirror
Q69. The relation between radius of curvature and focal length is:

(a) R = f
(b) R = 2f
(c) R = f/2
(d) R = 4f
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✔ Answer: (b) R = 2f
Q70. A concave mirror forms an erect and enlarged image when the object is:

(a) Beyond C
(b) At C
(c) Between C and F
(d) Between P and F
✔ Answer: (d) Between P and F
Q71. The SI unit of power of a lens is:

(a) metre
(b) centimetre
(c) dioptre
(d) watt
✔ Answer: (c) dioptre
Q72. A convex lens has:

(a) Negative focal length
(b) Positive focal length
(c) Zero focal length
(d) Infinite focal length
✔ Answer: (b) Positive focal length
Q73. A concave lens always forms an image that is:

(a) Real, inverted and enlarged
(b) Real and diminished
(c) Virtual, erect and diminished
(d) Virtual and enlarged
✔ Answer: (c) Virtual, erect and diminished
Q74. When light travels from air to glass obliquely, it bends:

(a) Away from normal
(b) Towards normal
(c) Along the surface
(d) Does not bend
✔ Answer: (b) Towards normal
Q75. The refractive index of a medium is given by:

(a) v/c
(b) c/v
(c) c × v
(d) c + v
✔ Answer: (b) c/v
Q76. The power of a lens of focal length 0.5 m is:

(a) +2 D for convex lens
(b) −2 D for convex lens
(c) +0.5 D
(d) −0.5 D
✔ Answer: (a) +2 D for a convex lens
Q77. If magnification produced by a mirror is negative, the image is:

(a) Virtual and erect
(b) Real and inverted
(c) Virtual and inverted
(d) Real and erect
✔ Answer: (b) Real and inverted

15. Case-Based Questions

Case Study 1 – Concave Mirror

A student places a candle at different positions in front of a concave mirror. He observes that the nature, position and size of the image change with the position of the object.

Q1. What happens when the object is placed at C?

Q2. What happens when the object is placed between C and F?

Q3. What happens when the object is placed between P and F?

Answers:
1. Image is formed at C, same size, real and inverted.
2. Image is formed beyond C, enlarged, real and inverted.
3. Image is formed behind the mirror, enlarged, virtual and erect.

Case Study 2 – Convex Mirror

A driver uses a convex mirror as a rear-view mirror to observe traffic behind the vehicle.

Q1. What type of image is formed?

Q2. Why is the image smaller?

Q3. Why does the driver get a wider field of view?

Answers:
1. Virtual and erect.
2. A convex mirror produces a diminished image.
3. Its outward curved surface provides a wider field of view.

Case Study 3 – Refraction Through Glass

A ray of light enters a rectangular glass slab from air and emerges again into air.

Q1. Does the ray bend towards or away from the normal when entering glass?

Q2. What happens when it emerges into air?

Q3. What is the direction of the emergent ray relative to the incident ray?

Answers:
1. It bends towards the normal.
2. It bends away from the normal.
3. The emergent ray is parallel to the incident ray but is laterally displaced.

Case Study 4 – Convex Lens

An object is placed at different positions in front of a convex lens. The image changes in size, position and nature.

Q1. Where should the object be placed to obtain an image of the same size?

Q2. Where is a virtual and enlarged image obtained?

Q3. What happens when the object is placed at F1?

Answers:
1. At 2F1.
2. Between F1 and the optical centre O.
3. The image is formed at infinity.

16. Quick Revision – Light Reflection & Refraction

Concept Key Point / Formula
Law of reflection ∠i = ∠r
Mirror relation R = 2f
Mirror formula 1/v + 1/u = 1/f
Mirror magnification m = h′/h = −v/u
Refraction Change in direction of light between media
Snell's law sin i / sin r = constant
Refractive index n = c/v
Lens formula 1/v − 1/u = 1/f
Lens magnification m = h′/h = v/u
Power of lens P = 1/f
Unit of power Dioptre (D)
Convex lens Positive focal length and positive power
Concave lens Negative focal length and negative power

17. Class 10 Board Exam Final Checklist

  • ☑ Learn both laws of reflection.
  • ☑ Learn all terms related to spherical mirrors: P, C, F, R and aperture.
  • ☑ Memorise the relationship R = 2f.
  • ☑ Practise all concave mirror image-formation cases.
  • ☑ Remember that a convex mirror always forms a virtual, erect and diminished image.
  • ☑ Learn the New Cartesian Sign Convention.
  • ☑ Practise the mirror formula.
  • ☑ Practise mirror magnification.
  • ☑ Learn the definition and laws of refraction.
  • ☑ Learn Snell's law.
  • ☑ Practise refractive-index numericals.
  • ☑ Learn refraction through a rectangular glass slab.
  • ☑ Differentiate between convex and concave lenses.
  • ☑ Learn all convex-lens image-formation cases.
  • ☑ Remember that a concave lens always forms a virtual, erect and diminished image.
  • ☑ Practise lens formula and magnification.
  • ☑ Learn power of lens and its unit, dioptre.
  • ☑ Practise numerical problems involving signs.
  • ☑ Practise MCQs and case-based questions.

18. Exam Strategy for Light – Reflection and Refraction

How to score better in this chapter?

1. For numerical questions: Write Given → Formula → Substitution → Calculation → Final Answer with unit.

2. For ray diagrams: Use a ruler and clearly mark P, F, C or F1, F2, 2F1, 2F2 wherever required.

3. For sign convention: Do not substitute values before deciding whether each quantity is positive or negative.

4. For theory questions: Use scientific keywords such as real, virtual, erect, inverted, diminished, enlarged, towards normal, away from normal, converging and diverging.

5. For 3-mark and 5-mark questions: Write the formula, explanation and conclusion clearly instead of giving only the final answer.

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