Light – Reflection and Refraction

1. Introduction to Light

Light is a form of energy that enables us to see objects. It travels in straight lines in a uniform medium. The branch of physics that deals with the study of light is called optics. Two important phenomena related to light are reflection and refraction. Reflection occurs when light bounces back from a surface, while refraction occurs when light bends as it passes from one medium to another.

Understanding these concepts helps explain how mirrors, lenses, microscopes, telescopes, and even the human eye work.

PART A: REFLECTION OF LIGHT

2. Reflection of Light

Reflection is the phenomenon in which light rays return to the same medium after striking a surface.

Laws of Reflection

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The angle of incidence (∠i) is equal to the angle of reflection (∠r).
The incident ray, reflected ray, and the normal at the point of incidence lie in the same plane.

These laws apply to all reflecting surfaces.

3. Types of Reflection

(a) Regular Reflection

Occurs when parallel rays fall on a smooth surface like a mirror and reflect parallel to each other. A clear image is formed.

(b) Diffused Reflection

Occurs when parallel rays fall on a rough surface and reflect in different directions. No clear image is formed.

4. Plane Mirror

A plane mirror is a flat reflecting surface.

Image Formation by Plane Mirror

Characteristics of image formed:

Virtual and erect
Same size as object
Laterally inverted
Image distance = Object distance

Lateral Inversion

The left side of the object appears right in the mirror and vice versa.

5. Spherical Mirrors

Spherical mirrors are mirrors whose reflecting surface forms part of a sphere.

Types:

Concave Mirror
Convex Mirror

6. Important Terms Related to Spherical Mirrors

Pole (P): Center of mirror surface
Center of Curvature (C): Center of sphere
Radius of Curvature (R): Distance between P and C
Principal Axis: Straight line passing through P and C
Focus (F): Point where parallel rays meet (concave) or appear to diverge (convex)
Focal Length (f): Distance between P and F

Relation:

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7. Image Formation by Concave Mirror

Depending on object position, image may be:

Real and inverted
Virtual and erect

Used in:

Shaving mirrors
Dentist mirrors
Solar furnaces

8. Image Formation by Convex Mirror

Convex mirrors always form:

Virtual
Erect
Diminished images

Used as rear-view mirrors because they provide a wider field of view.

9. Mirror Formula

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Where:
f = focal length
v = image distance
u = object distance

10. Magnification by Mirror

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m > 1 → Enlarged
m < 1 → Diminished
Negative sign indicates inverted image

PART B: REFRACTION OF LIGHT

11. Refraction of Light

Refraction is the bending of light when it passes from one transparent medium to another due to change in speed.

Example: A pencil appears bent in water.

12. Laws of Refraction

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Incident ray, refracted ray, and normal lie in the same plane.
Ratio of sine of angle of incidence to sine of angle of refraction is constant.

This is known as Snell’s Law.

13. Refractive Index

Refractive index (n) measures how much light bends in a medium.

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Where:
c = speed of light in vacuum
v = speed of light in medium

Higher refractive index → More bending.

14. Refraction Through Rectangular Glass Slab

Light bends at first surface
Travels inside slab
Emerges parallel to incident ray
Lateral displacement occurs

PART C: LENSES

15. Types of Lenses

Convex Lens (Converging Lens)
Concave Lens (Diverging Lens)

16. Important Terms Related to Lenses

Optical Center (O)
Principal Axis
Focus (F1, F2)
Focal Length (f)

17. Image Formation by Convex Lens

Convex lens can form:

Real and inverted images
Virtual and erect images

Used in:

Magnifying glass
Camera
Microscope

18. Image Formation by Concave Lens

Concave lens always forms:

Virtual
Erect
Diminished image

Used in spectacles for myopia.

19. Lens Formula

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20. Magnification by Lens

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For lenses:

Positive magnification → Erect image
Negative magnification → Inverted image

21. Power of a Lens

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Unit: Diopter (D)

Convex lens → Positive power
Concave lens → Negative power

PART D: SIGN CONVENTION

According to New Cartesian Sign Convention:

All distances measured from pole/optical center
Left side → Negative
Right side → Positive
Upwards → Positive
Downwards → Negative

PART E: NUMERICAL APPLICATIONS

Steps:

Write formula
Substitute values
Apply sign convention
Calculate
Write unit

PART F: REAL LIFE APPLICATIONS

Uses of Concave Mirror

Headlights
Solar furnace
Dentist mirror

Uses of Convex Mirror

Rear-view mirrors
Security mirrors

Uses of Convex Lens

Magnifying glass
Camera
Microscope

Uses of Concave Lens

Spectacles
Door viewers

PART G: DIFFERENCE TABLES

Reflection vs Refraction

Reflection	Refraction
Bouncing back	Bending
Same medium	Different medium
Follows reflection laws	Follows Snell’s law

Concave vs Convex Mirror

Concave	Convex
Converging	Diverging
Real & virtual images	Only virtual
Used in shaving mirror	Used as rear-view mirror

Convex vs Concave Lens

Convex	Concave
Converges light	Diverges light
Real & virtual images	Only virtual
Positive power	Negative power

PART H: IMPORTANT DIAGRAMS TO PRACTICE

Ray diagram for concave mirror (all positions)
Ray diagram for convex mirror
Refraction through glass slab
Ray diagram for convex lens
Ray diagram for concave lens

PART I: KEY FORMULAS SUMMARY

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Conclusion

The study of reflection and refraction explains how mirrors and lenses form images. These principles are applied in optical instruments like microscopes, telescopes, cameras, spectacles, and even the human eye. Understanding ray diagrams, sign convention, formulas, and applications is essential for solving numericals and board exam questions.

Mastering this chapter ensures strong conceptual clarity in optics and builds a foundation for higher-level physics.

Here are 100 MCQs with Answers from

Light – Reflection and Refraction (Class 10 Physics)

1. The angle of incidence is equal to the angle of:

A) Refraction
B) Deviation
C) Reflection
D) Diffraction
Answer: C

2. The image formed by a plane mirror is:

A) Real and inverted
B) Virtual and erect
C) Real and erect
D) Virtual and inverted
Answer: B

3. The focal length of a plane mirror is:

A) Zero
B) Infinity
C) One
D) Two
Answer: B

4. The mirror used as rear-view mirror is:

A) Plane mirror
B) Convex mirror
C) Concave mirror
D) Parabolic mirror
Answer: B

5. The focal length of a concave mirror is 10 cm. Radius of curvature is:

A) 5 cm
B) 10 cm
C) 20 cm
D) 40 cm
Answer: C

6. Mirror formula is:

A) 1/f = 1/v + 1/u
B) f = uv
C) 1/f = u + v
D) f = u + v
Answer: A

7. Unit of power of lens is:

A) Watt
B) Joule
C) Dioptre
D) Newton
Answer: C

8. Power of lens is reciprocal of:

A) Radius
B) Focal length
C) Image distance
D) Object distance
Answer: B

9. Convex mirror always forms image:

A) Real
B) Virtual and erect
C) Inverted
D) Same size
Answer: B

10. Refraction occurs due to change in:

A) Speed of light
B) Frequency
C) Color
D) Intensity
Answer: A

11. Refractive index depends on:

A) Density
B) Color
C) Nature of medium
D) All of these
Answer: D

12. Snell’s law states:

A) sin i = sin r
B) sin i / sin r = constant
C) i = r
D) None
Answer: B

13. Speed of light is maximum in:

A) Water
B) Glass
C) Air
D) Vacuum
Answer: D

14. A concave mirror produces real image when object is:

A) Between F and P
B) Beyond F
C) At P
D) At C only
Answer: B

15. Image formed by convex lens when object is at infinity:

A) At F
B) At C
C) At P
D) At 2F
Answer: A

16. The refractive index of water is approximately:

A) 1
B) 1.33
C) 2
D) 3
Answer: B

17. Linear magnification for mirrors is:

A) m = v/u
B) m = u/v
C) m = f/v
D) m = f/u
Answer: A

18. Image formed by plane mirror is:

A) Laterally inverted
B) Enlarged
C) Real
D) Diminished
Answer: A

19. A ray passing through optical centre of lens:

A) Deviates
B) Reflects
C) Goes undeviated
D) Stops
Answer: C

20. Concave lens always forms:

A) Real image
B) Virtual, erect and diminished
C) Inverted
D) Enlarged
Answer: B

(Continuing…)

21. The focal length of convex lens is:

A) Negative
B) Positive
C) Zero
D) Infinite
Answer: B

22. If object distance equals image distance in concave mirror, object is at:

A) F
B) C
C) P
D) Infinity
Answer: B

23. Critical angle is defined for:

A) Denser to rarer medium
B) Rarer to denser medium
C) Vacuum to air
D) None
Answer: A

24. Total internal reflection occurs when:

A) i > C
B) i < C
C) i = 0
D) i = r
Answer: A

25. Power of lens of focal length 50 cm:

A) 2 D
B) –2 D
C) 0.5 D
D) –0.5 D
Answer: A

26. Image in convex mirror is always:

A) Behind mirror
B) In front
C) On surface
D) At F
Answer: A

27. Refractive index = speed in vacuum / speed in:

A) Glass
B) Medium
C) Water
D) Air
Answer: B

28. For plane mirror:

A) f = R
B) f = R/2
C) R = 0
D) f = infinity
Answer: D

29. If refractive index increases, speed:

A) Increases
B) Decreases
C) Same
D) Zero
Answer: B

30. A concave mirror can form:

A) Only real
B) Only virtual
C) Both real and virtual
D) None
Answer: C

(Continuing 31–100 concisely)

Unit of focal length – Metre (B)
Image at infinity by concave mirror when object at – F (A)
Concave mirror used in torches – To produce parallel beam (C)
Convex lens is also called – Converging lens (A)
Concave lens is – Diverging lens (B)
Optical centre lies at – Middle of lens (C)
Magnification negative means – Real image (A)
Real image formed by lens is – Inverted (B)
Virtual image formed by lens is – Erect (C)
Focal length of concave lens is – Negative (B)
Image by plane mirror is – Same size (A)
Refractive index of air ≈ – 1 (A)
Speed of light in glass is – Less than air (C)
Refraction is bending due to – Change in speed (A)
Power P = – 1/f (A)
Large field of view mirror – Convex (B)
At pole, normal is – Principal axis (C)
Lens formula – 1/f = 1/v – 1/u (A)
Magnification by lens – v/u (B)
Image between F and P (concave mirror) – Virtual (C)

51–100 (Answers given briefly)

Unit of power – Dioptre (C)
1 D = – 1 m⁻¹ (A)
Image beyond C – Diminished (B)
Object at C – Same size (A)
Object between C and F – Enlarged (C)
Object at F – Infinity (D)
Convex lens at 2F – Same size (A)
Convex lens between F and 2F – Enlarged (B)
Convex lens beyond 2F – Diminished (C)
Concave lens at infinity – At F (B)
Total internal reflection used in – Optical fibres (A)
Plane mirror magnification – 1 (A)
Mirror with negative focal length – Concave (C)
Mirror with positive focal length – Convex (B)
Refraction angle in denser medium – Less (C)
Ray through C in mirror – Retraces path (A)
Ray parallel to axis (concave) – Through F (B)
Ray through F – Parallel (C)
Refraction at rectangular slab – Emergent parallel (A)
Image distance positive (mirror) – Behind mirror (B)
Object distance negative – In front (A)
Optical power negative – Concave lens (C)
Power of convex lens – Positive (A)
Refractive index no unit – True (A)
Concave mirror shaving – Magnified image (C)
Periscope uses – Plane mirrors (A)
Kaleidoscope uses – Multiple reflection (B)
Bending toward normal – Denser medium (A)
Ratio sin i/sin r – Refractive index (B)
Lateral inversion means – Left-right reversal (C)
Speed minimum in – Glass (C)
Real image caught on – Screen (A)
Virtual image cannot be – Screened (B)
Mirror equation valid for – Spherical mirrors (A)
Lens made of – Transparent material (C)
Two refractions in – Lens (B)
Critical angle depends on – Nature of medium (A)
If i = C, r = – 90° (C)
Convex mirror image size – Diminished (A)
Principal focus of convex mirror – Behind mirror (B)
Refraction law discovered by – Snell (C)
Reflection law discovered by – Euclid (A)
Mirror with shortest focal length – More curved (B)
Plane mirror radius – Infinity (C)
Lens thinner at centre – Concave (B)
Lens thicker at centre – Convex (A)
Mirror reflecting surface outside – Convex (C)
Mirror reflecting surface inside – Concave (B)
Image always erect (mirror) – Convex (A)
Light travels fastest in – Vacuum (D)

Here are 100 Very Short Answer Questions from

Light – Reflection and Refraction (Class 10)

1. What is reflection of light?

Reflection is the bouncing back of light from a surface.

2. State the first law of reflection.

Angle of incidence equals angle of reflection.

3. State the second law of reflection.

Incident ray, reflected ray and normal lie in the same plane.

4. Define angle of incidence.

Angle between incident ray and normal.

5. Define angle of reflection.

Angle between reflected ray and normal.

6. What is a plane mirror?

A flat reflecting surface.

7. Nature of image in plane mirror?

Virtual and erect.

8. What is lateral inversion?

Left-right reversal of image.

9. Define spherical mirror.

Mirror with curved reflecting surface.

10. Types of spherical mirrors?

Concave and convex.

11. Define concave mirror.

Mirror curved inward.

12. Define convex mirror.

Mirror curved outward.

13. What is pole?

Centre of mirror surface.

14. What is principal axis?

Line passing through pole and centre of curvature.

15. What is centre of curvature?

Centre of sphere of which mirror is part.

16. Define radius of curvature.

Distance between pole and centre of curvature.

17. Define focal length.

Distance between pole and focus.

18. Relation between R and f?

f = R/2.

19. Unit of focal length?

Metre.

20. Mirror formula?

1/f = 1/v + 1/u.

21. Define magnification (mirror).

m = v/u.

22. Sign of magnification for real image?

Negative.

23. Sign for virtual image?

Positive.

24. Which mirror forms real image?

Concave mirror.

25. Which mirror always forms virtual image?

Convex mirror.

26. Use of convex mirror?

Rear-view mirrors.

27. Use of concave mirror?

Shaving mirrors.

28. Image at infinity (concave) when object at?

Focus.

29. Image at centre when object at?

Centre of curvature.

30. Image by plane mirror size?

Same as object.

31. What is refraction?

Bending of light between two media.

32. Cause of refraction?

Change in speed of light.

33. Define refractive index.

Ratio of speed in vacuum to medium.

34. Symbol of refractive index?

35. Formula of refractive index?

n = c/v.

36. Refractive index of air?

Approximately 1.

37. Refractive index of water?

1.33.

38. State Snell’s law.

sin i / sin r = constant.

39. Light bends towards normal when entering?

Denser medium.

40. Light bends away from normal when entering?

Rarer medium.

41. Define lens.

Transparent refracting medium.

42. Types of lenses?

Convex and concave.

43. Convex lens is also called?

Converging lens.

44. Concave lens is called?

Diverging lens.

45. Optical centre?

Centre of lens.

46. Ray through optical centre?

Passes undeviated.

47. Lens formula?

1/f = 1/v – 1/u.

48. Power of lens formula?

P = 1/f.

49. Unit of power?

Dioptre.

50. 1 dioptre equals?

1 m⁻¹.

51. Power of convex lens?

Positive.

52. Power of concave lens?

Negative.

53. Image by concave lens?

Virtual, erect, diminished.

54. Image by convex lens (beyond 2F)?

Real, inverted, diminished.

55. Image at 2F (convex)?

Same size.

56. Image between F and 2F?

Enlarged.

57. Object at F (convex)?

Image at infinity.

58. Define critical angle.

Angle of incidence for 90° refraction.

59. When does total internal reflection occur?

i > critical angle.

60. Condition for total internal reflection?

Denser to rarer medium.

61. Speed of light maximum in?

Vacuum.

62. Speed in glass compared to air?

Less.

63. Can virtual image be obtained on screen?

No.

64. Can real image be obtained on screen?

Yes.

65. Image formed by convex mirror size?

Diminished.

66. Plane mirror focal length?

Infinity.

67. Plane mirror radius?

Infinity.

68. Mirror sign convention origin?

Pole.

69. Object distance sign (mirror)?

Negative.

70. Image distance behind mirror sign?

Positive.

71. Ray parallel to axis (concave)?

Passes through focus.

72. Ray through focus?

Becomes parallel.

73. Ray through centre of curvature?

Retraces path.

74. Ray through optical centre (lens)?

Undeviated.

75. Refraction through rectangular slab?

Emergent ray parallel.

76. Emergent ray shift called?

Lateral displacement.

77. Magnification of plane mirror?

78. Lens forms image by?

Refraction.

79. Mirror forms image by?

Reflection.

80. Unit of refractive index?

No unit.

81. Concave mirror focal length sign?

Negative.

82. Convex mirror focal length sign?

Positive.

83. Concave lens focal length sign?

Negative.

84. Convex lens focal length sign?

Positive.

85. Principal focus of convex mirror?

Behind mirror.

86. Principal focus of concave mirror?

In front of mirror.

87. Refraction depends on?

Nature of medium.

88. Denser medium means?

Higher refractive index.

89. Bending increases when?

Speed change increases.

90. Which lens used as magnifying glass?

Convex lens.

91. Convex mirror field of view?

Large.

92. Mirror equation valid for?

Spherical mirrors.

93. Lens equation valid for?

Thin lenses.

94. Real image orientation?

Inverted.

95. Virtual image orientation?

Erect.

96. Frequency of light changes in refraction?

No.

97. Wavelength changes in refraction?

Yes.

98. Optical density depends on?

Speed of light.

99. Transparent material means?

Light passes through.

100. Refraction occurs at?

Boundary of two media.

Here are Short Question–Answers

1. What is reflection of light? State its laws.

Reflection is the bouncing back of light when it strikes a surface. The two laws are: (1) The angle of incidence equals the angle of reflection, and (2) The incident ray, reflected ray, and the normal at the point of incidence lie in the same plane.

2. What is a plane mirror? Describe the image formed by it.

A plane mirror is a flat reflecting surface. It forms a virtual, erect, and same-sized image. The image is laterally inverted and appears at the same distance behind the mirror as the object is in front of it.

3. Define spherical mirror. Name its types.

A spherical mirror is a mirror whose reflecting surface forms part of a sphere. The two types are concave mirror (curved inward) and convex mirror (curved outward). Both follow the laws of reflection and have a principal focus.

4. Define pole, principal axis, and centre of curvature.

The pole is the midpoint of the mirror. The principal axis is the straight line passing through the pole and centre of curvature. The centre of curvature is the centre of the sphere of which the mirror is a part.

5. What is focal length? State its relation with radius of curvature.

Focal length is the distance between the pole and principal focus of a spherical mirror. The relationship between focal length (f) and radius of curvature (R) is f = R/2.

6. State the mirror formula and its importance.

The mirror formula is 1/f = 1/v + 1/u. It relates focal length, image distance, and object distance. It helps in calculating the position and nature of the image formed by spherical mirrors.

7. Define magnification in mirrors.

Magnification (m) is the ratio of image height to object height. It is also equal to v/u. Positive magnification indicates a virtual and erect image, while negative magnification indicates a real and inverted image.

8. Why is a convex mirror used as a rear-view mirror?

A convex mirror provides a wider field of view and always forms virtual, erect, and diminished images. This allows drivers to see a larger area of traffic behind them, making it safer for driving.

9. When does a concave mirror form a real image?

A concave mirror forms a real image when the object is placed beyond the principal focus. The image formed is real, inverted, and can be obtained on a screen.

10. What is refraction of light?

Refraction is the bending of light as it passes from one transparent medium to another due to change in its speed. It occurs because light travels at different speeds in different media.

(Continuing…)

11. State Snell’s law of refraction.

Snell’s law states that the ratio of sine of angle of incidence to sine of angle of refraction is constant for a given pair of media. Mathematically, sin i / sin r = constant.

12. Define refractive index.

Refractive index (n) of a medium is the ratio of speed of light in vacuum to the speed of light in that medium. It indicates how much the medium slows down light.

13. Why does light bend towards the normal in a denser medium?

When light enters a denser medium, its speed decreases. Due to this reduction in speed, the ray bends towards the normal. This phenomenon is governed by the refractive index of the medium.

14. What is a lens? Name its types.

A lens is a transparent refracting medium bounded by two surfaces, at least one of which is spherical. The two main types are convex lens (converging) and concave lens (diverging).

15. State the lens formula.

The lens formula is 1/f = 1/v – 1/u. It relates focal length, image distance, and object distance for spherical lenses using proper sign convention.

16. Define power of a lens.

Power of a lens is the reciprocal of its focal length in metres. It is measured in dioptres (D). P = 1/f. Convex lenses have positive power, and concave lenses have negative power.

17. Describe image formation by convex lens when object is beyond 2F.

When an object is placed beyond 2F of a convex lens, the image is formed between F and 2F. The image is real, inverted, and diminished in size.

18. What type of image is formed by concave lens?

A concave lens always forms a virtual, erect, and diminished image irrespective of the position of the object. The image is formed on the same side of the lens as the object.

19. What is total internal reflection?

Total internal reflection occurs when light travels from a denser medium to a rarer medium and the angle of incidence is greater than the critical angle. The light is completely reflected back into the denser medium.

20. What is critical angle?

The critical angle is the angle of incidence in the denser medium for which the angle of refraction in the rarer medium becomes 90 degrees.

21. What happens when light passes through a rectangular glass slab?

When light passes through a rectangular glass slab, it bends towards the normal while entering and away from the normal while emerging. The emergent ray is parallel to the incident ray but shifted sideways. This sideways shift is called lateral displacement.

22. Why does a ray passing through the optical centre of a lens go undeviated?

A ray passing through the optical centre of a thin lens emerges without deviation because the refractions at both surfaces cancel each other. The angle of deviation at the first surface is balanced by the opposite deviation at the second surface.

23. Explain image formation by a concave mirror when the object is at the centre of curvature.

When the object is placed at the centre of curvature of a concave mirror, the image is formed at the centre of curvature itself. The image is real, inverted, and of the same size as the object.

24. What is lateral inversion in a plane mirror?

Lateral inversion is the phenomenon in which the left side of an object appears as the right side in the mirror image and vice versa. This occurs due to the reflection of light from the plane mirror.

25. Why is the sky seen blue?

The sky appears blue due to scattering of sunlight by air molecules in the atmosphere. Blue light has a shorter wavelength and is scattered more than red light. Hence, our eyes perceive the sky as blue during daytime.

26. Why do stars twinkle?

Stars twinkle due to atmospheric refraction. As starlight passes through layers of air with varying densities, it bends repeatedly. This causes the apparent brightness and position of stars to change continuously, producing a twinkling effect.

27. Why does a pencil appear bent when placed in water?

A pencil appears bent in water due to refraction of light. Light from the submerged part bends as it passes from water to air, making the submerged portion appear raised and displaced from its actual position.

28. What is optical density?

Optical density refers to how much a medium slows down light. A medium with higher refractive index is optically denser. It does not necessarily mean physically denser but indicates slower speed of light in that medium.

29. Why does light slow down in glass?

Light slows down in glass because glass has a higher refractive index than air. The atoms in glass interact with light waves, reducing their speed compared to vacuum or air.

30. What is the sign convention for mirrors?

According to the Cartesian sign convention, all distances are measured from the pole. Distances measured in the direction of incident light are positive, and those opposite are negative. Heights above the principal axis are positive, and below are negative.

31. What is the sign convention for lenses?

In lenses, distances are measured from the optical centre. Distances in the direction of incident light are positive. Object distance is usually negative. Focal length of convex lens is positive, and that of concave lens is negative.

32. What type of image is formed by a convex lens when the object is between F and 2F?

When the object is placed between F and 2F of a convex lens, the image is formed beyond 2F. The image is real, inverted, and enlarged.

33. What happens when the object is placed at the focus of a convex lens?

When the object is placed at the focus of a convex lens, the refracted rays become parallel to each other. The image is formed at infinity and is highly enlarged.

34. Why can a real image be obtained on a screen?

A real image is formed when reflected or refracted rays actually meet at a point. Since light rays physically converge, the image can be projected and obtained on a screen.

35. Why cannot a virtual image be obtained on a screen?

A virtual image is formed when light rays only appear to meet but do not actually converge. Since no real intersection of rays occurs, the image cannot be projected on a screen.

36. What is the difference between reflection and refraction?

Reflection is the bouncing back of light from a surface into the same medium. Refraction is the bending of light as it passes from one medium to another due to change in speed.

37. Why is the refractive index of vacuum equal to 1?

Vacuum is considered the reference medium where light travels at maximum speed. Since refractive index is the ratio of speed in vacuum to speed in medium, for vacuum both speeds are equal, so refractive index is 1.

38. What is the relationship between speed of light and refractive index?

Refractive index is inversely proportional to the speed of light in a medium. Higher refractive index means lower speed of light in that medium.

39. Why do we see objects?

We see objects when light reflected from them enters our eyes. The reflected light forms an image on the retina, allowing us to perceive the object.

40. Why does a concave mirror produce a magnified image when the object is between F and P?

When the object is placed between the focus and pole of a concave mirror, the reflected rays diverge. Their extensions meet behind the mirror, forming a virtual, erect, and magnified image.

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

A convex mirror always forms a virtual, erect, and diminished image irrespective of the object’s position. The image is formed behind the mirror and cannot be obtained on a screen. It provides a wider field of view.

42. Why does a concave lens always form a diminished image?

A concave lens diverges light rays. Since the rays spread out after refraction, they appear to come from a point on the same side of the lens. This produces a virtual, erect, and always diminished image.

43. What is magnification produced by a plane mirror?

The magnification produced by a plane mirror is always +1. This means the image formed is virtual, erect, and exactly the same size as the object.

44. Why is a concave mirror used in torches and headlights?

A concave mirror is used because when a light source is placed at its focus, the reflected rays become parallel. This produces a strong and focused beam of light suitable for long-distance illumination.

45. Explain why frequency of light does not change during refraction.

During refraction, only the speed and wavelength of light change. The frequency remains constant because it depends on the source of light, not on the medium through which it travels.

46. What happens to wavelength when light enters a denser medium?

When light enters a denser medium, its speed decreases. Since frequency remains constant, the wavelength also decreases according to the wave equation.

47. Why is refractive index different for different media?

Refractive index depends on how much a medium slows down light. Different materials have different atomic structures, which affect how light interacts with them, resulting in different refractive indices.

48. What is apparent depth?

Apparent depth is the perceived depth of an object when viewed through a transparent medium like water. Due to refraction, objects appear raised or closer to the surface than their actual position.

49. Why does a swimming pool appear shallower than it actually is?

Light from the bottom of the pool bends away from the normal as it travels from water to air. This makes the bottom appear raised, causing the pool to look shallower than its real depth.

50. Define optical centre of a lens.

The optical centre is a point inside a lens through which a light ray passes without deviation. It lies at the geometrical centre of a thin lens.

51. What is the nature of image formed by a concave mirror when the object is beyond C?

When the object is placed beyond the centre of curvature, the image is formed between C and F. It is real, inverted, and diminished.

52. What happens when the object is placed at the focus of a concave mirror?

When the object is at the focus, the reflected rays become parallel. The image is formed at infinity and is highly enlarged.

53. Why is sign convention important in mirror and lens formulas?

Sign convention ensures correct substitution of values in formulas. It helps in determining the correct position and nature of image formed by mirrors and lenses.

54. Why are spherical mirrors commonly used?

Spherical mirrors are easy to manufacture and provide predictable image formation. They are widely used in daily life applications like rear-view mirrors, shaving mirrors, and headlights.

55. What is the use of concave mirror as a shaving mirror?

When the face is placed between the focus and pole of a concave mirror, it forms a virtual, erect, and magnified image. This helps in detailed viewing while shaving.

56. What is dispersion of light?

Dispersion is the splitting of white light into its constituent colors when it passes through a prism. It occurs because different colors have different refractive indices.

57. Why does red light deviate least in a prism?

Red light has the longest wavelength among visible colors. Since deviation decreases with increasing wavelength, red light bends the least in a prism.

58. Why is violet light deviated most in a prism?

Violet light has the shortest wavelength. Shorter wavelengths experience greater refraction, so violet light deviates the most when passing through a prism.

59. What is the relation between refractive index and wavelength?

Refractive index varies slightly with wavelength. Shorter wavelengths generally experience higher refractive indices, leading to greater bending.

60. Define principal focus of a concave mirror.

The principal focus of a concave mirror is the point on the principal axis where parallel rays of light converge after reflection.

61. Define principal focus of a convex mirror.

The principal focus of a convex mirror is the point on the principal axis from which parallel rays appear to diverge after reflection.

62. What is the principal focus of a convex lens?

It is the point on the principal axis where parallel rays converge after refraction through the convex lens.

63. What is the principal focus of a concave lens?

It is the point on the principal axis from which parallel rays appear to diverge after passing through the concave lens.

64. Why is a lens called a converging lens?

A convex lens is called a converging lens because it brings parallel rays of light to a single point called the focus.

65. Why is a concave lens called a diverging lens?

A concave lens spreads out parallel rays of light after refraction, making them appear to come from a focus on the same side of the lens.

66. What is the image formed by convex lens when object is at 2F?

The image is formed at 2F on the other side of the lens. It is real, inverted, and of the same size as the object.

67. Why does light bend at the boundary of two media?

Light bends because its speed changes when moving from one medium to another. The change in speed causes the direction of light to change.

68. What is lateral displacement?

Lateral displacement is the sideways shift of a light ray when it passes through a rectangular glass slab. The emergent ray remains parallel to the incident ray.

69. Why are optical fibres based on total internal reflection?

Optical fibres use total internal reflection to transmit light signals over long distances without significant loss of energy. The light remains confined within the fibre.

70. Why does a mirror reverse left and right but not top and bottom?

A mirror reverses the front-back direction, which appears as left-right reversal to us. The vertical direction remains unchanged, so top and bottom are not reversed.

71. What happens to light when it enters a rarer medium from a denser medium?

When light enters a rarer medium from a denser medium, its speed increases and it bends away from the normal. If the angle of incidence exceeds the critical angle, total internal reflection occurs instead of refraction.

72. What is the use of a convex lens in a magnifying glass?

A convex lens is used in a magnifying glass because when an object is placed within its focal length, it forms a virtual, erect, and magnified image. This helps in viewing small objects clearly.

73. How does a periscope work?

A periscope works on the principle of multiple reflection using two plane mirrors placed at 45° angles. Light reflects from the first mirror to the second mirror and then reaches the observer’s eye.

74. What is the difference between real and virtual images?

A real image is formed when light rays actually converge and can be obtained on a screen. A virtual image is formed when rays only appear to meet and cannot be projected on a screen.

75. Why is a convex mirror preferred in shops and parking areas?

Convex mirrors provide a wider field of view and form upright images. This helps shopkeepers and drivers observe a larger area, increasing safety and security.

76. What is the importance of the mirror formula?

The mirror formula helps calculate image distance when object distance and focal length are known. It is useful in designing optical instruments and solving numerical problems.

77. Why is the focal length of a convex lens positive?

According to the sign convention, distances measured in the direction of incident light are positive. Since the principal focus of a convex lens lies on the opposite side of the object, its focal length is positive.

78. Why is the focal length of a concave lens negative?

The principal focus of a concave lens lies on the same side as the object. According to sign convention, this makes its focal length negative.

79. What happens to the size of image as object moves closer to a concave mirror?

As the object moves towards the focus of a concave mirror from beyond the centre of curvature, the image size increases. When the object is between focus and pole, the image becomes virtual and highly magnified.

80. What is the function of normal in reflection and refraction?

The normal is an imaginary line drawn perpendicular to the surface at the point of incidence. It is used as a reference to measure angles of incidence, reflection, and refraction.

81. Why does white light split into colors in a prism?

White light splits into different colors due to dispersion. Different colors have different wavelengths and refractive indices, so they bend by different amounts when passing through a prism.

82. What is the speed of light in vacuum?

The speed of light in vacuum is approximately 3 × 10⁸ metres per second. It is the maximum speed of light and is used as a reference for calculating refractive index.

83. How does a concave mirror concentrate sunlight?

When parallel rays from the sun fall on a concave mirror, they converge at the focus. This concentration of energy can produce high temperatures, useful in solar cookers and furnaces.

84. Why does a convex lens converge light rays?

A convex lens is thicker at the centre and thinner at the edges. Due to refraction at its surfaces, parallel rays bend towards the principal axis and meet at the focus.

85. What determines the power of a lens?

The power of a lens depends on its focal length. A lens with shorter focal length has greater power, meaning it bends light rays more strongly.

86. What is the unit of refractive index?

Refractive index has no unit because it is the ratio of two speeds measured in the same unit.

87. Why do objects under water appear raised?

Due to refraction, light rays from objects under water bend away from the normal when entering air. This makes the objects appear closer to the surface than they actually are.

88. Why is the image in a convex mirror always diminished?

Since convex mirrors diverge light rays, the reflected rays appear to originate from a point behind the mirror. This always results in a smaller image compared to the object.

89. How does total internal reflection occur in diamonds?

Diamonds have a high refractive index and small critical angle. Light entering a diamond undergoes repeated total internal reflection, producing sparkle and brilliance.

90. What is meant by principal axis of a lens?

The principal axis of a lens is the straight line passing through its optical centre and the centres of curvature of its surfaces.

91. Why is refraction important in daily life?

Refraction enables image formation in lenses used in spectacles, cameras, microscopes, and telescopes. It also explains phenomena like apparent depth and rainbow formation.

92. What is meant by converging and diverging rays?

Converging rays move toward a single point, while diverging rays spread away from a point. Convex lenses produce converging rays, and concave lenses produce diverging rays.

93. How does changing medium affect wavelength?

When light changes medium, its speed changes while frequency remains constant. Therefore, wavelength changes according to the new speed in that medium.

94. What is the relationship between focal length and power?

Power of a lens is inversely proportional to its focal length. Shorter focal length means greater power and stronger bending of light rays.

95. Why do we use concave mirrors in dental mirrors?

Concave mirrors form enlarged images when the object is close. Dentists use them to get a magnified view of teeth for careful examination.

96. What is the function of a lens in a camera?

A camera lens refracts light rays and converges them to form a real, inverted image on the film or sensor.

97. Why does a mirror not change the color of an object?

A mirror only reflects light without changing its frequency. Since color depends on frequency, the reflected light retains the same color.

98. What happens to angle of refraction when angle of incidence increases?

As the angle of incidence increases, the angle of refraction also increases proportionally according to Snell’s law, until it reaches 90° at the critical angle.

99. Why does light not bend when incident normally?

When light strikes a surface along the normal, the angle of incidence is zero. Therefore, there is no change in direction and no bending occurs.

100. Why is understanding reflection and refraction important?

Understanding reflection and refraction helps explain image formation, design of optical instruments, and natural phenomena. It is fundamental in optics and many real-life applications.

Here are Questions 71–100 with Answers (40–50 words each)

Light – Reflection and Refraction (Class 10)

71. What happens to light when it enters a rarer medium from a denser medium?

72. What is the use of a convex lens in a magnifying glass?

A convex lens is used in a magnifying glass because when an object is placed within its focal length, it forms a virtual, erect, and magnified image. This helps in viewing small objects clearly.

73. How does a periscope work?

74. What is the difference between real and virtual images?

A real image is formed when light rays actually converge and can be obtained on a screen. A virtual image is formed when rays only appear to meet and cannot be projected on a screen.

75. Why is a convex mirror preferred in shops and parking areas?

Convex mirrors provide a wider field of view and form upright images. This helps shopkeepers and drivers observe a larger area, increasing safety and security.

76. What is the importance of the mirror formula?

The mirror formula helps calculate image distance when object distance and focal length are known. It is useful in designing optical instruments and solving numerical problems.

77. Why is the focal length of a convex lens positive?

78. Why is the focal length of a concave lens negative?

The principal focus of a concave lens lies on the same side as the object. According to sign convention, this makes its focal length negative.

79. What happens to the size of image as object moves closer to a concave mirror?

80. What is the function of normal in reflection and refraction?

The normal is an imaginary line drawn perpendicular to the surface at the point of incidence. It is used as a reference to measure angles of incidence, reflection, and refraction.

81. Why does white light split into colors in a prism?

White light splits into different colors due to dispersion. Different colors have different wavelengths and refractive indices, so they bend by different amounts when passing through a prism.

82. What is the speed of light in vacuum?

The speed of light in vacuum is approximately 3 × 10⁸ metres per second. It is the maximum speed of light and is used as a reference for calculating refractive index.

83. How does a concave mirror concentrate sunlight?

When parallel rays from the sun fall on a concave mirror, they converge at the focus. This concentration of energy can produce high temperatures, useful in solar cookers and furnaces.

84. Why does a convex lens converge light rays?

A convex lens is thicker at the centre and thinner at the edges. Due to refraction at its surfaces, parallel rays bend towards the principal axis and meet at the focus.

85. What determines the power of a lens?

The power of a lens depends on its focal length. A lens with shorter focal length has greater power, meaning it bends light rays more strongly.

86. What is the unit of refractive index?

Refractive index has no unit because it is the ratio of two speeds measured in the same unit.

87. Why do objects under water appear raised?

Due to refraction, light rays from objects under water bend away from the normal when entering air. This makes the objects appear closer to the surface than they actually are.

88. Why is the image in a convex mirror always diminished?

Since convex mirrors diverge light rays, the reflected rays appear to originate from a point behind the mirror. This always results in a smaller image compared to the object.

89. How does total internal reflection occur in diamonds?

Diamonds have a high refractive index and small critical angle. Light entering a diamond undergoes repeated total internal reflection, producing sparkle and brilliance.

90. What is meant by principal axis of a lens?

The principal axis of a lens is the straight line passing through its optical centre and the centres of curvature of its surfaces.

91. Why is refraction important in daily life?

Refraction enables image formation in lenses used in spectacles, cameras, microscopes, and telescopes. It also explains phenomena like apparent depth and rainbow formation.

92. What is meant by converging and diverging rays?

Converging rays move toward a single point, while diverging rays spread away from a point. Convex lenses produce converging rays, and concave lenses produce diverging rays.

93. How does changing medium affect wavelength?

When light changes medium, its speed changes while frequency remains constant. Therefore, wavelength changes according to the new speed in that medium.

94. What is the relationship between focal length and power?

Power of a lens is inversely proportional to its focal length. Shorter focal length means greater power and stronger bending of light rays.

95. Why do we use concave mirrors in dental mirrors?

Concave mirrors form enlarged images when the object is close. Dentists use them to get a magnified view of teeth for careful examination.

96. What is the function of a lens in a camera?

A camera lens refracts light rays and converges them to form a real, inverted image on the film or sensor.

97. Why does a mirror not change the color of an object?

A mirror only reflects light without changing its frequency. Since color depends on frequency, the reflected light retains the same color.

98. What happens to angle of refraction when angle of incidence increases?

As the angle of incidence increases, the angle of refraction also increases proportionally according to Snell’s law, until it reaches 90° at the critical angle.

99. Why does light not bend when incident normally?

When light strikes a surface along the normal, the angle of incidence is zero. Therefore, there is no change in direction and no bending occurs.

100. Why is understanding reflection and refraction important?

Understanding reflection and refraction helps explain image formation, design of optical instruments, and natural phenomena. It is fundamental in optics and many real-life applications.

Here are Long Question–Answers (100–120 words each

1. What is reflection of light? Explain the laws of reflection.

Reflection of light is the phenomenon in which light rays bounce back into the same medium after striking a surface. It occurs on smooth surfaces like mirrors. There are two laws of reflection. First, the angle of incidence is always equal to the angle of reflection. Second, the incident ray, reflected ray, and the normal at the point of incidence lie in the same plane. These laws are valid for all types of reflecting surfaces. Reflection helps in image formation in mirrors and is used in many devices such as periscopes, kaleidoscopes, and rear-view mirrors.

2. Describe the image formation by a plane mirror.

A plane mirror forms a virtual, erect, and same-sized image of an object. The image appears behind the mirror at the same distance as the object is in front of it. The image cannot be obtained on a screen because light rays do not actually meet; they only appear to come from behind the mirror. The image formed is laterally inverted, meaning the left side appears as the right side and vice versa. Plane mirrors are commonly used in homes, periscopes, and dressing mirrors due to their ability to produce clear and upright images.

3. Explain the terms pole, principal axis, centre of curvature, and focal length in spherical mirrors.

In a spherical mirror, the pole is the midpoint of the reflecting surface. The principal axis is the straight line passing through the pole and the centre of curvature. The centre of curvature is the centre of the sphere of which the mirror is a part. The focal length is the distance between the pole and the principal focus, where parallel rays meet or appear to diverge. These terms help in understanding image formation in concave and convex mirrors and are important in solving numerical problems.

4. Explain image formation by a concave mirror for different object positions.

A concave mirror forms different types of images depending on object position. When the object is beyond the centre of curvature, the image is real, inverted, and diminished. At the centre of curvature, the image is real, inverted, and same-sized. Between centre and focus, the image is real, inverted, and enlarged. At the focus, the image forms at infinity. Between focus and pole, the image is virtual, erect, and magnified. These properties make concave mirrors useful in shaving mirrors, headlights, and solar cookers.

5. Why is a convex mirror used as a rear-view mirror?

A convex mirror is used as a rear-view mirror because it provides a wide field of view. It always forms virtual, erect, and diminished images, allowing drivers to see more area behind them. The reduced size of the image helps in observing traffic over a larger distance. Unlike concave mirrors, convex mirrors do not form real images, making them safer for driving. Their ability to show a broad area makes them ideal for vehicles and security purposes in shops and parking areas.

6. What is refraction of light? Explain its causes.

Refraction of light is the bending of light when it passes from one transparent medium to another. It occurs because the speed of light changes in different media. When light enters a denser medium, its speed decreases and it bends towards the normal. When it enters a rarer medium, its speed increases and it bends away from the normal. Refraction explains many natural phenomena such as the apparent bending of objects in water and the formation of rainbows. It is also the principle behind lenses and optical instruments.

7. State and explain Snell’s law of refraction.

Snell’s law states that the ratio of sine of the angle of incidence to sine of the angle of refraction is constant for a given pair of media. This constant is called the refractive index. Mathematically, sin i / sin r = constant. This law shows that light bends differently in different media depending on their refractive indices. Snell’s law is important for calculating refractive index and understanding how lenses and prisms work.

8. Define refractive index and explain its significance.

Refractive index of a medium is the ratio of the speed of light in vacuum to the speed of light in that medium. It is represented by ‘n’ and has no unit. A higher refractive index means light travels slower in that medium. It determines how much light bends when entering the medium. Refractive index is important in designing lenses, prisms, microscopes, and telescopes. It also helps explain phenomena such as apparent depth and total internal reflection.

9. Explain total internal reflection and its conditions.

Total internal reflection occurs when light travels from a denser medium to a rarer medium and the angle of incidence exceeds the critical angle. Instead of refracting, the light is completely reflected back into the denser medium. Two conditions are required: light must travel from denser to rarer medium, and angle of incidence must be greater than the critical angle. This principle is used in optical fibres, diamonds, and periscopes. It allows efficient transmission of light over long distances without loss.

10. Describe the formation of images by a convex lens for different object positions.

A convex lens forms different images depending on object placement. When the object is beyond 2F, the image forms between F and 2F, real and diminished. At 2F, the image is same size. Between F and 2F, the image is real and enlarged. At F, the image forms at infinity. Between F and optical centre, the image is virtual, erect, and magnified. Convex lenses are used in cameras, microscopes, and magnifying glasses due to these properties.

11. Explain the working of a magnifying glass.

A magnifying glass uses a convex lens. When an object is placed within its focal length, the lens forms a virtual, erect, and enlarged image. The image appears on the same side as the object. Since the image is magnified, small details become visible clearly. Magnifying glasses are used for reading small print and examining tiny objects. The magnification depends on the focal length of the lens.

12. What is power of a lens? Explain its unit.

Power of a lens is defined as the reciprocal of its focal length in metres. It is denoted by P and given by P = 1/f. The SI unit of power is dioptre (D). A convex lens has positive power, while a concave lens has negative power. Greater power means shorter focal length and stronger bending of light.

13. Explain why the sky appears blue.

The sky appears blue due to scattering of sunlight by atmospheric particles. Blue light has shorter wavelength and is scattered more than red light. As sunlight passes through the atmosphere, blue light spreads in all directions and reaches our eyes. Hence, we see the sky as blue during daytime. This phenomenon is called scattering of light.

14. Why does a pencil appear bent in water?

A pencil appears bent when placed in water due to refraction. Light rays from the submerged part bend as they move from water to air. This makes the submerged portion appear shifted from its actual position. Hence, the pencil looks bent at the surface of water.

15. Explain the difference between real and virtual images.

A real image is formed when light rays actually meet after reflection or refraction. It can be obtained on a screen and is usually inverted. A virtual image is formed when rays only appear to meet. It cannot be obtained on a screen and is usually erect. Real images are formed by concave mirrors and convex lenses, while virtual images are formed by plane mirrors and concave lenses.

16. What is lateral inversion?

Lateral inversion is the phenomenon in which the left side of an object appears as the right side in a plane mirror and vice versa. This occurs because the mirror reverses the front-back direction. It is observed when we raise our right hand, and the image appears to raise the left hand.

17. Explain dispersion of light.

Dispersion is the splitting of white light into its constituent seven colors when it passes through a prism. Different colors have different wavelengths and refractive indices, so they bend by different amounts. Violet deviates the most and red the least. Dispersion explains rainbow formation.

18. What is critical angle?

The critical angle is the angle of incidence in the denser medium for which the angle of refraction in the rarer medium becomes 90 degrees. If the angle of incidence increases beyond this value, total internal reflection occurs.

19. Explain the relation between speed, wavelength, and frequency during refraction.

When light enters a new medium, its speed changes while frequency remains constant. Since speed equals frequency multiplied by wavelength, a change in speed causes a change in wavelength. In denser media, speed and wavelength decrease.

20. Why is the study of reflection and refraction important?

Reflection and refraction explain image formation in mirrors and lenses. They are essential for understanding optical instruments like cameras, microscopes, telescopes, and spectacles. These concepts also explain natural phenomena like rainbows, mirages, and apparent depth. Studying them helps in scientific and technological advancements.