💡 UNIT – 9: LIGHT – REFLECTION AND REFRACTION(Question)

💡 UNIT – 9: LIGHT – REFLECTION AND REFRACTION

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I. Multiple Choice Questions (1 mark each)

1️⃣ Which of the following is not a source of light?
A) Candle B) Sun C) Electric bulb D) Moon

2️⃣ The speed of light is maximum in
A) Air B) Water C) Glass D) Diamond

3️⃣ The angle between the incident ray and the reflected ray is
A) Angle of incidence B) Angle of reflection C) Angle between mirrors D) Angle of deviation

4️⃣ A ray of light incident on a plane mirror makes an angle of 30° with the mirror surface. The angle of reflection is
A) 30° B) 60° C) 90° D) 45°

5️⃣ The mirror used by dentists to examine teeth is
A) Convex mirror B) Plane mirror C) Concave mirror D) Cylindrical mirror

6️⃣ The mirror used in vehicles as a rear-view mirror is
A) Plane mirror B) Convex mirror C) Concave mirror D) Spherical mirror

7️⃣ Which of the following correctly represents the image formed by a convex mirror?
A) Real and inverted B) Virtual and erect C) Real and enlarged D) Diminished and inverted

8️⃣ The refractive index of water is 1.33. The speed of light in water is
A) 3 × 10⁸ m/s B) 2.25 × 10⁸ m/s C) 4 × 10⁸ m/s D) 1.33 × 10⁸ m/s

9️⃣ If the magnification of a mirror is +1, then the mirror must be
A) Concave B) Plane C) Convex D) Spherical

🔟 The focal length of a concave mirror is 20 cm. Its radius of curvature is
A) 40 cm B) 10 cm C) 20 cm D) 30 cm

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II. One Mark Questions

11. Define reflection of light.

12. State the laws of reflection.

13. Define refraction of light.

14. Write the mirror formula.

15. Define the refractive index.

16. What is the unit of power of a lens?

17. What type of mirror is used in solar cookers?

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

19. Why is the convex mirror preferred as a rear-view mirror?

20. What happens to the speed of light when it passes from air into glass?

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III. Two Mark Questions

21. What is meant by lateral inversion?

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

23. What is the sign convention used for spherical mirrors?

24. State the laws of refraction of light.

25. Define focal length and centre of curvature of a spherical mirror.

26. Write the mirror formula and explain its terms.

27. What is the nature of image formed when an object is placed between the pole and focus of a concave mirror?

28. Write the lens formula and magnification formula for a convex lens.

29. What happens when a ray of light passes through the optical centre of a lens?

30. Draw a neat diagram showing the refraction of light through a glass slab.

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IV. Three Mark Questions

31. Draw ray diagrams to show the formation of image by a concave mirror when
     (a) the object is placed at infinity,
     (b) the object is beyond the centre of curvature, and
     (c) the object is between the focus and pole.

32. Define focal length of a convex lens. Derive the lens formula.

33. Draw a neat labelled diagram of refraction through a convex lens when the object is between F and 2F.

34. Explain with ray diagrams the image formation by a convex mirror.

35. Define the power of a lens and its unit. How is it related to focal length?

36. State Snell’s law of refraction and write its mathematical form.

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V. Four Mark Questions

37. Derive the mirror formula for a concave mirror.

38. State the Cartesian sign conventions for spherical mirrors and lenses.

39. Explain the relationship between object distance, image distance, and focal length in spherical mirrors.

40. What is meant by refractive index? Write its formula and explain with an example.

41. Draw a neat labelled diagram showing the refraction of light through a glass slab.

42. What is total internal reflection? Explain its conditions and applications.

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💡 UNIT – 9 : LIGHT – REFLECTION AND REFRACTION – KEY ANSWERS

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I. Multiple Choice Questions (1 mark each)

Q.No	Correct Answer	Explanation
1	✅ D. Moon	Moon reflects light, not a source
2	✅ A. Air	Light travels fastest in rarer medium
3	✅ D. Angle of deviation	Between incident & reflected rays
4	✅ B. 60°	Angle of incidence = 60°, reflection = 60°
5	✅ C. Concave mirror	Produces magnified image of teeth
6	✅ B. Convex mirror	Gives wider field of view
7	✅ B. Virtual and erect	Image by convex mirror always virtual
8	✅ B. 2.25 × 10⁸ m/s	v = c/n = 3×10⁸ / 1.33
9	✅ B. Plane mirror	Magnification +1 = virtual, same size
10	✅ A. 40 cm	R = 2f ⇒ 2 × 20 = 40 cm

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II. One Mark Questions – Key Answers

11. Reflection of light: Bouncing back of light from a surface.

12. Laws of reflection:
     (i) Angle of incidence = Angle of reflection.
     (ii) Incident ray, reflected ray, and normal lie in same plane.

13. Refraction: Bending of light when it passes from one medium to another.

14. Mirror formula:
     1/f = 1/v + 1/u

15. Refractive index: Ratio of speed of light in vacuum to that in medium (n = c/v).

16. Unit of power: Dioptre (D).

17. Mirror used in solar cooker: Concave mirror (to focus sunlight).

18. Image by convex mirror: Virtual, erect, and diminished.

19. Convex mirror for vehicles: Provides wide field of view.

20. Speed of light: Decreases when passing from air to glass.

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III. Two Mark Questions – Key Answers

21. Lateral inversion: Left appears right and right appears left (in plane mirror).

22. Difference – Real vs Virtual Image:

Real	Virtual
Formed by actual intersection of rays	Formed by apparent intersection
Inverted	Erect

23. Sign conventions:

• All distances measured from pole.

• Left side (object side) – negative.

• Heights measured upward – positive.

24. Laws of refraction:
     (i) Incident ray, refracted ray, and normal lie in same plane.
     (ii) sin i / sin r = constant = n (Snell’s law).

25. Focal length: Distance between focus and pole.
    Centre of curvature: Centre of the sphere of which mirror is part.

26. Mirror formula: 1/f = 1/v + 1/u
    f = focal length, v = image distance, u = object distance.

27. Object between pole & focus (concave):
    Image – virtual, erect, and magnified.

28. Lens formula: 1/f = 1/v – 1/u
    Magnification: m = h₂/h₁ = v/u

29. Ray through optical centre: Passes undeviated.

30. Refraction through glass slab: Incident, refracted, and emergent rays parallel but displaced.

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IV. Three Mark Questions – Key Answers

31. Concave mirror image formation:

Object position	Nature of Image
At infinity	Real, inverted, at focus
Beyond C	Real, inverted, between F & C
Between F & P	Virtual, erect, magnified

(Draw 3 ray diagrams.)

32. Focal length (convex lens): Distance from optical centre to focus.
    Lens formula derivation: 1/f = 1/v – 1/u.

33. Refraction diagram (object between F & 2F):
    Image – beyond 2F, real, inverted, magnified.

34. Image by convex mirror:
    Always virtual, erect, and diminished (behind mirror).

35. Power of lens: P = 1/f (in metres), Unit – Dioptre (D).
    Positive for convex, negative for concave.

36. Snell’s law: sin i / sin r = constant (n).
    Shows relationship between angles and media refractive index.

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V. Four Mark Questions – Key Answers

37. Mirror formula derivation:
    Using geometry of similar triangles –
    1/f = 1/u + 1/v

38. Cartesian sign conventions:

• All distances from pole.

• Left side negative, right side positive.

• Heights upward positive, downward negative.

39. Relationship:
    Mirror equation 1/f = 1/u + 1/v shows relation between object, image, and focal length.

40. Refractive index:
    n = sin i / sin r = c/v
    Measures bending ability of medium.

41. Glass slab diagram:
    Incident ray → refracted ray → emergent ray (parallel, lateral shift shown).

42. Total Internal Reflection (TIR):
    Light reflects completely inside denser medium when angle of incidence > critical angle.
    Applications: Optical fibres, diamond sparkle, prism.