Light – Reflection and Refraction – Class 10 Science notes, important questions and MCQs

Chapter overview

Light travels in straight lines and bounces off mirrors (reflection) or bends when it passes from one medium to another (refraction). Mirror and lens formulas give image position and size.

Spherical mirrors

• Concave mirror: converging; forms real inverted images (and a virtual enlarged image when the object is within F). Used in torches, shaving mirrors, dentists' mirrors.
• Convex mirror: diverging; always virtual, erect, diminished image. Used as rear-view mirrors.
• f = R/2; mirror formula 1/v + 1/u = 1/f; magnification m = −v/u.

Refraction

• Light bends towards the normal entering a denser medium and away when leaving it.
• Snell's law: sin i / sin r = constant = refractive index n₂₁.
• Absolute refractive index n = c/v (speed in vacuum ÷ speed in medium).

Lenses

• Convex lens: converging; concave lens: diverging.
• Lens formula: 1/v − 1/u = 1/f; m = v/u.
• Power P = 1/f (f in metres), unit dioptre (D); convex lens has positive power.

Key terms

Principal focus

point where parallel rays meet (or appear to meet) after reflection/refraction

Refractive index

ratio of speed of light in vacuum to that in the medium

Real image

image that can be taken on a screen

Formulas

• 1/v + 1/u = 1/f (mirror)
• 1/v − 1/u = 1/f (lens)
• P = 1/f (metres), m = h′/h
• n = c/v

Important questions with answers

Quick revision

• Convex mirror → rear-view; concave mirror → torches and shaving.
• Light bends towards normal in a denser medium.
• Power in dioptres = 1/f in metres.

Exam tips

• Use the New Cartesian sign convention: distances measured against the incident light are negative.
• Ray-diagram questions: object beyond C, at C, between C and F, at F, within F — know each image.

Practice MCQs

1. The mirror used as a rear-view mirror in vehicles is

   1. concave
   2. cylindrical
   3. plane
   4. convex
   Show answer

   Answer: (d) convex. It gives a wider field of view.

2. The SI unit of power of a lens is

   1. metre
   2. watt
   3. dioptre
   4. joule
   Show answer

   Answer: (c) dioptre. Dioptre.

3. A ray going through the centre of curvature of a mirror is reflected

   1. along the same path
   2. parallel to the axis
   3. through F
   4. at 90°
   Show answer

   Answer: (a) along the same path. It strikes the mirror normally.

4. The focal length of a concave mirror of radius of curvature 30 cm is

   1. 30 cm
   2. 7.5 cm
   3. 60 cm
   4. 15 cm
   Show answer

   Answer: (d) 15 cm. f = R/2.

5. Power of a lens of focal length −50 cm is

   1. +2 D
   2. +0.5 D
   3. −0.5 D
   4. −2 D
   Show answer

   Answer: (d) −2 D. P = 1/(−0.5 m) = −2 D.

6. Speed of light in glass (n = 1.5) is

   1. 3 × 10⁸ m/s
   2. 2 × 10⁸ m/s
   3. 4.5 × 10⁸ m/s
   4. 1.5 × 10⁸ m/s
   Show answer

   Answer: (b) 2 × 10⁸ m/s. v = c/n = 2 × 10⁸ m/s.

7. An object is 20 cm in front of a concave mirror of focal length 10 cm. The image is at

   1. infinity
   2. 10 cm behind, virtual
   3. 20 cm in front, real
   4. 40 cm in front
   Show answer

   Answer: (c) 20 cm in front, real. 1/v = −1/10 + 1/20 = −1/20 ⇒ v = −20 cm (at C).

8. A convex lens forms a real image the same size as the object at 40 cm. Its focal length is

   1. 40 cm
   2. 20 cm
   3. 10 cm
   4. 80 cm
   Show answer

   Answer: (b) 20 cm. Same-size real image when object and image are at 2F: 2f = 40.