CMDLearn

Learn

Class 10 Science: Light, Reflection and Refraction, 8 practice questions with answers

CMDLearn ·

These are 8 practice questions on NCERT Class 10 Science Chapter 9, Light: Reflection and Refraction, each with a worked answer and the textbook page it comes from. Two are mirror formula numericals and one is a lens formula numerical; the rest cover the radius of curvature, power, refractive index and a rear-view mirror. Try each one on paper before you read the answer. Reflection is not only for light: sound reflects too, and echoes are in the Class 9 Sound Waves questions.

These questions are our own, drafted with AI help. Every number was re-computed by a test script, but no practising teacher has reviewed them yet. If you find a mistake, please tell us.

The formulas and the sign convention

Facts from the NCERT textbook (Science, Textbook for Class X, Reprint 2026-27, Chapter 9, printed pages 134 to 160):

Mirrors

Question 1. A concave mirror has a radius of curvature of 30 cm. How far is its focus from the mirror, in cm?

Answer: 15 cm. Focal length = radius of curvature ÷ 2 = 30 ÷ 2 = 15 cm. By the sign convention a concave mirror's focal length is written −15 cm. (R = 2f, page 137.)

Question 2. A concave mirror has a focal length of 15 cm. A candle stands 30 cm in front of it. How far from the mirror is the image formed?

Answer: 30 cm in front of the mirror. Use 1/v + 1/u = 1/f with u = −30 cm and f = −15 cm: 1/v = −1/15 + 1/30 = −1/30, so v = −30 cm. The image forms 30 cm in front of the mirror, at the centre of curvature. (Mirror formula, page 143.)

Question 3. A concave mirror has a focal length of 20 cm. An object 5 cm tall stands 30 cm from it. How tall is the image, in cm?

Answer: 10 cm, inverted. With u = −30 cm and f = −20 cm: 1/v = −1/20 + 1/30 = −1/60, so v = −60 cm. Magnification m = −v/u = −(−60)/(−30) = −2. Image height = m × 5 = −10 cm, which means 10 cm tall and inverted. (Mirror formula and magnification, pages 143-144.)

Question 4. Why is a convex mirror used as the rear-view mirror of a vehicle?

Answer: A convex mirror always gives an erect, though diminished, image, and it has a wider field of view, so the driver can see more of the traffic behind. (Uses of convex mirrors, page 142.)

Lenses and refraction

Question 5. A convex lens has a focal length of 10 cm. An object 4 cm tall is placed 15 cm from it. How tall is the image, in cm?

Answer: 8 cm, inverted. With u = −15 cm and f = +10 cm: 1/v − 1/u = 1/f gives 1/v = 1/10 − 1/15 = 1/30, so v = +30 cm. Magnification m = v/u = 30/(−15) = −2. Image height = −2 × 4 = −8 cm: 8 cm tall and inverted. (Lens formula, page 155; lens magnification, page 156.)

Question 6. A convex lens has a focal length of 50 cm. What is its power, in dioptres?

Answer: +2 D. Power P = 1/f with f in metres: f = 50 cm = 0.5 m, so P = 1/0.5 = +2 D (positive for a convex lens). (Pages 157-158.)

Question 7. Two thin lenses of power +2.5 D and −0.5 D are placed in contact. What is the focal length of the combination, in cm?

Answer: 50 cm. Powers of lenses in contact add: P = 2.5 + (−0.5) = 2 D. Focal length f = 1/P = 1/2 m = 0.5 m = 50 cm, a converging combination. (Page 158.)

Question 8. Light travels at 3 × 10⁸ m/s in air and at 2 × 10⁸ m/s in a kind of glass. What is the refractive index of this glass with respect to air?

Answer: 1.5. Refractive index = speed of light in air ÷ speed of light in the glass = (3 × 10⁸) ÷ (2 × 10⁸) = 1.5. It has no unit. (The textbook defines refractive index through the speed of light in vacuum; summary, page 159. Air is taken as very nearly the same as vacuum for this question.)

Practise more, and check your marks

Where CMDLearn fits

CMDLearn is building an AI tutor for Classes 1 to 10 that explains a step and checks the child's working, but leaves the writing to the child. It is still in development, and no child uses it yet. If you would like to hear when there is something to try, you can join the waitlist.

Sources

All opened and read on 21 October 2026. Page numbers are the printed page numbers of the textbook.

The formulas and definitions are from the textbook pages above. The questions, worked answers and checks are our own, and every number was re-computed by script. This guide is general information; follow your teacher and the textbook.