Class 10 · NCERT Science

Chapter 9

Light: Reflection and Refraction

Solve mirror and lens problems using ray diagrams and formulae. Learn the ideas, see how they connect, and practise until you can choose the next step without help.

MirrorsLensesRay diagrams

Reviewed by Eduro educators · Built for independent practice

Study board

Quick reference
Concept 01Mirrors
Concept 02Lenses
Concept 03Ray diagrams

NCERT chapter notes

Light: Reflection and Refraction, clearly explained

This chapter covers reflection of light by spherical mirrors and refraction through lenses. It develops the mirror formula and lens formula, magnification, sign conventions, and the idea of refractive index and power of a lens.

Numericals using the mirror and lens formulae, plus reasoning about which mirror or lens to use where, carry most of the marks.

Core concepts

Key concepts and formulae

01

Laws of reflection and spherical mirrors

The angle of incidence equals the angle of reflection, and the incident ray, reflected ray, and normal lie in one plane. For spherical mirrors the focal length is half the radius of curvature: f = R/2.

02

Mirror formula and magnification

The mirror formula is 1/v + 1/u = 1/f, using the New Cartesian sign convention. Magnification m = -v/u = h'/h; a negative m means a real, inverted image and a positive m means a virtual, erect image.

03

Uses of concave and convex mirrors

Concave mirrors are used in torches, shaving mirrors, and by dentists (they can form magnified, erect images of nearby objects). Convex mirrors are used as rear-view mirrors in vehicles because they form erect, diminished images and give a wider field of view.

04

Refraction and refractive index

Light bends when it passes from one medium to another. It bends towards the normal when entering a denser medium and away from the normal when entering a rarer medium. Refractive index n = sin i / sin r (Snell's law); a higher n means a denser (optically) medium.

05

Lens formula and magnification

For thin lenses, 1/v − 1/u = 1/f, and magnification m = v/u. A convex (converging) lens has positive focal length; a concave (diverging) lens has negative focal length.

06

Power of a lens

Power P = 1/f (with f in metres); its unit is the dioptre (D). A convex lens has positive power and a concave lens has negative power. Powers of lenses in contact add up.

Exam practice

Important questions, with worked answers

Use each answer to check the method and wording—not as a paragraph to memorise.

Q1Why is a convex mirror preferred as a rear-view (wing) mirror in vehicles?

Answer

A convex mirror always forms an erect, diminished (smaller) image and has a wider field of view, so the driver can see a larger area of traffic behind the vehicle.

Q2An object is placed 10 cm in front of a concave mirror of focal length 15 cm. Find the image distance.

Answer

Using 1/v + 1/u = 1/f with u = −10 cm, f = −15 cm: 1/v = 1/f − 1/u = (−1/15) − (−1/10) = −1/15 + 1/10 = 1/30. So v = +30 cm (a virtual, erect, magnified image behind the mirror).

Q3Define refractive index. Does light bend towards or away from the normal when going from air into glass?

Answer

Refractive index n = speed of light in vacuum / speed of light in the medium = sin i / sin r. Going from air (rarer) into glass (denser), light bends towards the normal.

Q4The power of a lens is +2 D. What is its focal length and what type of lens is it?

Answer

P = 1/f, so f = 1/P = 1/2 = 0.5 m = 50 cm. Since the power is positive, it is a convex (converging) lens.

Q5What does the negative sign in the value of magnification of a mirror indicate?

Answer

A negative magnification indicates that the image is real and inverted with respect to the object.

Q6Why do we prefer a concave mirror as a shaving mirror?

Answer

When the face is held within the focal length of a concave mirror, it forms an enlarged, erect, virtual image, which helps in shaving.

Fast recall

Key terms to remember

Focal length: Distance between the pole and the principal focus of a mirror or lens; for mirrors f = R/2.

Refractive index: Ratio sin i / sin r; measures how much a medium bends light.

Dioptre: SI unit of lens power; 1 D = 1 m⁻¹.

Real image: Image that can be caught on a screen; formed by actually converging rays (inverted).

Magnification: Ratio of image height to object height; tells the size and nature of the image.

A repeatable method

How to learn Light: Reflection and Refraction properly

  1. 1

    Start with the chapter promise

    Solve mirror and lens problems using ray diagrams and formulae. Before solving, the student should be able to say what the chapter is trying to teach and which kind of problem it helps solve.

  2. 2

    Build the core vocabulary

    The important words for this chapter are Mirrors, Lenses and Ray diagrams. Eduro should make the student define each one in simple language, then use it in a question or explanation.

  3. 3

    Move from recognition to recall

    Recognition means the answer makes sense after seeing it. Recall means the student can produce the next step independently. This page is built for recall, because that is what tests reward.

  4. 4

    Close the loop with practice

    A strong study session ends with concept and diagram practice, not only reading. The student should solve, review the mistake, and then attempt a similar question before moving on.

Before the test

Where students usually lose marks

Look for the first wrong decision—not only the final wrong answer. That is the mistake worth repairing.

Mistake 1

Knowing Mirrors only after seeing the solution

This is the most common hidden gap. The student feels confident while reading, but cannot choose the starting step alone. Eduro should ask a short diagnostic question before explaining the method.

Mistake 2

Treating Light: Reflection and Refraction as a memory chapter

Even memory-heavy chapters need reasoning. A memorised line becomes fragile when the question changes. The student should explain why the answer works, not only what the answer is.

Mistake 3

Skipping the checking step

concept-to-example gap usually survives because the student finishes the answer and moves on. Eduro should make review part of the answer: what was asked, what was used, and whether the final response fits.

Try it with Eduro

Practice that builds real confidence

Say it naturally, as you would to a patient teacher. Each prompt below is designed to make understanding visible.

Prompt 1

Ask Eduro to explain Light: Reflection and Refraction through Mirrors before showing any solved answer.

Prompt 2

Create five questions that separately test Mirrors, Lenses and Ray diagrams.

Prompt 3

Give one wrong answer from Light: Reflection and Refraction and ask the student to find the first incorrect step.

Prompt 4

End with a mixed mini-test where Eduro does not reveal which skill is being tested.

For parents

A better way to ask, “Is the chapter done?”

For Class 10 Science, a good sign is not that the child says 'Light: Reflection and Refraction is done.' A better sign is that they can explain Mirrors, solve one fresh question, and correct one mistake without panic.

Created by

Eduro Editorial Team

CBSE learning research and AI tutoring design

Reviewed by

Eduro Academic Review

Curriculum and accuracy review

Freshness

Reviewed for the 2025-26 CBSE/NCERT study cycle

Reviewed quarterly and after major CBSE/NCERT updates

Quick help

Common questions

How can I study Light: Reflection and Refraction for NCERT Class 10?

Start with the NCERT examples, understand the key ideas in Mirrors, Lenses, Ray diagrams, then practice exercise questions and ask Eduro where you get stuck.

Can Eduro help with Light: Reflection and Refraction?

Yes. Eduro can explain Light: Reflection and Refraction step by step, answer follow-up doubts, and help students practice related Science questions.