Chapter 6 of 25

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Tamil Nadu (TNBSE)Class 9 Science★ 6-8 marks in assessments

Light

Samacheer Kalvi (TN State Board) Class 9 Science — Physics Chapter 6, Light. Light enables us to see the world. Understanding reflection, refraction, lenses, and spherical mirrors explains how cameras, microscopes, telescopes, eyeglasses, and car mirrors work. Notes, worked examples, common mistakes, and quick revision.

⏱ 22 min readHard difficulty✓ Free · NCERT-aligned

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Study Guide

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Key points, glossary and the questions most likely to show up.

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By the end of this chapter you'll be able to…

1Differentiate concave and convex mirrors and draw ray diagrams
2Use the mirror formula and magnification equations
3Explain refraction of light and refractive index
4Understand lens formula, power of lenses, and eye defects

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Why this chapter matters

Light enables us to see the world. Understanding reflection, refraction, lenses, and spherical mirrors explains how cameras, microscopes, telescopes, eyeglasses, and car mirrors work.

Before you start — revise these

A 5-minute refresher here will save you 30 minutes of confusion below.

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Class 8 Science — Light

Reflection laws and plane mirrors.

Light — Class 9 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 9 Science, Physics — Chapter 6. Light enables us to see the world. Understanding reflection, refraction, lenses, and spherical mirrors explains how cameras, microscopes, telescopes, eyeglasses, and car mirrors work.

1. About this chapter

This chapter covers reflection and refraction of light, spherical mirrors, lenses, magnification, refractive index, and power of lenses.

2. Spherical Mirrors

Concave Mirror: Converging mirror. Forms real/inverted images (except when object is very close, which forms virtual/erect).
Convex Mirror: Diverging mirror. Always forms virtual, erect, and diminished images.
Mirror Formula: $\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$ .

3. Refraction and Snell's Law

Refraction: Bending of light as it passes from one medium to another.
Snell's Law: $\frac{s i n i}{s i n r} = constant = \frac{n _{2}}{n _{1}}$ .
Refractive Index: $n = c / v$ (ratio of speed of light).

4. Lenses

Convex Lens: Converging lens.
Concave Lens: Diverging lens.
Lens Formula: $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$ .

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Relation between Focal Length and Radius

f = R / 2

R = radius of curvature.

Mirror Formula

1/f = 1/v + 1/u

u = object distance, v = image distance, f = focal length. Uses sign conventions.

Refractive Index

n = c / v

c = speed of light in vacuum, v = speed in medium.

Lens Formula

1/f = 1/v - 1/u

Notice the minus sign compared to mirror formula.

Power of a Lens

P = 1 / f (in metres)

Measured in dioptres (D).

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Incorrect sign convention for object/image distances.

✓ Object distance (u) is always negative. Focal length (f) is negative for concave mirror/lens, positive for convex mirror/lens.

WATCH OUT

✗ Confusing Mirror and Lens formulas.

✓ Mirror uses $1/f = 1/v + 1/u$; Lens uses $1/f = 1/v - 1/u$.

NCERT exercises (with solutions)

Every NCERT exercise from this chapter — what it covers and how many questions to expect.

Multiple Choice Questions

Questions on lens types, image properties, and refractive index.

Questions

Numerical Problems

Calculations on mirror/lens focal lengths and magnification.

Questions

Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1MEDIUM· Numerical

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

▶ Show solution

u = -10, f = -15. 1/v = 1/f - 1/u = 1/(-15) - 1/(-10) = -1/15 + 1/10 = (-2 + 3)/30 = 1/30. v = +30 cm (Virtual, erect, behind the mirror).

Q2EASY· Concept

Define refractive index and give its formula.

▶ Show solution

Refractive index is the ratio of the speed of light in vacuum to the speed of light in a medium ($n = c / v$). It has no unit.

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Concave mirrors converge; convex mirrors diverge.
•Mirror formula: $1/f = 1/v + 1/u$. Lens formula: $1/f = 1/v - 1/u$.
•Snell's law of refraction: $\sin i / \sin r = \text{constant} = n_2/n_1$.
•Power of lens $P = 1/f$ (in meters, unit: dioptre).

Tamil Nadu (TNBSE) marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Typical chapter weightage: 6-8 marks in assessments

Question type	Marks each	Typical count	What it tests
MCQ	1	1-2	Base concepts and definitions
Short Answer	2-3	1-2	Descriptive and application points

Prep strategy

Understand core definitions and solve standard textbook problems.
Review common mistakes to avoid losing easy marks.

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Spectacles

Concave lenses correct myopia (nearsightedness), while convex lenses correct hypermetropia (farsightedness).

Optical Fibers

Use total internal reflection to carry high-speed internet data across global networks.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

Write definitions precisely as defined in the textbook.
Draw neat, labeled diagrams for biology and physics chapters.

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

Read advanced reference materials to explore concepts beyond the school syllabus.

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

Class 9 Annual ExamsHigh

NTSE Stage 1Medium

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Questions students ask

The real ones — pulled from the Q&A community and tutor sessions.

Convex mirrors always form an erect, diminished image and cover a wider field of view, helping drivers see traffic behind them easily.

When light travels from a denser to a rarer medium and the angle of incidence exceeds the critical angle, the light is entirely reflected back into the denser medium.

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