Light — Reflection and Refraction · Class 10 Science: quick revision, 79 bites

Reflection of Light

Why can you see anything at all?

**Reflection** is the bouncing back of light into the same medium when it strikes a surface. Every object you can see is either giving out light or reflecting it into your eyes.

Takeaway. Seeing = light from an object reaching your eyes, usually by reflection.

The Vocabulary of Reflection

Five names you must know cold

Takeaway. Every angle in optics is measured from the normal.

First Law of Reflection

Two laws run this whole chapter

Whatever angle the light arrives at (measured from the normal), it leaves at exactly the same angle on the other side of the normal.

Takeaway. ∠i = ∠r — always, on every surface.

Second Law of Reflection

The forgotten second law

The **incident ray**, the **reflected ray** and the **normal** at the point of incidence all lie in a **single plane**.

Takeaway. All three — incident ray, reflected ray, normal — share one plane.

Angle from the Normal, NOT the Mirror

The #1 mark-loser in this chapter

Examiners set this trap every year. Read whether the angle is given from the *surface* or from the *normal* before you write anything.

Takeaway. Angle with surface + angle of incidence = 90°.

Regular vs Diffuse Reflection

Mirror vs wall — same laws, different result

Takeaway. Roughness scatters rays because each point's normal points a different way — the laws never break.

How a Plane Mirror Forms an Image

Where exactly IS your reflection?

Rays from your face reflect off the mirror obeying ∠i = ∠r. Your eye receives the *diverging* reflected rays, and your brain extends them **backwards in straight lines** — they appear to meet behind the mirror.

Takeaway. A plane-mirror image is formed where reflected rays APPEAR to meet, behind the mirror.

Properties of a Plane Mirror Image

Five properties, guaranteed one exam question

Takeaway. Virtual, erect, same size, equal distance, laterally inverted.

Real vs Virtual Images

Real or virtual? Screen test decides

Takeaway. Real = rays truly meet (screen ✓). Virtual = rays traced backwards (screen ✗).

Lateral Inversion

Why mirror-you raises the 'wrong' hand

In a plane mirror, **left and right are interchanged** — your left hand becomes the image's right hand. This left–right swap is called **lateral inversion**.

Takeaway. Lateral inversion = left↔right swap only.

Lateral Inversion at Work

Why is AMBULANCE written backwards?

Takeaway. Pre-inverting text cancels the mirror's inversion.

The Ray That Hits Head-On

A free mark, if you remember one line

A ray travelling **along the normal** has ∠i = 0°, so ∠r = 0° too — it reflects **straight back along its own path**.

Takeaway. ∠i = 0° → the ray retraces its own path.

The Mirror Rotation Result

Rotate the mirror, double the swing

Turning the mirror changes both the normal direction AND the reflection geometry — the two effects add, so the reflected ray swings twice as far.

Takeaway. Mirror rotates θ → reflected ray rotates 2θ.

The Periscope

Seeing over walls with two mirrors

Takeaway. Periscope = two 45° plane mirrors; used in submarines and bunkers.

Multiple Reflections

Infinite images from two mirrors

Two mirrors facing each other reflect each other's reflections — producing a theoretically infinite series of images. A **kaleidoscope** uses mirrors at an angle to turn bits of coloured glass into symmetric patterns.

Takeaway. Angled mirrors multiply images: 90° gives 3 images, 60° gives 5.

Moonlight is Reflected Sunlight

The Moon is a giant mirror (a bad one)

The Moon emits no light of its own — it reflects sunlight, and poorly (only about 12% of light is reflected; that's diffuse reflection from lunar dust). That reflected light takes about **1.3 seconds** to reach Earth.

Takeaway. Non-luminous bodies like the Moon are visible only by reflection.

Remember: SAME PLANE, SAME ANGLE

One phrase, both laws

**SAME PLANE** → incident ray, reflected ray and normal lie in one plane (Law 2). **SAME ANGLE** → ∠i = ∠r (Law 1). Say it twice; it's both laws in four words.

Takeaway. SAME PLANE (Law 2), SAME ANGLE (Law 1).

Board-Exam Question Patterns

How this topic is actually asked

Takeaway. Master the five properties + surface-vs-normal conversion and this topic is full marks.

Concave vs Convex

A mirror cut from a sphere

Takeaway. Concave converges, convex diverges — everything else follows.

P, C, R, F and the Principal Axis

Six words that unlock every diagram

Takeaway. The principal axis is the line through P and C — all measurements ride on it.

R = 2f

The most-used relation in the chapter

Given R = 30 cm? Then f = 15 cm — instantly. Half the numericals in this chapter start with this one line.

Takeaway. Focal length is always half the radius of curvature.

Parallel Ray → Through the Focus

Rule 1 of 4

A ray travelling **parallel to the principal axis** reflects and passes **through F** (concave) — or appears to come from F behind the mirror (convex).

Takeaway. Parallel in → focus out.

Through the Focus → Parallel

Rule 2 — Rule 1 in reverse

A ray passing **through F** reflects **parallel to the axis**. Light paths are reversible — this is Rule 1 backwards.

Takeaway. Focus in → parallel out (the torch principle).

Through C → Retraces Its Path

Rule 3 — the boomerang ray

A ray directed through the **centre of curvature** strikes the mirror along its **normal** (every radius is perpendicular to the sphere's surface), so ∠i = 0° and it reflects straight back on itself.

Takeaway. Via C → hits normally → comes straight back.

At the Pole → Symmetric Reflection

Rule 4 — the pole bounce

A ray striking the **pole** reflects making an equal angle on the other side of the principal axis — the axis acts as the normal at P, and plain old ∠i = ∠r applies.

Takeaway. At P, the principal axis is the normal; reflect symmetrically.

How to Draw Any Ray Diagram

Two rays are enough

Takeaway. Tip-to-tip with two rays — never more.

Concave Mirror: All Six Cases

The table the board loves

Takeaway. Five real cases, one virtual — and the virtual one is the shaving mirror.

Convex Mirror: Always VED

One mirror, one answer, always

For **every** object position, a convex mirror's image is **Virtual, Erect and Diminished**, formed between P and F *behind* the mirror.

Takeaway. Convex = Virtual, Erect, Diminished. Always.

f = R/2, never 2R

Half or double?

Takeaway. Focal length is the HALF, radius is the DOUBLE.

Concave Mirrors at Work

Why headlights throw a beam

Takeaway. Every concave use is Rule 1 or Rule 2 wearing a costume.

Why Vehicles Use Convex Mirrors

Objects in mirror are closer than they appear

A convex mirror's diminished, erect image squeezes a **wide field of view** into a small mirror — the driver sees far more road than a plane mirror of the same size would show.

Takeaway. Convex trades size for field of view — perfect for rear-view.

Object Moves In, Image Moves Out

Watch the image travel

Bring an object from infinity towards a concave mirror's focus: the real image starts at F and moves **away** from the mirror — F → between F and C → C → beyond C → infinity — growing all the way.

Takeaway. Object toward F ⇒ real image away and bigger; inside F ⇒ virtual flip.

'Which Mirror Is It?' Questions

Identify the mirror in one line

Takeaway. Erect-small = convex; anything real or magnified = concave.

Memory Pack

Three tricks, whole topic

Takeaway. Three phrases cover 80% of mirror MCQs.

The New Cartesian Sign Convention

Turn the mirror into a graph

It's just coordinate geometry with the pole as origin and incident light as +x.

Takeaway. Pole = origin; incident light direction = positive.

u Is Always Negative

The one sign everyone forgets

Takeaway. Write u with a minus before anything else.

Signs of f: Concave −, Convex +

Which f gets the minus?

Takeaway. CONcave = CONfirmed negative f.

The Mirror Formula

One line, whole chapter

Valid for both mirrors and every object position, provided signs go in by the convention.

Takeaway. Mirror = plus. (Lens = minus — never mix.)

Magnification

How big, which way up

One number answers three questions: real or virtual (sign), erect or inverted (sign), bigger or smaller (magnitude).

Takeaway. m carries the whole story of the image.

Reading Magnification

Decode m in two seconds

Takeaway. Sign → nature; magnitude → size. Read both, always.

Worked: Object at 30 cm, Concave f = 15 cm

The classic solved cleanly

Object at 2f (= C), image at C — the numbers reproduce the ray-diagram table perfectly.

Takeaway. Object at C ⇒ v = u, m = −1. Numbers agree with diagrams.

Worked: Convex Mirror Check

VED, now with numbers

Takeaway. Convex always lands at 0 < m < 1 — virtual, erect, diminished.

Mirror vs Lens Formula

Plus or minus? Careers divide here

Takeaway. Mirrors add, lenses subtract — memorise the plus.

Always Interpret, Never Just Compute

Where the last mark hides

Boards award the final mark for the WORDS: 'the image is real, inverted, magnified 2 times, formed 40 cm in front of the mirror.' A bare 'v = −40 cm' loses it.

Takeaway. Finish every numerical with one interpretation sentence.

The Range of m per Mirror

One mirror, every m

Takeaway. If |m| ≥ 1 or m is negative, the mirror can only be concave.

Reverse-Identification Questions

Spot the mirror from m alone

'A mirror forms an image with m = +0.4 — which mirror?' Positive and less than 1 → **convex** (a concave mirror's virtual images are always MAGNIFIED, m > +1). 'm = −2?' Real and double → **concave**, object between C and F.

Takeaway. 0 < m < 1 ⇒ convex; m > 1 or any negative m ⇒ concave.

Remember the Sign of m

Movie vs Mirror-selfie

**Minus m = Movie image** — real, inverted, could be thrown on a screen like a film. **Plus m = selfie in the mirror** — virtual, erect, only you can see it in there.

Takeaway. −m projects like a movie; +m poses like a selfie.

Sanity Checks for Answers

Numbers that check themselves

Takeaway. Every answer can be cross-checked against the mirror table in seconds.

Question Patterns to Expect

How boards ask this topic

Takeaway. Master forward AND backward use of m = −v/u.

Refraction of Light

Why the pencil looks broken

**Refraction** is the change in direction of light when it passes obliquely from one transparent medium to another — caused by the change in light's **speed** between the media.

Takeaway. Different medium → different speed → bent path.

Why Speed Change Bends Light

The car-in-sand picture

Picture a car driving from road onto sand at an angle: the wheel that hits sand first slows first, and the car swings. A light beam entering glass obliquely does the same — the slowed edge drags the ray **towards the normal**.

Takeaway. One edge slows before the other — the ray swings.

The Bending Rules

Which way does it bend?

Along the normal (i = 0°): no bending at all — only the speed changes.

Takeaway. Into dense: towards normal. Out of dense: away. Head-on: straight through.

Snell's Law

The second law has a name

Law 1 (same as reflection): incident ray, refracted ray and normal lie in one plane.

Takeaway. sin i / sin r is constant — that constant IS the refractive index.

Absolute Refractive Index

One number rates every medium

Takeaway. n = c/v. Water 1.33, glass 1.52, diamond 2.42 — memorise these three.

Relative Refractive Index

Comparing two media directly

And the pair property: n₂₁ × n₁₂ = 1. Water-to-glass and glass-to-water indices are reciprocals.

Takeaway. Watch the flip: n₂/n₁ but v₁/v₂.

Optical Density ≠ Mass Density

Kerosene floats — but wins at optics

Takeaway. Never argue optical density from floating and sinking.

Ray Through a Glass Slab

The experiment in every board paper

Takeaway. The slab shifts the ray without changing its direction.

Lateral Displacement

The sideways shift has a name

The perpendicular distance between the incident ray's original line and the emergent ray is the **lateral displacement**. It increases with: slab **thickness**, angle of **incidence**, and refractive **index**.

Takeaway. Thicker slab, steeper angle, denser glass → bigger shift.

Angle of Emergence = Angle of Incidence

A one-mark gift

The refractions at the two parallel surfaces are mirror events — what bends in, bends back out.

Takeaway. For a slab, e = i. Free mark, every time it's asked.

Apparent Depth

The pool is lying to you

Takeaway. Water hides a quarter of its depth — never judge a pool by eye.

Atmospheric Refraction

Stars don't actually twinkle

Takeaway. The atmosphere is a giant, restless refracting medium.

Frequency Never Changes

What refraction can't change

On refraction, **speed** changes and **wavelength** changes with it — but **frequency stays constant** (the wave can't gain or lose oscillations at the boundary). That's why colour doesn't change when light enters glass.

Takeaway. v and λ change; f (and colour) never does.

The Three Numerical Patterns

Numericals here are two-liners

Takeaway. Every refraction numerical is one of these three lines.

DENSE = TENSE

Never mix the bend again

Entering a DENSER medium, light gets TENSE — slows down and clings closer to the normal. Leaving into a rarer medium it relaxes — away from the normal. One rhyme, whole rule.

Takeaway. Dense = tense = towards the normal.

Convex vs Concave Lens

Thick middle or thin middle?

Takeaway. Convex converges (+f), concave diverges (−f).

Optical Centre and the Two Foci

A lens has TWO foci

Light can cross a lens from either side, so a lens carries **two principal foci** — F₁ (incident side) and F₂ — equidistant from the **optical centre O**, through which rays pass undeviated.

Takeaway. O in the middle, F₁ and F₂ at distance f on either side.

The Lens Ray Rules

Three rays rule the lens world

Takeaway. Parallel→focus, centre→straight, focus→parallel.

Convex Lens: All Six Cases

The lens table the board loves

Takeaway. Same shape as the concave-mirror table — converging twins.

Concave Lens: Always VED

The other always-VED device

A concave lens forms a **virtual, erect, diminished** image between F₁ and O for EVERY object position — the mirror-image twin of the convex mirror's behaviour.

Takeaway. Concave lens = the convex mirror of refraction.

The Lens Formula

Mind the minus

Takeaway. Lens = Less: subtract. And convex lens f is POSITIVE.

Lens Magnification

No minus this time

Negative m → real & inverted; positive m → virtual & erect; |m| compares sizes. Only the FORMULA changed, not the interpretation.

Takeaway. Mirrors: m = −v/u. Lenses: m = v/u. Interpretation identical.

Worked: Object 15 cm, Convex f = 10 cm

One numerical, full method

Takeaway. Between F₁ and 2F₁ → real, inverted, magnified — numbers match the table.

Power of a Lens

The optician's unit

A convex lens has positive power, a concave lens negative — that's why myopia prescriptions read like −2.5 D.

Takeaway. P = 1/f in METRES; sign follows the lens.

Convert f to Metres First

The centimetre trap

Takeaway. Dioptres demand metres.

Powers Add in Contact

Stacking lenses, adding numbers

+4 D touching −1.5 D behaves as +2.5 D. This additive simplicity is exactly why opticians work in power rather than focal length.

Takeaway. In contact, powers just add — with their signs.

One Convex Lens, Three Machines

Camera, projector, magnifier — one lens

Takeaway. Same lens — the OBJECT POSITION chooses the machine.

Spot the Lens from the Image

Instant device identification

Takeaway. Erect-small = concave lens; anything real = convex lens.

Spectacle Numbers Are Lens Powers

Your prescription decoded

Takeaway. Sign of the prescription = type of the lens.

Lens Memory Pack

Four anchors for the finale

Takeaway. Four lines that prevent every classic lens error.