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Grade 10 / Science / Physics / Light
Chapter 9 · NCERT Science 086

Light — Reflection
& Refraction

Why does a mirror flip your image? Why does a straw look bent in water? This chapter answers those — and gives you the formulas to prove it.

📐 5 topics · ⏱ ~2 hours · 📝 20 quiz questions
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Topics

Work through each topic

What happens when light hits a mirror?

Light travels in straight lines. When it hits a smooth, shiny surface (a mirror), it bounces off. This bouncing is called reflection.

Key Concept

The angle of incidence (∠i) always equals the angle of reflection (∠r). Both angles are measured from the normal (an imaginary perpendicular line to the surface).

Interactive: Reflection Simulator

Drag the light source to change the angle of incidence. Watch the reflected ray update in real time.

Drag the slider to change the angle. The reflected ray always mirrors the incident ray.

Image formed by a plane mirror

  • The image is virtual (you can't project it on a screen)
  • The image is laterally inverted (left ↔ right swapped)
  • The image distance from the mirror = object distance from the mirror
  • The image is the same size as the object
Formula

∠i = ∠r

Angle of incidence = Angle of reflection (Law of Reflection)

Concave vs Convex

A concave mirror curves inward (like a cave). It converges light — used in torches, shaving mirrors, and satellite dishes.

A convex mirror curves outward. It diverges light — used as rear-view mirrors in vehicles because it gives a wider field of view.

Key Concept

For a concave mirror, real images form in front of the mirror. Virtual images form behind it (when the object is between the pole and focus).

Mirror Formula

1/f = 1/v + 1/u

f = focal length, v = image distance, u = object distance

Magnification

m = h'/h = −v/u

h' = image height, h = object height. Negative m means inverted image.

Worked example · concave mirror

An object is placed 30 cm in front of a concave mirror of focal length 20 cm. Find the image distance and magnification.

  1. Apply signs: u = −30 cm (real object, in front), f = −20 cm (concave).
  2. Mirror formula: 1/v = 1/f − 1/u = 1/(−20) − 1/(−30) = −1/20 + 1/30 = −1/60.
  3. So v = −60 cm — a real image, 60 cm in front of the mirror.
  4. m = −v/u = −(−60)/(−30) = −2 → real, inverted, and twice the size.
Common mistake: dropping the signs. A real object's distance u is always negative, and a concave mirror's focal length is negative. Put the signs in first, then solve — sign errors are the #1 reason marks are lost here.

Why does a straw look bent in water?

When light passes from one medium to another (like air → water), it changes speed. This change in speed causes it to bend. This bending is called refraction.

Key Concept

Light bends towards the normal when entering a denser medium (air → water) and away from the normal when entering a rarer medium (water → air).

Light slows down in water — watch it bend toward the normal.

🔬 Open the full Refraction Lab →

In the lab: drag the angle and refractive index, see Snell's law update live, push past the critical angle for Total Internal Reflection — and ask Buffy anything.

Snell's Law

n₁ sin θ₁ = n₂ sin θ₂

n₁, n₂ = refractive indices of the two media; θ₁, θ₂ = angles with the normal

Refractive Index

n = c / v

c = speed of light in vacuum (3×10⁸ m/s), v = speed in the medium

Worked example · Snell's law

Light goes from air into glass (n = 1.5) at an angle of incidence of 45°. Find the angle of refraction.

  1. n₁ sin θ₁ = n₂ sin θ₂ → 1 × sin 45° = 1.5 × sin θ₂.
  2. sin θ₂ = sin 45° / 1.5 = 0.707 / 1.5 = 0.471.
  3. θ₂ = sin⁻¹(0.471) ≈ 28° — the ray bends toward the normal (entering a denser medium).
Common mistake: measuring the angle from the surface. Both θ₁ and θ₂ are always measured from the normal (the perpendicular), never from the boundary surface.

Convex lens — the converging lens

A convex lens is thicker in the middle. It bends parallel light rays to meet at a single point — the principal focus (F).

A concave lens is thinner in the middle and diverges light rays.

Lens Formula

1/f = 1/v − 1/u

Same as mirror formula but with a minus sign for v.

Power of a Lens

P = 1/f (in metres)

Unit: Dioptre (D). A convex lens has positive power; concave has negative.

Worked example · convex lens

An object is 30 cm from a convex lens of focal length 20 cm. Find the image distance, its power, and the image nature.

  1. Signs: u = −30 cm, f = +20 cm (convex is positive).
  2. Lens formula: 1/v = 1/f + 1/u = 1/20 + 1/(−30) = 1/20 − 1/30 = 1/60 → v = +60 cm (real, other side).
  3. m = v/u = 60/(−30) = −2 → real, inverted, twice the size.
  4. Power P = 1/f (metres) = 1/0.20 = +5 D.
Common mistake: mixing up the two formulas. The lens formula is 1/v − 1/u = 1/f (minus), while the mirror formula is 1/v + 1/u = 1/f (plus). Use the right one for the right device.

The Cartesian Sign Convention

  • All distances are measured from the pole (mirror) or optical centre (lens)
  • Distances in the direction of incident light are positive
  • Distances against the direction of incident light are negative
  • Heights above the principal axis are positive, below are negative
Pro Tip for Numericals

Always draw a diagram first. Mark the given values with correct signs. Then substitute into the formula. This catches sign errors — the #1 mistake in board exams.

Why this matters

Where you'll actually use Light

Reflection and refraction aren't just exam topics — they run the internet, fix eyesight, and keep cars safe. Here's where this chapter shows up in the real world.

Optical fibres — the internet runs on this

Total Internal Reflection traps a beam inside a hair-thin glass fibre, bouncing it thousands of times so a signal travels hundreds of kilometres with almost no loss. Every video call, broadband line and undersea cable depends on it — and doctors use the same trick in an endoscope to see inside your body without surgery.

Total Internal Reflection
A convex lens focuses rays onto the retina

Spectacles — lenses that fix your eyes

If your eye focuses light in front of or behind the retina, the world looks blurry. A convex lens (for long-sight) or a concave lens (for short-sight) bends the rays by exactly the right amount so they meet on the retina. Billions of people read, drive and work thanks to the lens formula in this chapter.

Lenses & Power
📷 Cameras & phone lenses

A convex lens projects a real, inverted image onto a sensor — the exact image rules you just learned, in your pocket.

☀️ Solar cookers

A concave mirror concentrates sunlight to a focus hot enough to cook — converging reflection doing real work.

🌈 Rainbows & mirages

Refraction and dispersion split sunlight into seven colours; bending through hot air creates desert mirages.

🔒 More real-world applications

Cameras, solar cookers, rainbows, periscopes and medical endoscopy — each explained with a diagram. Free to unlock.

Create a free account to unlock →

Competency-Based Quiz

Test yourself

20 questions — MCQ, assertion-reason, case-study, and source-based. Just like your board exam.

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