Unit 10, lesson 46. About 12 minutes.

Refraction

Why does a straw look bent in a glass of water?

Play

Getting the experiment ready…

Explain

Light bends when it goes into water.

Straws look bent in a drink.

Light travels more slowly in water and glass than in air.

When light enters at an angle, it changes direction. This bending is called refraction.

A lens is a curved piece of glass that uses refraction to bend light. Glasses and cameras use lenses.

Snell's law: n₁ sin θ₁ = n₂ sin θ₂, where n is the refractive index.

A larger n means light travels more slowly, and bends more. Water is 1.33, glass about 1.5, and diamond about 2.4.

Worked example: light enters glass at 45°. sin θ₂ = sin 45° / 1.5, so θ₂ ≈ 28°.

Light enters water
It slows down
Its path bends

Want the full story?

Light going straight in along the normal doesn't bend. Light going from glass to air at a large angle can't get out at all, which is called total internal reflection. Fibre optic cables use this to carry the internet.

The Formula

bend depends on the angle and on how much the material slows light

n_1 \sin\theta_1 = n_2 \sin\theta_2

$n$: the refractive index, which has no unit, which has no unit
$\theta_1$: the angle in air, from the normal line, measured in degrees (°)
$\theta_2$: the angle in the material, from the normal line, measured in degrees (°)

Try the numbers

Angle in air: 45°Refractive index: 1.5

Angle in the material: 28.13°

Try it at home

You need a clear plastic cup, water, a straw or pencil.

You need: a clear plastic cup, water, a straw or pencil.

Put a straw in water. It looks bent!

Half-fill the cup with water.
Put a straw in it.
Look from the side, and move your head around.

What's happening: light from the straw bends as it leaves the water, which makes the straw look broken.

Physics is life

Your body

Your eyes bend light.

The front of your eye bends light so that it lands on the back of your eye.

Daily life

Glasses bend light.

Glasses, cameras and magnifying glasses all use lenses.

Nature

Pools look shallow.

A pool looks shallower than it really is, because light bends as it leaves the water.

Space

Telescopes use lenses too.

Many telescopes use lenses to bend light. Air also bends starlight a little, and that makes stars twinkle.

Quick check

Question 1 of 3

Why does light bend in water?

Words to know

Refraction: Light bending as it goes from one material into another.

Refraction: Light bending as it goes from one material into another.
Lens: A curved piece of clear material that bends light.
Light: Energy that travels as waves you can see.

Next up

Rainbows and colour