Refraction

Pick two materials and shine the torch across the boundary between them.

Into slower stuff: the light bends towards the normal. Like a toy car rolling at a slant off the road onto sand: the wheel that reaches the sand first slows down first, and the car turns.

Angle of incidence: 40°
Angle of refraction: 29°
The light bends by: 11°
Critical angle: none

What’s happening

Light is the fastest thing there is, but it slows down inside water, glass and diamond. When it crosses into a slower material at a slant, one side of the beam gets there first and slows down first, so the beam turns, the way a toy car turns when one front wheel rolls onto sand before the other. This bending is called refraction. Going into slower stuff, light bends towards the normal (the line standing straight up from the boundary); coming out into faster stuff, it bends away from it. A little of the light always bounces back too, like your reflection in a window.

Try this

Shine the light straight down. Does it bend?
Which bends light more, water or diamond?
Put glass on top and air below, then move the torch lower and lower. What happens to the light?
Swap top and bottom. Does the light take the same path backwards?
At home: put a coin in an empty mug and step back until the rim just hides it. Keep still while a friend slowly pours in water. Where does the coin go?

Learn more: Snell’s law, trapped light, and a lifeguard’s shortcut

Each material has a number called its refractive index, $n_1$ for the top and $n_2$ for the bottom. It says how many times slower light goes in it than in empty space:

n = \frac{\text{speed of light in empty space}}{\text{speed of light in the material}}

Air is 1.00 (light hardly slows down at all), water 1.33, glass about 1.5 and diamond 2.42: light in diamond goes at well under half its top speed. With $\theta_1$ the angle of incidence and $\theta_2$ the angle of refraction (θ is the Greek letter “theta”), both measured from the normal, the bending follows Snell’s law, named after Willebrord Snellius, who found it in 1621 (Ibn Sahl in Baghdad had found it over 600 years before him):

n_1 \sin\theta_1 = n_2 \sin\theta_2

Trapped light. Going from slower stuff into faster stuff, light bends away from the normal. At one angle, the critical angle $\theta_c$ , it would leave skimming along the boundary; any more slanted and it can’t get out at all, so all of it bounces back. This is total internal reflection:

\sin\theta_c = \frac{n_2}{n_1}

For glass into air that is 42°, for water into air 49°, and for diamond only 24°. The Total Internal Reflection demo shows what that trapped light does next.

Why does it bend? Two ways of seeing it:

The quickest path: a lifeguard running to someone in the sea doesn’t go in a straight line, but runs further along the sand, where they are fast, and swims less, where they are slow. Light does the same: of all the paths from the torch to a point under the water, the bent one takes the least time.
Light as a wave: the crests of a light wave are like a line of people marching side by side. The end of the line that crosses into the slow stuff first slows down first while the rest keep going, so the whole line swings round to a new direction. The waves also bunch up closer together, but just as many arrive each second, so the color stays the same.

Bent straws and shallow pools: light from under water bends as it comes out, so a straw in a glass looks broken and a pool looks shallower than it really is.
Lenses: glasses, cameras, microscopes and your own eyes use curved glass or jelly to bend light to a focus.
Colors: each color bends by a slightly different amount, which is how a prism and a raindrop split sunlight (see Dispersion).

Refraction

What’s happening

Try this

See it at Science Playground