Refraction and refractive index

Put a pencil in a glass of water: it looks bent at the surface. A coin at the bottom of a bucket seems raised, and a pond looks shallower than it is. Light changes direction when it passes at a slant from one medium (such as air) into another (such as water or glass). This is refraction.

"Why does light bend?" Because its speed changes from one medium to another. It travels fastest in a vacuum (3 × 10⁸ m/s), almost as fast in air, and more slowly in water and glass.

Through a glass slab:

• entering the glass (into the slower medium), the ray bends towards the normal;
• leaving it, the ray bends away from the normal;
• the ray that comes out (the emergent ray) is parallel to the ray that went in, just shifted sideways.

A ray that hits the surface straight on, at 90°, does not bend at all.

The laws of refraction:

• the incident ray, the refracted ray and the normal lie in one plane;
• for light of a given colour and a given pair of media, sin i / sin r is constant. This is Snell's law.

In plain words: same two media and same colour gives the same bending, anywhere, any day. Change either medium, or the colour of light, and the bending changes.

The refractive index says how much a medium slows light:

n = speed of light in air (or vacuum) ÷ speed of light in the medium = c/v

The medium with the larger refractive index is optically denser: light is slower in it. Going from rarer to denser, light slows down and bends towards the normal.

Careful: optical density is not mass density. Kerosene has a higher refractive index than water, so it is optically denser, yet it is lighter: it floats on water.