Light changes speed
Light is refracted only when it crosses the boundary at an angle — not when it arrives along the normal — and the reason is that its speed changes from medium to medium.
In a vacuum light travels at 3 × 108 m s-1. In any other medium it is slower.
Rarer medium
Of two media, the one in which light travels faster.
Denser medium
Of two media, the one in which light travels slower.
The speed of light in some media
| Medium | Speed (km s-1) |
|---|---|
| Air | 300 000 |
| Water | 225 000 |
| Glass | 197 000 |
| Perspex | 201 000 |
| Diamond | 124 000 |
Going from Perspex into glass, which way does a ray bend? Light is slower in glass, so glass is the denser of the two and the ray bends towards the normal.