Refraction

When a wave enters a denser medium, it slows down and bends toward the normal. When it enters a less dense medium, it speeds up and bends away from the normal.

Snell’s law relates the angles and speeds:

$n_1 \sin \theta_1 = n_2 \sin \theta_2$

The refractive index of a material is:

$n = \frac{\text{speed of light in vacuum}}{\text{speed of light in material}}$

Refraction explains why swimming pools look shallower than they are, and why lenses can focus light.