The wave nature of light leads to two very important properties: refraction, where the direction of light propagation is altered at the boundary between media of different densities, and diffraction, which has as a consequence that light can "bend around corners".

If we adopt the convention that light passes from medium 1 into medium 2, refraction is (1) away from the perpendicular if medium 2 is less dense than medium 1, and (2) toward the perpendicular if medium 2 is more dense than medium 1.

In the adjacent example, the refraction is away from the perpendicular because air is less dense than water. Such effects form the basis of the refracting telescope.