Question

What causes light to refract?

Answer 1

Light enters a medium of different optical density and this causes its speed to change and hence it bends or refracts.

Explanation:

When light passes from an optically less dense to an optically more dense medium (example from air to glass since the refractive index `n_(air) < n_(glass)`) its speed decreases and hence it refracts towards the normal. (line drawn perpendicularly to the plane of the surface).

When light passes from an optically more dense to an optically less dense medium (example from water to air) its speed decreases and hence it refracts towards the normal. (provided the angle of incidence does not exceed the critical angle for the 2 media otherwise total internal reflection will occur).

In each case, the wave equation `v=f lambda` is still satisfied.