Refraction

When light passes from air into water it bends. This sudden change in direction is called refraction. The degree that light bends in a clear material is called the refractive index of that material. When light passes through two materials with different refractive indices refraction occurs.

The light will bend towards the perpendicular if it goes from a material with a low refractive index to material with a high refractive index. Look at the animation on the right of light passing from air into glass and out again. Glass has a higher refractive index than air and so light bends towards the perpendicular as it enters the glass. It bends away from the perpendicular as it passes from glass into air.