Refraction of light

Light waves slow down when they travel from air into glass. If they are at an angle to the glass, they bend towards the normal. When the light rays travel out of the glass into the air, their speed increases and they bend away from the normal. If the block of glass has parallel sides, the light resumes its original direction. This is why a sheet of window glass has so

little effect on the view beyond. However, the view is shifted slightly sideways if you are looking through the glass at an angle.

The angle of incidence i, is the angle between the incident light ray and the normal to the surface. The angle of refraction r is the angle between the refracted light ray and the normal to the surface inside the material.