Refraction is the bending of light as it passes through a material with a different refractive index. When light passes through a glass slab it is refracted or bent because the light travels at a different speed through the glass than it does through air. This difference in speed causes the light to change direction as it passes through the glass which results in the bending of the light. The amount of bending depends on the thickness of the glass slab and the angle at which the light strikes the surface of the glass.