When a light ray is incident on a rectangular glass slab, refraction occurs twice, once at the interface at which the light is incident and second at the interface through which the ray leaves the slab. At the interface through which light ray enters the slab, light moves from rarer to denser medium such that it is refracted towards the normal. Again, when it leaves the slab, it experiences the same change in refractive indices. This causes the angle of incidence at the entrance end to become equal to the angle of refraction at the leaving end. Hence, the light ray emerges parallel to itself with a slight offset from its original path.