Draw a ray diagram to show the refraction of light through a glass prism. Mark on it (a) the incident ray, (b) the emergent ray and (c) the angle of deviation.

When light enters a glass prism from air, it slows down and bends towards the normal at the first surface ($AB$). Upon exiting the prism into the air at the second surface ($AC$), it speeds up and bends away from the normal. Because the two refracting surfaces are not parallel (they are inclined at an angle $A$), the emergent ray is not parallel to the incident ray. The total bending suffered by the light ray is called the angle of deviation ($\\angle D$).