A light beam passes through a thick slab of material whose index of refraction is n₂.Show that the emerging beam is parallel to the incident beam.

Light sound, and other waves travel at different speeds, depending on the media (air, water, wood, and so on) through which they pass. Suppose that light travels from a point A in one medium, where its speed is v₁, to a point B in another medium, where its speed is v₂. Refer to the figure, where the angle θ₁ is called the angle of incidence and the angle θ₂ is the angle of refraction. Snell's Law, which can be proved using calculus, states that

The v_l/v₂ is called the index of refraction. Some values are given in the table shown to the right.

sin 01 sin 02 V2 Angle of incidence A. Incident ray, speed v, Refracted ray, speed v2 "Angle of refraction