A light ray with a wavelength of 589 nanometers (produced by a sodium lamp) traveling through air makes an angle of incidence of 30° on a smooth, flat slab of crown glass. Find the angle of refraction.

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.

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Angle of incidence A. Incident ray, speed v, Refracted ray, speed v2 "Angle of refraction