The speed of yellow sodium light (wavelength, 589 nanometers) in a certain liquid is measured to be 1.92 × 10⁸ meters per second. What is the index of refraction of this liquid, with respect to air, for sodium light? The speed of light in air is approximately 2.998 × 10⁸ meters per second.

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