Rectilinear propagation

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Rectilinear propagation describes the tendency of electromagnetic waves (light) to travel in a straight line. Light does not deviate when travelling through a homogeneous medium, which has the same refractive index throughout; otherwise, light experiences refraction. Even though a wave front may be bent, (e.g. the waves created by a rock hitting a pond) the individual rays are moving in straight lines. Rectilinear propagation was discovered by Pierre de Fermat.^[1]

Rectilinear propagation is only an approximation.^{[citation needed]} The rectilinear approximation is only valid for short distances, in reality light is a wave and have a tendency to spread out over time. The distances for which the approximation is valid depends on the wavelength and the setting being considered. For everyday usages, it remains valid as long as the refractive index in the medium is constant.

The more general theory for how light behaves is described by Maxwell's equations.