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Rectilinear propagation describes the tendency of Electromagnetic waves (light) to travel in a straight line. Light only deviates from a straight line when the medium it is travelling through changes density. This is called refraction. Light does not deviate when travelling through a homogenous medium, which has the same refractive index throughout.
Even though a wave front may be bent, (e.g. the waves created by a rock hitting a pond) the individual waves are moving in straight lines. In the sense of the scattering of waves by an inhomogeneous medium, this situation corresponds to the case n ≠ 1, where n is the refractive index of the material. An experiment can be set up to prove this. Three cardboard squares are aligned with a small hole in the centre of each. A light is set up behind the cardboard. The light appears through all three holes from the other side. The light is blocked if any one of the cardboard squares are moved even a tiny bit. This proves that waves travel in straight lines and this helps to explain how humans see things, among other uses. It has a number of applications in real life as well.
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