The principle that any point on a wave front of light may be regarded as the source of secondary waves and that the surface that is tangent to the secondary waves can be used to determine the future position of the wave front.
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Huygens\' principle. (n.d.). Retrieved October 23rd, 2018, from https://www.yourdictionary.com/huygens-principle
The principle that any point on a wave front may be regarded as a source of spherical or circular wavelets. The sum of all such wavelets emanating from the front is same as the wavefront itself. This principle can be used to derive the laws of reflection and refraction.