Huygens principle, in optics, a statement that all points of awave frontof light in a vacuum or transparent medium may be regarded as new sources of wavelets that expand in every direction at a rate depending on their velocities. Proposed by the Dutch mathematician, physicist, and astronomer,Christiaan Huygens, in 1690, it is a powerful method for studying various optical phenomena.

A surface tangent to the waveletsconstitutesthe new wave front and is called theenvelope of the wavelets. If a medium ishomogeneousand has the same properties throughout (i.e.,is isotropic), permitting light to travel with the same speed regardless of its direction ofpropagation, the three-dimensional envelope of a point source will be spherical; otherwise, as is the case with many crystals, the envelope will be ellipsoidal in shape (seedouble refraction). An extended light source will consist of aninfinitenumber of point sources and may be thought of as generating a plane wave front.