# Translations:The wave equation/15/en

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When irregular discontinuities exist in a nonuniform medium, they give rise to complex reflection, refraction, and diffraction phenomena. Huygens’ principle (or construction) is particularly useful for treating problems of this type. Huygens’ principle states that every point on a primary wavefront serves as the source of spherical secondary wavelets, and the primary wavefront at some later time is the envelope of these wavelets. Moreover, the wavelets advance with a speed and frequency equal to those of the primary wave at each point in space. If the medium is homogeneous, the wavelets can be constructed with finite radii, whereas if the medium is inhomogeneous, the wavelets must have infinitesimal radii.