Huygen's Principle

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Wavefront

Wavefront is the set of all points where a wave has the same phase of the sinusoid. Wavelets are wave-like oscillations having an amplitude that beginning at zero, gradually increasing and decreasing back to zero.

Huygens principle

The medium is assumed to be homogeneous (independent of position) and isotropic (independent of direction). For light of a given frequency, every point on a wavefront acts as a secondary source of spherical wavelets with the same frequency and the same initial phase. The wavefront at a later time and position is then the linear superposition of all of these wavelets. The presence of the backwave is avoided by assuming that the amplitude of the secondary wavelets is not uniform in all directions. It is maximum in the forward direction and zero in the backward direction.