Huygens' principle

"Each point of the wavefront, reached by the wave at a certain moment, can be considered as the source of the elementary wave, which propagates from it in elementary wavefronts. The wavefront at the next moment in time is the outer enveloping surface of all elementary wavefronts.[1]"

In an isotropic medium waves propagate in all directions at the same speed. Waveforms are represented by wavefronts. A wavefront is a set of points oscillating with the same phase. From the source, the wave arrives at a certain wavefront in time "t". In an isotropic environment, the individual points of the wavefront are arranged in such a way that they form the surface of a sphere (they have the same distance from the wave source). The radius of this sphere is characterized by:
 * $$r=v\cdot t,$$

where v is the speed of propagation of the wave and t is the time taken by the wave to travel from the source of the wave to a point on the surface of this sphere. The direction of wave propagation' is determined at each instant by the ray, which is normal to the wavefront. A plane wavefront only occurs if the wave source is plane. The rays are parallel to each other (see figure). A wavefront located at a great distance from the wave source has such a large radius that part of it can also be considered aplane wavefront. The elementary wavefronts cancel each other by interference at all points except the outer envelope surface. This mechanism of wave propagation can be used even if the exact position of the source cannot be determined, but we know the shape of the wavefront at a certain previous moment. Using Huygens' principle, reflection and refraction of light, wave propagation, bending at an obstacle can be explained - events that occur when waves hit the interface between two environments.

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