What is the Huygens Principle

Huygens principle

Huygens principle, a principle for the qualitative construction of the wave fronts in the case of wave propagation in homogeneous media. It goes back to the Dutch physicist Christiaan Huygens.

To construct the new wavefront, all points of the original wavefront become more spherical as sources Elementary waves considered with the same frequency and propagation speed as the primary wave. The shape of the wave front at a later point in time is then given by the enveloping surface of all elementary waves (Fig. 1).

Fresnel extended this principle to Huygens-Fresnel principle (or Fresnel-Huygens principle) by including the interference of elementary waves (interference of waves) with various path differences.

With the help of Huygens principle it is e.g. possible to qualitatively describe the diffraction of waves (Fig. 2): in the vicinity of obstacles some of the elementary waves fail. That is why others can penetrate the geometric shadow without being obliterated by destructive interference.

With his principle, Huygens also explained the reflection and refraction on surfaces (Reflection of waves and refraction of waves) and also birefringence in crystals.

With this intuitive formulation of Huygens' principle, however, the amplitude, phase and directional dependence of the elementary waves remain open (obviously they must not, for example, propagate in a backward direction). These connections were first derived and clarified by Gustav Kirchhoff.