Wavefront

[1] The term is generally meaningful only for fields that, at each point, vary sinusoidally in time with a single temporal frequency (otherwise the phase is not well defined).

In classical physics, the diffraction phenomenon is described by the Huygens–Fresnel principle that treats each point in a propagating wavefront as a collection of individual spherical wavelets.

[2] The characteristic bending pattern is most pronounced when a wave from a coherent source (such as a laser) encounters a slit/aperture that is comparable in size to its wavelength, as shown in the inserted image.

This is due to the addition, or interference, of different points on the wavefront (or, equivalently, each wavelet) that travel by paths of different lengths to the registering surface.

If there are multiple, closely spaced openings (e.g., a diffraction grating), a complex pattern of varying intensity can result.

However, given the above simplifications, Huygens' principle provides a quick method to predict the propagation of a wavefront through, for example, free space.

[4] However, there may be more complex sources of aberrations such as in a large telescope due to spatial variations in the index of refraction of the atmosphere.

That said, those wavefront sensors suffer from linearity issues and so are much less robust than the original SHWFS, in term of phase measurement.