Quasioptics

Quasioptics concerns the propagation of electromagnetic radiation when the size of the wavelength is comparable to the size of the optical components (e.g. lenses, mirrors, and apertures) and hence diffraction effects become significant. It commonly describes the propagation of Gaussian beams where the beam width is comparable to the wavelength. This is in contrast to geometrical optics, where the wavelength is small compared to the relevant length scales.