Propagation of optical radiation.

The electromagnetic waves of optical radiation have short wavelengths between 0.1 und 1 mym. As long as the transverse dimensions of an optical system are large compared with the wavelength, one may neglect the finiteness of the optical wavelength. Under this assumption the light propagation is adequately described by "geometrical optics". Many optical problems can be solved by this approximation. The wave is replaced by a beam which consists of a bundle of rays. Obstacles and apertures within the beam cause sharp shadows or spots which can be accurately determined by the geometry of the ray path. However, if the transverse dimensions of these obstacles and apertures are comparable with the wavelength, diffraction effects occure, which are typical for the wave properties of the radiation. Such phenomena are neglected in geometrical optics and will be discussed in Section 2.4. The main purpose of geometrical optics is the calculation of the lightpath in optical instruments.