Mirror is any surface reflecting light. As light is an electromagnetic radiation in the visible range of waves, the conclusions derived from analyses of parabolic mirror are true for communication antennas as well. Only the size of an antenna must be tailored to wavelength of the radiation.
The white lines in Fig. 17 illustrate the rays of the light. The parabolic mirror is illuminated by a plane wave and the reflected rays concentrate in a point on the axis of the mirror. This point is labelled as focus. The reflection obeys the low of geometric optics: the angle of incidence equals to the angle of reflection. As all the rays meet in one point on the mirror axis, the focal length is independent on the aperture of the mirror.
Geometric optic does not solve the distribution of the light along the axis in a vicinity of the focus. Distribution of light along the axis is solved by the QED
Fig. 17 Probability pattern for low (red), middle (green) and high (blue) frequency
There are shown three probability patterns for lights of red, green and blue frequencies in Fig. 17. As the photon travels over paths always in the same medium, no dispersion effect takes place and the focuses are not separated in contrast with the focuses separation by lens. Nevertheless, the width of the probability pattern is getting larger with decreasing photon frequency. A focus calculated by the
Parabola is defined by the equation x = Ky2. The parameter K is optional in the program