In this section linear, circular and elliptical polarisations will be described in terms of the electric field of a monochromatic plane wave propagating in the +z direction which can be written
The actual field is
Depending on the ratio of E0, y to E0, x and the relative phase (ϐy – ϐx) the monochromatic plane wave will be linearly, circularly or elliptically polarised.
The laser (light amplification by stimulated emission of radiation) has many practical appli-cations and works on the same principle as the astrophysical maser just discussed. The main differences are that transitions typically in the optical or infrared are used, and various methods (depending on the medium) are used to pump the upper (metastable) level. Also, in the laser, the enormous path lengths needed are achieved by having a laser cavity containing the laser medium between two parallel mirrors in which the light is reflected back and forth, with some small fraction escaping from one of the mirrors (which is partially transmitting) to form a laser beam.
The subject of coherence (and the closely-related field of quantum optics) is important in physical optics and is a field in its own right of which we have just skimmed the surface, but suffice it to say here that the electromagnetic field in the centre of a laser beam is an excellent approximation to that of a monochromatic EM waves, and that many of the results I shall derive in the remainder of this chapter for monochromatic plane waves will apply also to partially coherent quasi-monochromatic beams.