Regular and quasiregular spectra of disordered layer structures

Two examples of problems solvable by statistical optics methods are considered. They are quite clear, they can be interpreted in a transparent manner, and they do not require time-consuming calculations. It is shown how the solution of the first problem — the influence of random variations of the optical thickness of a Fabry—Perot interferometer on its transmission spectra — helps to solve the second promlem: the 'mystery' of the experimental spectra of polydomain KDP crystals. These examples may be useful in tackling other statistical problems.