Fig. 1	Plane-parallel plates can have the following applications: - separation of two optical mediums with different characteristics (pressure, temperature, chemical composition, etc.); - as input/output windows;- as substrates for filters, beamsplitters and polarizers; - for compensation of spherical aberration, etc.         In figure 1 the path of rays through window, which separates two optical mediums with different refractive indexes, is shown. The law of refraction (Snell’s Law) in this case can be represented as follows:     sin Qi/sin Qr= N12• N23=n3/n1 Where N12, N23, n1 and n3 – relative and absolute refractive indexes of the corresponding mediums.         Usually the part of light reflects from the surface of a plate. An angle between an incident ray and a normal to the surface is equal to an angle between a normal to the surface and the reflected ray. This equation is known as the law of refraction.
Fig. 2	When an angle between the reflected and the refracted rays is 90 deg, the reflected light is completely polarized (Fig.2). The angle of incidence, at which this correlation between the reflected and the reflected rays is fulfilled, is called the Brewster angle.          The Brewster angle can be determined from the formula:  tan QБ=n2/ n1         At the Brewster angle incidence, polarization of refracted rays is partial, but at the same time maximal in comparison with the other angles of incidence. If these refracted rays pass through the set of plates, arranged at the Brewster angle, the value of their polarization will increase. The corresponding plates are called Brewster windows; they are used for generation of polarized light.
Fig. 3	There is one more phenomenon which is usually reviewed in the section of ray optics. It is called the total internal reflection. This phenomenon also takes place when light passes a plane boundary between optical mediums, but it occurs in prisms, not in plates.          The total internal reflection effect means that at the definite angle of incidence all incident rays reflect from the surface completely. The angle of the total internal reflection (or the critical angle of incidence) is given by:          sin Qt=n2/n1 , (n2<n1)      It is clear out of the formula, that the critical angle of incidence exists at n2 < n1 only, i.e. the total internal reflection is possible if the light goes from optically more consistent material to optically less consistent one.

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