Pile of Plates

in optics, a set of flat transparent plates mounted at some angle to incident light. Such a pile is a simple polarization device. In accordance with the Fresnel equations, the transmittance and reflectance are different for the two polarization components of the incident light—that is, for the components polarized parallel and perpendicular to the plane of incidence. Natural light is therefore polarized when it passes through the pile; the component whose electric vector lies in the plane of incidence predominates. The greater the inclination of the rays to the pile, the greater the degree of polarization p. The optimal angle of incidence, however, is the Brewster angle (seeBREWSTERS LAW), at which the transmittance is at a maximum (approximately 50 percent).

For light in the visible region of the spectrum, the plates are made of optical glass. In order to reduce losses because of absorption, the plates are of very small thickness. When the refractive index n of the glass is 1.5, a pile of 16 plates yields practically complete polarization (p = 0.99). For the infrared region, piles composed of, for example, lithium-fluoride or fluorite plates with thin selenium, germanium, or silicon coatings are used. The high values of n (~2–4) of such coatings permit the required p to be obtained with a small number of plates.