Dependence of the refractive index on crystal thickness due to a Pekar extra light wave

Transmission spectra of CdS crystals of different thickness are calculated in the region of An=1 exciton using the Pekar theory taking into account an extra light wave. It is shown that the dependence of the refractive index variance on the thickness of superthin crystals at low temperatures, which has been observed earlier, is associated with interference of the extra light wave, to which the crystal is opaque, with the fundamental wave transmitted by the crystal.