Thermo-optic behaviour of solids

Summary

A phenomenological theory has been put forward for the variation of the refractive indices of solids with temperature, based on the idea that the refractive index can be expressed in terms of the number of dispersion centres and the polarizabilities of these centres, which are again dependent on the dispersion frequencies. The refractive index variation can then be represented as the sum of two terms, one arising from the change in the number of dispersion centres and the other from the variation of the frequencies. Accurate data of the thermal variation of the refractive index of diamond have been obtained for the first time from — 180° C. to 450° C. for three wavelengths, 4358, 5461 and 5893 Å.U. A new method was employed for this purpose which measuresdn/dt directly and is capable of general application. When applied to diamond, the theoretical ideas bring out the interesting fact that the rate of change of the dispersion frequencies with temperature is correlated to the rate of change of the fundamental lattice frequency of the crystal.