The thermo-optic behaviour of quartz

Abstract

The temperature variation of the refractive indices of α -quartz contains a positive part, though it is completely submerged by the predominant negative part due to the thermal expansion. At the α -β transition temperature of 573° C., the structural changes represented by the α -β transformation cause a change in a direction opposite to that of the density change,i.e., an increase in the refractive indices. The rate of change of the electronic dispersion wave-lengths decreases with decreasing temperatures tending to zero at very low temperatures. The thermo-optic behaviour of quartz and vitreous silica bear a close resemblance, the difference in the two cases being due to the difference in the thermal expansions.