In this paper we present a theory to explain the observed lens effect produced by a photocurrent in Cds. An applied dc voltage causes local heating due to the presence of a localized photocurrent produced by a focused laser beam. The local heating causes an increase in the index of refraction and hence focusing. The theory predicts that the magnification should be linear in the power dissipated by the photocurrent and these predictions fall very near the measured values of magnification for different values of photoconductance, with no fitted parameters. In order to explain the threshold effect observed in some Cds samples the change in the index of refraction with temperature of Cds is measured over a temperature range of 20°–350° and is found to bedN/dT=1.5×10−4(°C)−1.