What effect does light have on the conductivity of photoconductive material?

The correct answer is that light increases the conductivity of photoconductive materials. Photoconductive materials are semiconductor materials that exhibit a change in their electrical conductivity when exposed to light. This phenomenon occurs because photons from the light have enough energy to excite electrons from the valence band to the conduction band within the material, creating free charge carriers (electrons and holes). As more electrons are excited by incoming light, the number of charge carriers increases, leading to enhanced electrical conductivity.

In contrast, other options present scenarios that do not align with the behavior of photoconductive materials. For instance, a decrease in conductivity when exposed to light is contrary to the fundamental principles of photoconductivity. Similarly, if conductivity were to remain the same or fluctuate, that would imply no significant interaction between light and the material's ability to conduct electricity, which is not the case for materials designed to utilize the photoconductive effect. Thus, it is the increase in conductivity under illumination that characterizes the reaction of photoconductive materials to light.