613. What is a photoconductor? — MN, Chicago, IL
A photoconductor is a material that behaves as an electric insulator in the dark but becomes an electric conductor when exposed to light. An insulator is unable to transport electric charges because its own electrons can't respond to modest electric forces. Because of quantum physics, electrons can only follow specific paths called "levels" as they move through a material and all of the easily accessible levels in an insulator are completely filled. For reasons of symmetry, there are always as many electrons traveling to the right in an insulator as are traveling to the left, so that on average, no electrons move anywhere, even when they are exposed to electric forces. But when light energy shifts some of the electrons from the filled levels to a collection of formerly unoccupied levels that previously weren't accessible, these shifted electrons can respond to electric forces and transport electric charge through the material. In the light, a photoconductor stops acting as an insulator and starts acting as a conductor. Such photoconductors are the basis for xerographic copiers and laser printers.