Photoconductive cells alter the resistance of an electrical circuit in relation to the amount of light hitting the cell. This is why photoconductive cells are sometimes called photoresistive devices. Photoconductive cells are frequently used in applications such as street lamps, which come on as the ambient light levels drop below a certain threshold.
Visible light is a form of electromagnetic radiation. As electromagnetic radiation carries energy and light is a form of electromagnetic radiation, light carries energy and can be used to cause an electrical current. This is called the photoelectric effect. When photons of light land on a conductive surface, they deliver their energy to the surface, which causes the atoms to lose electrons and generate electrical current. This phenomenon was first discovered by Heinrich Hertz in 1887, but it was not fully understood until Albert Einstein concluded his research on the phenomenon in 1905. In 1921, Einstein was awarded the Nobel Prize for his efforts to describe the photoelectric effect.
Photoconductive cells feature a lens on their top surface, which works to concentrate the light where it is needed. Under the lens, a small piece of light-receptive calcium sulfide or similar substance bears electrical connectors, which facilitate placing the cell in a circuit. When the light level reaching the cell rises, the resistance in the cell drops, which allows current to flow through the circuit.