The photocell is a device that converts light energy into electrical energy. It works on the principle of the emission of electrons by incident radiation. When light falls on the surface of a photocell, it knocks electrons out of the surface of the material. These electrons can be collected as a current if the photocell is connected to an external circuit. The current generated by the photocell can be used for various applications, including light sensors, solar panels, and even in some medical devices. The other options, such as voltaic cell, emission of protons by incident electrons, and photographic plate, are not directly related to the operation of a photocell.