A photocell is a technological application of the photoelectric effect.It is a device whose electrical properties are affected by light.A photocell consists of a semi cylindrical photo sensitive metal plate(emitter) and a wire loop(collector) supported in a evacuated glass.When light of suitable wavelength falls on the emitter,photoelectrons are emitted and are drawn to the collector and photocurrent is obtained.A photocell converts a change in intensity of illumination into a change in photocurrent.This current can be used to operate control systems and in light measuring devices.in scientific work,photocells are used to measure the intensity of light.The photocells inserted in the door light electric circuits,are used as automatic door opener.They are used in a control of a counting device which records every interruption of the light beam caused by a person.So photocells help count the persons entering an auditorium provided they enter the hall one by one.In burglar alarm,ultraviolet light is continuously made to fall on a photocell installed at the doorway.A person entering the door interrupts the beam falling on the photocell.The abrupt change in photocurrent is used to start an electric bell ringing.Photocells are used in the reproduction of sound in motion pictures and in the television camera for scanning and telecasting scenes.