Can Someone Explain The Concept Of Photocell?

A photocell uses the concept of the photoelectric effect. It manly consists of an evacuated glass bulb with a thin anode rod and a cathode of an appropriate metal surface. The material chosen for the cathode should be such that it suits to the frequency range of incident radiation over which the cell is operated. For example sodium or potassium cathode emits electrons foe visible light, caesium coated oxidized silver emits electrons for infra-red light and some other metals respond to ultraviolet light.
Now when a photo-emissive surface is exposed to appropriate light, electrons are emitted and a current flows in the external circuit which increases with the increase in light intensity. The flow of current stops when the light beam is interrupted.

There are various applications of photocell in most of which are a part of out daily lives. They are as follows:
Security systems deployed in building utilize the concept of photocell. As the light beam is interrupted it sounds an alarm.
Counting system
Automatic door system
Automatic street lighting
Exposure meter for photography
Sound track for movies

To get a better understanding of how a photocell works, please do go through photoelectric effect. Clear concepts of photoelectric effect would definitely help.