Many public building have doors that open automatically as people approach. What is the basic concept involved in it?

Explanation

The automatic opening of the doors in a building is often operated by "electric eyes". On the door of a public bulding, a light beam from a strong source of light shines across the door opening on to a photo voltaic cell. It causes the ejection of electrons, resulting in photoelectric current in the circuit which keeps the door closed. When some person approaches the door for entering into the building, the light beams is prevented from falling on photovotaic cell. Due to it, the current stops in the cell circuit. A mechanism is triggered that opens the door automatically.