A photocell is a practical application of the phenomenon of photoelectric cell.
A photocell consists of an evacuated sealed glass tube containing a wire anode and a concave cathode of suitable emitting material such as Cesium (Cs) The material of cathode responds to a given frequency range.
When light of frequency greater than the threshold frequency of the cathode material falls on the cathode, photoelectrons are emitted. These electrons are collected by the anode and an electric
  current starts flowing in the external circuit. The current increases with the increase in the intensity
  of light. The current would stop, if the light does not fall on the cathode.
  A photocell can be used in any situation where beam of light falling on it is interrupted or broken by any
  To count vehicles passing a road.
  To count items running on a conveyer belt.
  To open doors automatically in a building such as banks or other commercial buildings or offices.
  To operate burglar alarms.
  To produce sound in movies.
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