Photocell
(Photoelectric cell)
A device which converts light energy into electrical
energy is called photo- electric cell or simply photo cell. It is also known as
electric eye.
ΓΌ Photoelectric
cell works on the principle of photoelectric effect.
It
consists of an evacuated glass or quartz bulb containing cathode C (emitter) and a wire loop anode A (collector). The cathode is a
semi-cylindrical photosensitive metal plate C on which a layer of photo-
sensitive alkali metal is deposited.
The external circuit connected
to anode and cathode consists of a high tension battery and a micro ammeter () as shown in figure.
When light is incident on the
cathode, it emits photoelectrons which are attracted by the anode. The
photoelectrons constitute a small current which flows through the external circuit.
Note:
(a) The photoelectric current can be increased by filling some inert gas like
Argon into the bulb. The photo-electrons emitted by cathode ionize the gas by
collision and hence the current is increased. The positive ions move towards
the cathode while negative ions and electrons move towards the anode.
(b)
In the absence of light there will be no conduction in the photo tube.
Applications:
(1)Automatic fire alarm: When fire breaks
out, light falls on the photo cell and causes the photo emission. Photoelectrons
cause the flow of current and a bell connected to it starts ringing.
(2) Automatic burglar alarm: when a burglar
(thief) crosses the beam of U.V. light, the emission of photoelectrons stops
and bell start ringing.
(3) Automatic door opener: A person
approaching a doorway interrupt light beam which is incident on a photocell. The
change in photocurrent is used to start a motor which opens the door.
(4) To measure the intensity of light: Light
meters in Photographic cameras make use of photocells to measure the intensity
of light.
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