Photo Electric Emission

Electron emission from a metallic surface by the application of light is known as photo
electric emission.

When a beam of light strikes the surface of certain metals (e.g. potassium,
sodium, cesium), the energy of photons of light is transfer to the free electrons within
the metal.

If the energy of the striking photons is greater than the work function of the
metal then free electrons will be knock out from the surface of the metal. The emitted
electrons are know as photo electrons and the phenomenon is know as photoelectric
emission. The amount of photoelectric emission depends upon the intensity of light
falling upon the emitter and frequency of radiations. The greater the intensity and
frequency of radiations, the greater is the photo electric emission. Photo-electric
emission is utilize in photo tubes which form the basis of television and sound films.

Fig. 2.6 illustrates the phenomenon of photoelectric emission. The emitter E and
anode A are enclose in an evacuate glass envelope G. A battery B maintains the
anode at positive potential w.r.t, emitter.

When light of suitable intensity and frequency
falls on the emitter, electrons are eject from its surface. These electrons are attract
by the positive anode to constitute current in the circuit. It may be note that current will
exist in the circuit so long as illumination is maintain.
(An interesting aspect of secondary emission is that a high-speed bombarding electron
may liberate as many as 10 “secondary electrons”. This amounts to a multiplication of
electron flow by a ratio as great as 10 and is utilize in current multiplier devices.)

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