PHOTO ELECTRIC EFFECT
DEFINITION
 
When ultraviolet light falls on certain metals, electrons are emitted. This phenomenon in which certain metals emit electrons when exposed to light of suitable frequency, is called PHOTO ELECTRIC EFFECT.
In short, ejection of electrons by means of light is called 'photo electric effect'.
EXPERIMENTAL ARRANGEMENT
TO OBSERVE
PHOTOELECTRIC EFFECT
 
If suitable light is allowed to fall on plate 'P', it will give out photo electrons as shown in the figure. The photo electrons are attracted by the collector 'C' connected to the +ve terminal of a battery. The glass tube is evacuated. When the collector 'C' is kept at +ve potential, the photo electrons are attracted by it and a current flows in the circuit which is indicated by the galvanometer.
THRESHOLD FREQUENCY
 
Threshold frequency is defined as the minimum frequency of incident light which can cause photo electric emission i.e. this frequency is just able to eject electrons with out giving them additional energy.
It is denoted by .
 
WORK FUNCTION
 
  Minimum amount of energy which is necessary to start photo electric emission is called Work Function. If the amount of energy of incident radiation is less than the work function of metal, no photo electrons are emitted.
It is denoted by . Work function of a material is given by .
It is a property of material. Different materials have different values of work function. Generally, elements with low I.P values have low work function such as Li, Na, K, Rb, and Cs.
 
 
STOPPING POTENTIAL
 
  The negative potential of the plate 'C' at which the photo electric current becomes zero is called stopping potential or cut-off potential. Stopping potential is that value of retarding potential difference between two plates which is just sufficient to halt the most energetic photo electrons emitted.
It is denoted by "Vo".
 
DEPENDENCE OF PHOTO ELECTRONS
 
  The number of photo electrons depends upon:
The nature of material
The frequency of incident radiation
The intensity of incident radiation
Potential difference b/w the electrons
 
EFFECT OF FREQUENCY
 
  The graph shows that a threshold frequency '' is a minimum frequency below which no electrons escape from the metal surface. The photo electric effect occurs above the frequency while ceases below this frequency.
 
 
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