Photoemissivity

 Some  materials  liberate  electrons  when  subjected  to  the  radiation  of  light.
The  liberation  of  electrons  from  a  metallic  surface  and  the  generation  of  electron-
hole  pairs  in  the  semiconductors,  when  these solids  are  subjected to  light  is  called
Photoemissive effect.

The cathode is made up of photosensitive material which acts as a photoemitter. The
anode acts as an electron collector which collects the liberated electrons. The anode
is  maintained  at  some  definite  positive  potential  with  respect  to  cathode.  When
cathode  is  subjected  to  radiation,  it  emits  the  electrons  which  possess  a  range  of
initial velocities.  These velocities vary  between zeros  to  a definite  maximum  value.
The maximum value of velocity is denoted as Vmax and given by,

When electrons move from cathode to anode, a current is established in the circuit
called photocurrent.


Some other characteristics of Photoemissivity are,

1.The  saturated  photoelectric  current  increases  as  the  intensity  of  the  light
increases. Thus the saturation photoelectric current is directly proportional to
the intensity of the light.


2.The  photoelectric  characteristics  are independent  of  the temperature  for the
wide ranges of temperature.





3.The  emission  of  the  electrons  from  the  photosensitive  surface  is  immediate
when  it  is  subjected  to  the  light.  Experimentally  the  time  required  to  emit
electron from surface after light is incident on it is less than 3 nsec, The time
lag is very small and photo devices can be fast responding devices.





4.The photoelectric devices are selective devices. Means if it is subjected to the
light of of one particular wavelength then number of electrons liberated is not
equal to that when subjected to the light of another wavelength. So number
of electrons liberated due to say red light is not equal to number of electrons
liberated due to say blue light.