Class 12 Physics Dual Nature Radiation Matter Experimental Observations of Photoelectric Effect

Experimental Observations of Photoelectric Effect:

Variation of photoelectric current with intensity of radiation absorbed:

  • When the values of photoelectric current were plotted against the different values for intensity of light, it was observed to be a straight line passing through the origin.
  • It proved that Photoelectric current, which is number of photoelectrons flowing per unit time, is directly proportional to the intensity of incident light

Variation of photoelectric current with the potential applied:

Case-1-When collector plate was kept at higher potential (accelerating potential) with respect to the emitter.

  • As the positive potential of collector rises, photoelectric current rises for a certain period of time because the electrons emitted experience a strong attractive force by the collector
  • On further increasing the positive potential, photoelectric current reaches a maximum value, beyond which it remains fixed even when positive potential is increased. This is because number of free electrons in a metal surface is always fixed.
  • This maximum value of photoelectric current beyond which it remains fixed, no matter how high the positive potential gets, is called the saturation current.

Case-2-When collector plate was kept at lower potential (retarding potential) with respect to the emitter.

  • As the negative potential of collector rises, photoelectric current falls. This is because the electrons emitted will experience a strong repulsive force from the collector
  • As the retarding potential is further increased, even the electrons having maximum kinetic energy will be repelled by the repulsive force of the collector, and hence, the photoelectric current becomes zero
  • This corresponding minimum value of retarding (negative) potential for which the photoelectric current becomes zero is called stopping potential, or cut-off potential. It is denoted by (V0).
  • Mathematically, stopping potential could be expressed as 

Case-3-When the variation of photoelectric current was plotted against the potential for 3 different values of intensity of incident light (keeping the frequency constant).

  • It was observed that stopping potential was constant for all the values of intensity. Thus, stopping potential is independent on the intensity of incident light.
  • On the other hand, saturation current got higher for higher values of intensity of absorbed light

Variation of photoelectric current with the frequency of incident light:

  • The variation of photoelectric current was plotted against the potential for 3 different values of frequency of incident light (keeping the intensity constant)
  • It was observed that saturation current was constant for all the values of frequency. Thus, saturation current is independent on the frequency of incident light.
  • However, stopping potential got higher in the negative direction for higher values of frequency of absorbed light

  • It was also found that, after reaching a certain minimum frequency value, stopping potential followed a linear relationship with the frequency of incident light.
  • This minimum value of frequency of incident light required for photoelectric emission to take place was called as threshold frequency
  • The value of threshold frequency is fixed for a specific material, and it changes from one material to another

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