The Photoelectric Effect

It is observed that when a metal surface is irradiated with electromagnetic waves of certain frequencies, electron are ejected from the surface. The ejection of these electrons is observed in the form of a current. This is called photoelectric effect. This was one of the factors that led to the development of quantum physics.

Several experimental modifications lead to the following results :
  1. Variation of frequency - It is observed that emission of electrons i.e. photoelectric effect takes place only if the frequency of the incident electromagnetic wave is higher than a particular magnitude of frequency. If the frequency is lower than that threshold value, photoelectric effect is not observed no matter how large the intensity of the light or the time for which the metal is irradiated.

  2. Application of opposing voltage - It is observed that if an opposing voltage is applied to the electrons coming out of the metal surface, the current decreases and at a particular voltage, vanishes.

    Moreover, this particular voltage is the same for a metal for a given frequency no matter what the intensity of light. However, the value of this voltage increases if the frequency is increased.

    Now since by applying an opposing voltage, we are actually reducing the kinetic energy of the electrons, the above observations show that it is the frequency of the incident light, and not the intensity, which determines the maximum kinetic energy of the electrons. This result cannot be explained by the earlier electromagnetic wave theory of light.

  3. Variation of Intensity - It is observed that by applying a higher intensity of incident light, the number of electrons ejected is increased. (Shown by an increased value of current.) However, as mentioned above, the maximum kinetic energy of electrons does not change. The electromagnetic wave model would predict an increase in the kinetic energy of the electrons with an increase in the intensity of light, and hence cannot explain this observation.

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