Observations on Photo Electric Effect

Physics

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Hertz experiment in production of electromagnetic waves

To test Maxwell's hypothesis, Hertz used an oscillator made of polished brass knobs, each connected to an induction coil and separated by a tiny gap over which sparks could leap. To confirm this, Hertz made a simple receiver of looped wire,its ends separated by a tiny gap. The receiver was placed several yards from the oscillator.  Hertz reasoned that, if Maxwell's predictions were correct, electromagnetic waves would be transmitted during each series of sparks,that would induce a current in the loop that would send sparks across the gap. This occurred when Hertz turned on the oscillator, producing the first transmission and reception of electromagnetic waves. Hertz also noted that electrical conductors reflect the waves and that they can be focused by concave reflectors. He found that nonconductors allow most of the waves to pass through.

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Hertz's observation

The phenomenon of photoelectric emission was discovered in 1887 by Heinrich Hertz, during electromagnetic wave experiments.He observed that when light falls on a metal surface, some electrons near the surface absorb enough energy from the incident radiation to overcome the attraction of the positive ions in the material of the surface.

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Lenard's experiment

Wilhelm Hallwachs and Philipp Lenard investigated the phenomenon of photoelectric emission in detail during 1886-1902.
Their observations can be summarised as below:
  • When light is incident on a metal plate, it may cause the electrons to come out of the metal.
  • Hallwachs and Lenard also observed that when the light fell on the emitter plate, no electrons were emitted at all when the frequency of the incident light was smaller than a certain minimum value. This frequency is called threshold frequency.
  • He also observed that different metals have different threshold frequencies.

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Experimental study of photoelectric effect

Let us study an experimental setup as shown in the figure. Here a photosensitive C plate is irradiated with light of particular frequency. The plate A is maintained at some potential difference with plate C. Let us study the effects of different factors such as: 1) intensity of light 2) frequency of light 3) potential difference between the plates

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Effect of intensity of light on photocurrent

let us say that the plate A is given a positive potential with respect to the plate C. When the light is incident on the plate C, electrons will be attracted to plate A. More the intensity of the light, more will be the electrons ejected and more will be the photocurrent.

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Effect of potential on photoelectric current

  • More the potential difference between the plate C and A is increased, the phoelectric current will also increase.
  • At some stage, all the photoelectrons ejected by the metal will be accumulated by the collector plate and a saturation in current occurs. This current is called saturation current. After this stage the photocurrent does not increase no matter how much intensity of light is used.
  • If we increase the intensity of the light the saturation current also increases.Let initial intensity be and now it is increased to .Hence the saturation current of light with intensity is more than that of
  • We now apply a negative (retarding) potential to the plate A with respect to the plate C and make it increasingly negative gradually.  When the polarity is reversed, the electrons are repelled and only the most energetic electrons are able to reach the collector A.  The photocurrent is found to decrease rapidly until it drops to zero at a certain sharply defined, critical value of the negative potential on the plate A. For a particular frequency of incident radiation, the minimum negative (retarding) potential given to the plate A for which the photocurrent stops or becomes zero is called the cut-off or stopping potential.
 

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Experimental Study of Photoelectric Effect

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