Experimental Study of Photoelectric Effect

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12th physics. NCERT notes

We can use the experimental arrangement to study the variation of photocurrent with

  1. Intensity of radiation,
  2. Frequency of incident radiation,
  3. The potential difference between the plates A and C,
  4. The nature of the material of plate C.

Light of different frequencies can be used by putting appropriate colored filter or colored glass in the path of light falling on the emitter C. The intensity of light is varied by changing the distance of the light source from the emitter.

Effect of intensity of light on photocurrent:

The photocurrent is directly proportional to the number of photoelectrons emitted per second. This implies that the number of photoelectrons emitted per second is directly proportional to the intensity of incident radiation. It is found that the photocurrent increases linearly with intensity of incident light as shown graphically in Fig.

Effect of potential on photoelectric current:

It is found that the photoelectric current increases with increase in accelerating (positive) potential. At some stage, for a certain positive potential of plate A, all the emitted electrons are collected by the plate A and the photoelectric current becomes maximum or saturates. If we increase the accelerating potential of plate A further, the photocurrent does not increase. This maximum value of the photoelectric current is called saturation current. Saturation current corresponds to the case when all the photoelectrons emitted by the emitter plate C reach the collector plate A.

For a particular frequency of incident radiation, the minimum negative (retarding) potential V0 given to the plate A for which the photocurrent stops or becomes zero is called the cut-off or stopping potential.

This shows that more electrons are being emitted per second, proportional to the intensity of incident radiation. But the stopping potential remains the same as that for the incident radiation of intensity I1, as shown graphically in Fig.

In other words, the maximum kinetic energy of photoelectrons depends on the light source and the emitter plate material, but is independent of intensity of incident radiation.

Effect of frequency of incident radiation on stopping potential:

The resulting variation is shown in Fig. We obtain different values of stopping potential but the same value of the saturation current for incident radiation of different frequencies. The energy of the emitted electrons depends on the frequency of the incident radiations. The stopping potential is more negative for higher frequencies of incident radiation.

This implies that greater the frequency of incident light, greater is the maximum kinetic energy of the photoelectrons. Consequently, we need greater retarding potential to stop them completely.

If we plot a graph between the frequency of incident radiation and the corresponding stopping potential for different metals we get a straight line, as shown in (fig. below)

The graph shows that:

  1. The stopping potential V0 varies linearly with the frequency of incident radiation for a given photosensitive material.
  2. There exists a certain minimum cut-off frequency ν0 for which the stopping potential is zero.

These observations have two implications:

  1. The maximum kinetic energy of the photoelectrons varies linearly with the frequency of incident radiation, but is independent of its intensity.
  2. For a frequency ν of incident radiation, lower than the cut-off frequency ν0, no photoelectric emission is possible even if the intensity is large.
    This minimum, cut-off frequency ν0, is called the threshold frequency. It is different for different metals

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