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By learning Physics Topics, we can gain a deeper appreciation for the natural world and our place in it.
What are the Uses of Photoelectric Cell? Why Wave Theory Failed?
Definition: Cells, designed to convert light energy to electrical energy, based on the principle of photoelectric effect, are photoelectric cells.
Photo-voltaic cell: In this cell, an emf is developed directly from photoelectric effect. This cell is literally an electric cell as it works as a source of emf without any aid of an auxiliary cell.
Description: A copper plate is taken and on one surface, a layer of cuprous oxide (Cu2O) is deposited by the method of oxidation [Fig.]. Over this layer of Cu2O, using evaporation technique, a very thin coating of gold or silver is applied. This coating is so thin that light, specially ultraviolet rays, can easily penetrate it and reach the layer of Cu2O.
Working principle: When light falls on Cu2O, photoelectrons are emitted. These electrons instantaneously spread over the gold or silver coating. As a result, this coating acheives a negative potential with respect to the copper plate, i.e., an effective emf is developed between the copper plate and the gold or silver coating. This emf sets up a current in the load resistance (RL) used in external circuit. This current is directly proportional to the intensity of incident light.
Use of photoelectronic cells:
- Automatic switch: In fire alarm, railway signals, street light, etc., automatic switches are made using photoelectric cells.
- Sound recording and reproduction: Photoelectric cells are used in sound recording and in its reproduction, on the sound track of cinema and television.
- Solar cell: Photo-voltaic cell is used to construct a solar battery. This battery is indispensable in space ships and in artificial satellites.
- Television camera: In this device, photoelectric cell is used to convert optical image into video signal for television broadcast.
- Automatic counting device: Photoelectric cell is used to make automatic counting device. The number of viewers entering into or emerging from a hall can be counted with this device.
- Automatic camera: Photoelectric cell is used in automatic camera where the controlling of picture quality depends on the intensity of light.
Failure of Wave Theory of Light
Photoelectric effect cannot be explained by the concept of wave theory of light. Following observations are in contradiction with the wave nature of light:
i) Maximum kinetic energy of photoelectrons: According to wave theory, energy carried by light waves increases with increase in intensity of light. Hence, when the material surface is illuminated with highly intense light, the kinetic energy of emitted photoelectrons will be very high and it will not have any upper limit. But from the value of stopping potential V0, we see that, kinetic energy of photoelectron cannot be more than eV0.
ii) Threshold frequency: A highly intense light wave, even of frequency less than threshold frequency f0, carry a large amount of energy. This energy is sufficient to cause photo electric emission. But no photoelectron is emitted in such cases. On the other hand, even a very low intensity light beam but of frequency greater than f0, can start photoelectric emission.
iii) Photoelectric effect is instantaneous: Energy carried by light waves incident on the surface of a substance needs a little time to be centralised in the limited space of an electron. Hence, there should be a time gap between incidence of light wave on the surface and emission of photoelectron from the surface. But photoelectric effect is instantaneous; that means there is no time gap between incidence and emission.