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Einstein and The Photoelectric Effect

When light strikes a suitable metal surface, electrons are sometimes emitted from the surface. This phenomenon is known as photoelectric effect. The electrons thus emitted are called photoelectrons and the current that arises due to the flow of photoelectrons is known as photoelectric current.
Experimental Study of Photoelectric Effect
Figure shows an apparatus for studying the photoelectric effect. An evacuated glass or quartz tube contains a metallic plate C that is connected to the negative terminal of the battery (cathode) and another metallic plate A that is connected to the positive terminal of the battery (anode). When the tube is kept in the dark, the ammeter reads zero, indicating the absence of current in the circuit. When plate C is illuminated by light having a particular wavelength that depends on the metal used to make C, a current is detected by the ammeter, indicating a flow of charges between C and A.
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Photoelectric Effect
This current arises from photoelectrons emitted from the negative plate and collected by the positive plate.

Experimental Results of Photoelectric Effect

Albert Einstein explained the phenomenon of photoelectric effect by using the quantum theory of light. When light is incident on a metal, there is a collision between photons and electrons. During such a collision the photon transfers all its energy ‘hγ’ to the electron. The electron uses this energy, partly to come out of the metal (W) and the rest is retained as kinetic energy Description: 39312.png.
Hence, Einstein gave a simple equation which can be written as Description: 39333.png 
  1. Photoelectric effect is instantaneous.
  2. For a given frequency of radiation and plate potential, the photoelectric current is proportional to the intensity of radiation.
  3. For a given photo-sensitive cathode, there is a particular frequency of radiation below which no photoelectric emission takes place. This minimum frequency is called the threshold frequency.
  4. As the plate potential is increased, the photoelectric current I also increases and attains a constant value called the saturation current.
  5. Stopping potential: It is the negative voltage on the collector plate at which the photoelectric current just becomes zero.

Einstein's Photoelectric Equation

Description: 39340.png
Description: 39348.png
where h is the Planck’s constant, W is the work function, γ is the frequency of the incident light and γ0 is threshold frequency.
Description: 39356.png is the maximum kinetic energy of the electron.

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