Photoelectric effect - Threshold frequency

When a metal is exposed to a beam of light electrons are ejected out of it. The phenomenon of ejection of an electron from a metal surface upon exposure to a beam of light is known as a Photoelectric effect. The phenomenon of photoelectric effect was first noticed by H.Hertz in the year 1887. Some of the conclusions of this experiment were:

photoelectric effect

  1. There is no time lag between the striking of the light beam and the ejection of electrons from the metal surface.
  1. The number of electrons ejected upon striking of light beam depends on the intensity or brightness of the light.
  1. Each and every metal have a unique value of frequency better called as the threshold frequency below which they don’t exhibit the photoelectric effect. At a frequency greater than the threshold frequency, the electrons are ejected with certain kinetic energy. The kinetic energy of these electrons is directly proportional to the frequency of the light used.

Explanation of photoelectric effect by Einstein:=

Einstein took the help of Planck’s quantum theory of electromagnetic radiation to explain the photoelectric effect. According to Plank’s quantum theory, each radiation of light can be seen as shooting particles called photons which carry a discreet amount of energy. When this photon having sufficient amount of energy strikes an electron present in an atom of the metal, energy transfer takes place between the photon and the electron during the collision which excites the electron. This excited electron is finally ejected without any time lag or delay. Thus, the energy possessed by the photon is directly proportional to the kinetic energy of the ejected electron. Hence, we can conclude that the kinetic energy of the ejected electron is proportional to the frequency of the radiation striking the surface. He gave a general equation to explain this correlation mathematically:

hν = hνo + 1/2 2


h= Plank’s constant

ν = frequency of the striking photon,

νo = threshold frequency

m= mass of electron

v= velocity of ejected electron

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Photo cell is  a device to