Explain the phenomenon of photoelectric effect on the basis of quantum theory of radiations by Max Planck.

The Quantum Theory of Radiations by Max Planck can explain the photoelectric effect very well. Max Planck says that light is made of bundles of energy known as photons, energy of each photon being equal to hγ, γ being the frequency of the light. So when a photon of frequency being equal to the threshold frequency of the metal strikes the metal surface, it imparts its whole energy to an electron and hence helps the electron to overcome the forces of attraction from the nucleus of the atom. In this way an electron is ejected out of a metal surface when a photon strikes it. If the frequency of the photon is less that γ₀ (threshold frequency), then no photoelectric effect is observed.

But if γ>γ₀ , then some of its energy (which is equal to the ionization energy) is consumed in ejecting the electron from the metal and the remaining is used to impart some kinetic energy to the ejected electron.

Hγ= Ф + (½)mv²

Here, Ф is the ionization energy, v is the velocity imparted to the ejected electron, m is the mass of the electron.

As Ф=hγ₀ , hγ= hγ₀ + (½)mv²

Rearranging,  (½)mv² = h (γ-γ₀)………….(1)

Equation (1) is called Einstein photoelectric equation.