Planck put forward the idea that the emission of radiation is not continuous, but takes place in the form of small bundles or packets of definite amount of energy which are called ‘quanta’ or ‘photons’.

The energy associated with each photon (quantum) is hv where v is the frequency of radiation and h is Planck’s constant. Thus, emitted energies may be hv, 2hv 3hv, …, but not in between.

Suppose, the wavelength of some visible light radiation is λ = 5000 Å. According to the formula c = v λ, where c is the speed of light.

The frequency of this radiation is

V = C/ λ = 3.0 × 10⁸/ 5000 × 10^-10

= 6.0 × 10^14 per sec.

According to Planck’s hypothesis, the energy of each bundle (photon) of this radiation is hv= (6.6 x 10^-34) x (6.0 x 10^14 ) = 4.0 × 10-19 J

Hence, according to Planck’s hypothesis, the exchange of energy by radiation of wavelength 5000 A possible only in integral multiples of 4.0 × 10-19 J.