Frequency of radiation 
Energy (E) of source = Nhν
Where,
N is the no. photons emitted
h is Planck’s constant
ν denotes the frequency of the radiation
Substituting these values in the expression for (E):
Hence, the energy of the source (E) is 
Lifetimes of the molecules in the excited states are often measured by using pulsed radiation source of duration nearly in the nanosecond range. If the radiation source has the duration of 2 ns and the number of photons emitted during the pulse source is 