Lifetimes of the molecules in the excited states are often measured by using pulsed radiation source of duration nearly in the nano second range

Q- 2.49: Lifetimes of the molecules in the excited states are often measured by using pulsed radiation source of duration nearly in the nano second range. If the radiation source has the duration of 2 ns and the number of photons emitted during the pulse source is 2.5 × 10¹⁵, calculate the energy of the source.
Answer
Frequency of radiation (ν),

ν = 5.0 × 10⁸ s^-1
Energy (E) of source = Nhν
Where,
N = number of photons emitted
h = Planck’s constant
ν = frequency of radiation
Substituting the values in the given expression of (E): E = (2.5 × 10¹⁵) (6.626 × 10^-34 Js) (5.0 × 10⁸ s^-1) E = 8.282 × 10^-10 J
Hence, the energy of the source (E) is 8.282 × 10^-10 J.