Ncert Exercises & Solutions

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

Frequency of radiation (ν),
$v = \frac{1}{2.0 x 10^{- 9} s}$
ν = 5.0 × $10^{8} s^{- 1}$
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 × $10^{15}$ ) (6.626 × $10^{- 34}$ Js) (5.0 × $10^{8} s^{- 1}$ )

E = 8.282 × $10^{- 10}$ J
Hence, the energy of the source (E) is 8.282 × $10^{- 10}$ J.

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