Question

Assume that a 100 W sodium vapour lamp rediates its energy uniformly in all directions in the form of photons with an associated wavelength of 589 nm.
At what rate are photons emitted from the lamp?

Answer 1

Energy of single photon is given as,

$E=\frac{hc}{\lambda }$

Let $N$ be the number of photons emitting per second.

Thus, power $P=\frac{Nhc}{\lambda }⟹N=\frac{P\lambda }{hc}$

$⟹N=\frac{100\times 589\times {10}^{-9}}{6.63\times {10}^{-34}\times 3\times {10}^8}=2.96\times {10}^{20}photons/second$