Explanation

(b) From Einstein’s photoelectric effect concept the energy of these photons, for light of frequency v is $E = hv$
where $h$ is Planck’s constant.
Also, frequency = $\frac{velocity}{wavelength\ } = \frac{c}{\lambda}$
$\therefore$ = $\frac{hc}{\lambda}$
Energy of n photons is $E = \frac{nhc}{\lambda}$
Given, $E = 10^{- 7}J,\ \lambda = \ 5000\ \mathring{\mathrm{A}}$
$= \ 5000 \times 10^{- 10}\ m$
$\Rightarrow \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ n = \frac{E\lambda}{hc}$
$= \ \frac{10^{- 7} \times 5000 \times 10^{- 10}}{6.6 \times 10^{- 34} \times 3 \times 10^{8}}$
$= \ 2.5 \times 10^{11}$