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  4. The number of photons falling per second on a completely darkened plate to produce a force of 6.62 × 10-5 N is ‘n' If I the wavelength of the light falling is 5 × 10-7 m, then n = .... × 1022. (h = 6.62 × 10- 34 J-s)

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Q. The number of photons falling per second on a completely darkened plate to produce a force of $6.62 \times 10^{-5} \, N$ is $‘n'$ If I the wavelength of the light falling is $5 \times 10^{-7} \, m,$ then $n = .... \times 10^{22}$. (h = $6.62 \times 10^{- 34} $ J-s)

KCETKCET 2018Dual Nature of Radiation and Matter

Solution:

$P = \frac{n}{t} . \frac{hc}{\lambda}$
$ FC = \frac{n}{t} . \frac{hC}{\lambda} $
$ \frac{n}{t} = \frac{F \lambda}{h}$
$ \frac{n}{t} = \frac{6.62 \times10^{-5} \times5 \times10^{-7}}{6.62 \times10^{-34}} $
$ = 5\times10^{22}$