Question

Light of wavelength K = 4000 A and intensity $ 100\text{ }W/{{m}^{2}} $ is incident On a plate of threshold frequency $ 5.5\times {{10}^{14}} $ Hz. Find the number of photons incident $ {{m}^{2}} $ per sec.

• A. $ {{10}^{21}} $
• B. $ 3.0\times {{10}^{19}} $
• C. $ 2.02\times {{10}^{20}} $
• D. $ 2.02\times {{10}^{21}} $

Answer 1

Answer: (C)

Let number of photons emitted per second be $ n $ .
Then intensity $ =100\text{ }W/{{m}^{2}} $
$ \frac{nhc}{\lambda }=100 $
$ n=\frac{100\times \lambda }{hc} $
$ =\frac{100\times 4000\times {{10}^{-10}}}{hc} $
$ =\frac{100\times 4000\times {{10}^{-10}}}{6.6\times {{10}^{-34}}\times 3\times {{10}^{8}}} $ $ =2.02\times {{10}^{20}} $