Question

The number of photons emitted per second by a $60\, W$ source of monochromatic light of wavelength $663\, nm$ is $ (h=6.63\times {{10}^{-34}}Js) $

• A. $ 4\times {{10}^{-20}} $
• B. $ 1.5\times {{10}^{20}} $
• C. $ 3\times {{10}^{-20}} $
• D. $ 2\times {{10}^{20}} $
• E. $ 1\times {{10}^{-20}} $

Answer 1

Answer: (D)

Energy, $ E=\frac{nhc}{\lambda } $
$ \Rightarrow $ $ 60\times 1\,Js=\frac{n\times 6.63\times {{10}^{-34}}Js\times 3\times {{10}^{8}}m}{663\times {{10}^{-9}}m} $
$ \left[ \because power=\frac{energy}{time} \right] $
$ \therefore $ $ n=\frac{60\times 1\times 663\times {{10}^{-9}}}{6.63\times {{10}^{-34}}\times 3\times {{10}^{8}}} $
$ =2\times {{10}^{20}} $