Question

$A160$ W light source is radiating light of wavelength $6200\mathring{A}$ uniformly in all directions. The photon flux at a distance of $1.8m$ is of the order of (Planck’s con stant $=6.63\times 10^{-34}J-s$)

• A. $10^2{m}^{-2}{s}^{-1}$
• B. $10^{12}{m}^{-2}{s}^{-1}$
• C. $10^{19}{m}^{-2}{s}^{-1}$
• D. $10^{25}{m}^{-2}{s}^{-1}$

Answer 1

Answer: (C)

Photon flux $=$ Number of photons per unit area per unit time
$=\frac{I}{\left(\frac{hc}{\lambda }\right)}=\frac{I\lambda }{hc}=\frac{P\lambda }{4\pi r^{2}hc}$
$=\frac{160\times 6200\times 10^{-10}}{4\times \pi \times {\left(1.8\right)}^2\times 6.63\times 10^{-34}\times 3\times 10^8}$
$=1.22\times 10^{19}{m}^{-2}{s}^{-1}$