Question

A photo sensitive surface is receiving light of wavelength $5000\stackrel{o}{\text{A}}$ at the rate of ${10}^{-7}\text{J/s}$. The number of photons received per second is:

• A. $2.5\times {10}^{11}$
• B. $3.0\times {10}^{32}$
• C. $2.5\times {10}^{18}$
• D. $2.5\times {10}^9$

Answer 1

The correct option is A $2.5\times {10}^{11}$

Given: $\text{Wavelength}\left(\lambda \right)=5000\stackrel{o}{\text{A}}$
$\text{Energy rate}\left(E\right)={10}^{-7}\text{J/s}$
We know that, $E=n\frac{hc}{\lambda }$
where, $h=6.626\times {10}^{-34}\text{J s}$
$c=3\times {10}^8\text{m/s}$

$\therefore n=\frac{E\lambda }{hc}$

$\Rightarrow n=\frac{{10}^{-7}\times 5000\times {10}^{-10}}{6.626\times {10}^{-34}\times 3\times {10}^8}=2.5\times {10}^{11}$