Question

A photosensitive surface is receiving light of wavelength $5000\mathring{A},$ at the rate of $10^{-7}J/s$ The number of photons ejected per second is

• A. $2.5\times 10^{12}$
• B. $2.5\times 10^{11}$
• C. $2.5\times 10^{10}$
• D. $2.5\times 10^9$

Answer 1

Answer: (B)

According to Planck's quantisation law,
$E=nhv=n\left(\frac{hc}{\lambda }\right)$
$\Rightarrow \frac{E}{t}=\left(\frac{n}{t}\right)\left(\frac{hc}{\lambda }\right)$
$\Rightarrow 10^{-7}=\left(\frac{n}{t}\right)\frac{6.626\times 10^{-34}\times 3\times 10^8}{5000\times 10^{-10}}$
$\Rightarrow \frac{n}{t}=\frac{5000\times 10^{-10}\times 10^{-7}}{6.626\times 10^{-34}\times 3\times 10^8}$
$\Rightarrow \frac{n}{t}=2.5\times 10^{11}s$