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,
$E =n h v=n\left(\frac{h c}{\lambda}\right)$
$\Rightarrow \frac{E}{t}=\left(\frac{n}{t}\right)\left(\frac{h c}{\lambda}\right)$
$\Rightarrow 10^{-7} =\left(\frac{n}{t}\right) \frac{6.626 \times 10^{-34} \times 3 \times 10^{8}}{5000 \times 10^{-10}}$
$\Rightarrow \frac{n}{t} =\frac{5000 \times 10^{-10} \times 10^{-7}}{6.626 \times 10^{-34} \times 3 \times 10^{8}}$
$\Rightarrow \frac{n}{t} =2.5 \times 10^{11} s$