Question

A beam of light incident on a surface has photons each of energy $1mJ$ and intensity $25w/c{m}^2$. Find number of photons incident per second if surface area is $25c{m}^2$.

• A. $6.25\times {10}^5{s}^{-1}$
• B. $8.25\times {10}^5{s}^{-1}$
• C. $6.25\times {10}^4{s}^{-1}$
• D. $5.25\times {10}^5{s}^{-1}$

Answer 1

The correct option is A $6.25\times {10}^5{s}^{-1}$

$I\times A=n\times Energyofeachphoton$

$n=\frac{IA}{energyofeachphoton}$

$\frac{25\times 25}{1}\times {10}^{-3}$

$=6.25\times {10}^{-5}J{S}^{-1}$