Question

Light of wavelength $5000A^o$ falls on a sensitive surface. If the surface has received ${10}^{-7}J$ of energy, then the number of photons incident on the surface is nearly (given $h=6.6\times {10}^{-34}$ Js $c=3\times {10}^8$ m/s)

• A. $2.5\times {10}^{11}$
• B. $5.0\times {10}^{11}$
• C. $3.5\times {10}^{11}$
• D. $5.0\times {10}^{10}$

Answer 1

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


Energy= nhcλNumber of photons $N=\frac{E}{\frac{hc}{\lambda }}=\frac{E\lambda }{hc}$

where $E={10}^{-7}J$ and $\lambda =5000\times {10}^{-10}m$

$\therefore$ $N=\frac{{10}^{-7}\times 5000\times {10}^{-10}}{(6.6\times {10}^{-34})(3\times {10}^8)}=2.5\times {10}^{11}$

So, the answer is option (A).