Question

Light of wavelength K = 4000 A and intensity $100W/m^2$ is incident On a plate of threshold frequency $5.5\times {10}^{14}$ Hz. Find the number of photons incident $m^2$ per sec.

• A. ${10}^{21}$
• B. $3.0\times {10}^{19}$
• C. $2.02\times {10}^{20}$
• D. $2.02\times {10}^{21}$

Answer 1

Answer: (C)

Let number of photons emitted per second be $n$ .
Then intensity $=100W/m^2$
$\frac{nhc}{\lambda }=100$
$n=\frac{100\times \lambda }{hc}$
$=\frac{100\times 4000\times {10}^{-10}}{hc}$
$=\frac{100\times 4000\times {10}^{-10}}{6.6\times {10}^{-34}\times 3\times {10}^8}$ $=2.02\times {10}^{20}$