Question

Light of wavelength $\lambda =4000A^o$ and intensity $100W/m^2$ is incident on a plate the 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)

$2.02\times {10}^{20}$

Let number of photons emitted per second be $n.$

Then intensity $=100W/m^2$

$\frac{nhc}{\lambda }=100$

$n=\frac{100\times \lambda }{hc}$

$=\frac{100\times 4000\times {10}^{-10}}{6.6\times {10}^{-34}\times 3\times {10}^8}$

$=2.02\times {10}^{20}$