Question

Light of wavelength $\lambda=4000 \,\mathring{A}$ and intensity $100\, W / m ^{2}$ is incident on a plate of threshold frequency $5.5 \times 10^{14}\, Hz$. Find the number of photons incident per $m ^{2}$ 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 sec be $n$. Then, intensity
$=100\, W / m^{2}$
$\Rightarrow \frac{n h c}{\lambda}=100 $
$\Rightarrow n=\frac{100 \times \lambda}{h c} $
$=\frac{10 \times 4000 \times 10^{-10}}{6.6 \times 10^{-34} \times 3 \times 10^{8}} $
$=2.02 \times 10^{20}$