Question:
A 50 watt bulb emits monochromatic red light of wavelength of $795 \mathrm{~nm}$. The number of photons emitted per second by the bulb is $\mathrm{x} \times 10^{20}$. The value of $x$ is
$\left[\right.$ Given : $\mathrm{h}=6.63 \times 10^{-34} \mathrm{JS}$ and $\left.\mathrm{c}=3.0 \times 10^{8} \mathrm{~ms}^{-1}\right]$
Solution:
Total energy per sec. $=50 \mathrm{~J}$
$50=\frac{\mathrm{n} \times 6.63 \times 10^{-34} \times 3 \times 10^{8}}{795 \times 10^{-9}}$
$\mathrm{n}=1998.49 \times 10^{17}[\mathrm{n}=$ no. of photons per second $]$
$=1.998 \times 10^{20}$
$\simeq 2 \times 10^{20}$
$=x \times 10^{20}$
$x=2$
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