Question

A $50$ watt bulb emits monochromatic red light of wavelength of $795 \,nm$. The number of photons emitted per second by the bulb is $x \times 10^{20}$, The value of $x$ is _____.
[Given : $h =6.63 \times 10^{-34} \,J s $ and $c -3.0 \times 10^{8}\, ms ^{-1}$ ]

Answer 1

Total energy per sec. $ = 50\,J$
$50=\frac{ n \times 6.63 \times 10^{-34} \times 3 \times 10^{8}}{795 \times 10^{-9}}$
$n =1998.49 \times 10^{17}$
$[ n =$ no. of photons per second $]$
$=1.998 \times 10^{20}$
$\simeq 2 \times 10^{20}$
$= x \times 10^{20}$
$x =2$