Question

A $100\text{watt}$ bulb emits monochromatic light of wavelength $792\text{nm}$. Calculate the number of photons emitted per second by the bulb.

• A. $4.0\times {10}^{20}$
• B. $3.25\times {10}^{20}$
• C. $3.75\times {10}^{20}$
• D. $3.0\times {10}^{20}$

Answer 1

The correct option is A $4.0\times {10}^{20}$

We know that,
$E=h\nu =\frac{hc}{\lambda }$
$=\frac{6.6\times {10}^{-34}\times 3\times {10}^8}{792\times {10}^{-9}}\text{J}$
$=2.5\times {10}^{-19}\text{J}$
We know that,
$100\text{watt}=100\text{J/s}$ is the total energy of all the photons emitted in one second.
$\therefore$ Number of photons emitted per second
$=\frac{100}{2.5\times {10}^{-19}}=4\times {10}^{20}$