Question

A 100 watt bulb emits monochromatic light of wavelength 400 nm. Calculate the number of photons emitted per second by the bulb.

• A. $4.969\times {10}^{20}$
• B. $2.012\times {10}^{20}$
• C. $4.969\times {10}^{18}$
• D. $2.012\times {10}^{21}$

Answer 1

The correct option is B $2.012\times {10}^{20}$

We know Power = $\frac{Energy}{Time}$
According to the given data
Power of the bulb = 100 watt
$=100J{s}^{-1}$
Energy of one photon $\left(E\right)=hv$
$=\frac{hc}{\lambda }\because \lambda \nu =c$
$=\frac{6.626\times {10}^{-34}Js\times 3\times {10}^{8}m{s}^{-1}}{400\times {10}^{-9}m}$
$=4.969\times {10}^{-19}J$
Number of photons emitted per second = $\frac{Totalenergyemittedpersecond}{Energyofonephoton}$
$=\frac{100}{4.969\times {10}^{-19}}$
$=2.012\times {10}^{20}$