Question

A bulb emits light of wave length 396 nm. The bulb is rated as 100 watt and 10% of the energy is emitted as light. How many photons are emitted by bulb per second?

• A. ${10}^{18}$
• B. $2\times {10}^{19}$
• C. $2\times {10}^{18}$
• D. ${10}^{19}$

Answer 1

The correct option is B $2\times {10}^{19}$

Energy of the bulb = 100 watt
= 100 J per second
Percentage of energy emitted as light = 10%
Energy emitted as light per second = 10 J
Energy of the photon = $\frac{hc}{\lambda }$
$=\frac{6.626\times {10}^{-34}\times 3\times {10}^8}{3960\times {10}^{-10}}$
=$0.5\times {10}^{-18}$ J
Number of photons = $\frac{\text{total energy emitted per second}}{\text{energy of one photon}}=\frac{10}{0.5\times {10}^{-18}}$
= $2\times {10}^{19}$