Question

A bulb emits light of wavelength $\lambda =4500\mathring{A}$. The bulb is rated as $150$ watt and $8$% of the energy is emitted as light. How many photons are emitted by the bulb per second?

Answer 1

Energy emitted per second by the bulb $=150\times \frac{8}{100}J$
Energy of $1$ photon $=\frac{hc}{\lambda }=\frac{6.626\times {10}^{-34}\times 3\times {10}^8}{4500\times {10}^{-10}}$
$=4.42\times {10}^{-19}$ joule
Let $n$ photons be evolved per second.
$\therefore n\times 4.42\times {10}^{-19}=150\times \frac{8}{100}$
$n=27.2\times {10}^{18}$.