Question

A bulb emits light of wavelength $4500\mathring{A}$, 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

Electrical energy in joules = Power in watts $\times$ Time in seconds

Thus $150$ watt = $150$ joules of energy emitted per second.

$\therefore$ Energy emitted as light

$=\frac{8}{100}\times 150=12J$

$E=nh\nu =nh\frac{c}{\lambda }$

$\therefore$ $n=\frac{E\times \lambda }{h\times c}$

$=\frac{\left(12J\right)\times (4500\times {10}^{-10}m)}{(6.626\times {10}^{-34}Js)\times (3\times {10}^{8}m{s}^{-1})}$

$=2.717\times {10}^{19}$