Question

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

• A. 2.715 x 10¹⁹
• B. 4.715 x 10¹⁹
• C. 5.615 x 10¹⁹
• D. 7 x 10¹⁹

Answer 1

The correct option is A

2.715 x 10¹⁹

Always keep in mind.

An "X" Watt bulb emits "X" Joules of energy per second.Therefore,

A 150 W bulb emits 150 J of energy per second.

But only 8% of the total energy is emitted as light (given). Thus energy emitted by the bulb (per second) as light is:

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

Let the bulb emit n photons per second.

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

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

= $\frac{12J\times 4500\times {10}^{-10}m}{3\times {10}^{8}m{s}^{-1}\times 6.63\times {10}^{-34}Js}$

= 2.715 $\times$ ${10}^{19}$