Question

A 150 watt bulb emits light of wavelength 6600 $mathringA$ and only 8% of the energy is emitted as light. How many photons are emitted by the bulb per second?

• A. $4\times 10^{19}$
• B. $3.24\times 10^{19}$
• C. $4.23\times 10^{20}$
• D. $3\times 10^{20}$

Answer 1

Answer: (A)

Power of the bulb = $150watt=150J{s}^{-1}$
As only 8% of the energy is emitted as light so, the total energy emitted per second.
$=\frac{150J\times 8}{100}=12J$
Energy of one photon, $E=hv=\frac{hc}{\lambda }$
$=\frac{\left(6.626\times 10^{-34}Js\right)\times \left(3\times 10^{8}m{s}^{-1}\right)}{6600\times 10^{-10}m}$
$=3.0118\times 10^{-19}J$
$\therefore$ Number of photons emitted
$=\frac{12J}{3.0118\times 10^{-19}J}=3.98\times 10^{19}\approx 4.0\times 10^{19}$