Question

A bulb emits $\lambda =4500\stackrel{o}{A}$. The bulb is rated as 150 watts and 8 percent of the energy is emitted as light. The number of photons emitted by the bulb per second is:

• A. $27.2\times {10}^{18}$
• B. $28.9\times {10}^{18}$
• C. $31.2\times {10}^{18}$
• D. $23.5\times {10}^{18}$

Answer 1

Answer: (A)

$27.2\times {10}^{18}$

Power of bulb = $150watt=150J/s$

Only 8% of this energy is emitted as light.

So, $E=nh\nu =\frac{nhc}{\lambda }=0.08\times 150=12J/s$

$n$ is the number of photons.

$\Rightarrow n=\frac{12\times \lambda }{hc}$

$\Rightarrow n=\frac{12\times 4500\times {10}^{-10}}{6.626\times {10}^{-34}\times 3\times {10}^8}$

$\Rightarrow n=27.2\times {10}^{18}$

Hence, the correct option is $A$