Question

A bulb emits light having $\lambda =4500\stackrel{\circ }{A}$. 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?

• A. $2.72\times {10}^{11}$
• B. $5.44\times {10}^{11}$
• C. $2.72\times {10}^{19}$
• D. $5.44\times {10}^{19}$

Answer 1

The correct option is C $2.72\times {10}^{19}$

Given,
the power of the bulb $=150\text{watt}=150{\text{J s}}^{-1}$
So, the energy released by the bulb in the form of light per second $=150\times \frac{8}{100}\text{J}=12\text{J}$
Again, the wavelength of emitted light, $\lambda =4500\stackrel{\circ }{A}$
We know,
$E=nhv=n\times \frac{hc}{\lambda }=\text{Energy emitted by bulb in the form of light}$
Thus,
$n\times \frac{hc}{4500\times {10}^{-10}}=12\text{J}$

$\Rightarrow n\times \frac{6.62\times {10}^{-34}\times 3\times {10}^8}{4500\times {10}^{-10}}=12\text{J}$

$\Rightarrow n=2.72\times {10}^{19}$