Question

A bulb emits light having $\lambda =5500\stackrel{\circ }{A}$. The bulb is rated as $100$ watt and 15% of the energy is emitted as light. How many photons are emitted by the bulb per second?

• A. $4.15\times {10}^{11}$
• B. $2.68\times {10}^{11}$
• C. $4.15\times {10}^{19}$
• D. $2.68\times {10}^{19}$

Answer 1

The correct option is C $4.15\times {10}^{19}$

Given,
Power of the bulb $=100\text{watt}=100{\text{J s}}^{-1}$
So, the energy released by the bulb in the form of light per second $=100\times \frac{15}{100}\text{J}=15\text{J}$
Again, the wavelength of emitted light, $\lambda =5500\stackrel{\circ }{A}$
We know,
$E=nhv=n\times \frac{hc}{\lambda }=\text{Energy emitted by bulb in the form of light}$
Thus,
$n\times \frac{hc}{5500\times {10}^{-10}}=15\text{J}$

$\Rightarrow n\times \frac{6.62\times {10}^{-34}\times 3\times {10}^8}{5500\times {10}^{-10}}=15\text{J}$

$\Rightarrow n=4.15\times {10}^{19}$