If $5\%$ of the energy supplied to an incandescent light bulb is radiated as visible light, how many visible light photons are emitted by 100 Watt bulb. Assume wavelength of all visible photons to be 5600 Å. Given $h=6.625\times {10}^{-14} \mathrm{Js}$.

Energy of one photon is

$E=\frac{\text{ hc }}{\lambda }$

$\therefore E=\frac{6.625\times {10}^{-34}\times 3\times {10}^8}{5600\times {10}^{-10}}=3.5\times {10}^{-19} \mathrm{J}$

Since a 100 W bulb supplies $100 \mathrm{J}$ of energy per second. Light energy released per second as visible photons is

$E^{\text{'}}=\frac{100\times 5}{100}=5 \mathrm{J}$

Number of photons emitted per second as visible light

$\frac{E}{E}=\frac{5}{3.5\times {10}^{-19}}=1.43\times {10}^{19}$