A bulb is labelled as 150 watt, and only 8% of the energy is emitted as light of wave length 4500A°. The no.of photons emitted by the bulb in one second is

Total energy of light radiation emitting large number of photons = nhc/λ
where,
n=number of photons emitted per sec
Energy of each photon of wavelength λ=4500 A^o,
$\left(\mathrm{E}\right)=\frac{\mathrm{hc}}{\mathrm{\lambda }}=\frac{6\times {10}^{-34}\times 3\times {10}^8}{4500\times {10}^{-10}}=4\times {10}^{-19} \mathrm{J}$
Energy emitted by 150 watt with emission of light = 8 %=     $\frac{150\times 8}{100}=12 \mathrm{W}=12 \mathrm{J}/\mathrm{s}$               
But 1 watt=1 J/sec
Let n be the number of photons of light emitted.
n × energy of a photon = Total energy emitted by bulb as light radiation
          n×4×10⁻¹⁹=12
Number of photons emitted/second=n=$\frac{12}{4\times {10}^{-19}}=3\times {10}^{19}$