A 200 W bulb emits monochromatic light of wavelength 400 nm. The number of photons emitted per second by bulb is

Power of the bulb = 200 watt =200 Js^-1

Energy of photon  E = hv = $\frac{\mathrm{hc}}{\mathrm{\lambda }}$

E = $\frac{(6.626\times {10}^{-34}\mathrm{Js})\times (3\times {10}^8{\mathrm{ms}}^{-1})}{400\times {10}^{-9}\mathrm{m}}$

E = $4.969\times {10}^{-19}\mathrm{J}$

 Number of photons emitted  $=\frac{200\mathrm{Js}}{4 .969 \times {10}^{-19} \mathrm{J} }$

                                                 = 40.249 $\times$10¹⁹

                                                 = 4.0249 $\times$10²⁰ s^-1