A 100 watt bulb emits monochromatic light of wavelength 400nm. Calculate the number of photons emitted per second by the bulb

Power of the bulb = 100 watt = 100 T/sec

Energy of photo $E=h\vartheta$

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

$=\frac{6.626\times {10}^{-34}1.5\times 3\times {10}^{8}n/\sec }{400\times {10}^{-9}nm}=4.969\times {10}^{-19}J$

Number of photon emitted = $\frac{100J/\sec }{4.969\times {10}^{-19}J}=2.012\times {10}^{20}\sec$