$A$ $100 \, W$ bulb emits monochromatic light of wavelength $400 \, nm$. Calculate the number of photons emitted per second by the bulb.

Power of the bulb $P = 100 \, W = 100 \, J \, s^{-1}$.
Energy of one photon $E = \frac{hc}{\lambda}$.
$E = \frac{6.626 \times 10^{-34} \, J \, s \times 3 \times 10^{8} \, m \, s^{-1}}{400 \times 10^{-9} \, m} = 4.9695 \times 10^{-19} \, J$.
Number of photons emitted per second $n = \frac{P}{E}$.
$n = \frac{100 \, J \, s^{-1}}{4.9695 \times 10^{-19} \, J} \approx 2.012 \times 10^{20} \, s^{-1}$.