A 25 watt bulb emits monochromatic yellow light of wavelength of 0.57μm.

Solution:

Power of bulb, $P=25 \mathrm{Watt}=25 \mathrm{~J} \mathrm{~s}^{-1}$

Energy of one photon, $E=h v=\frac{h c}{\lambda}$

Substituting the values in the given expression of $E$ :

$E=\frac{\left(6.626 \times 10^{-34}\right)\left(3 \times 10^{8}\right)}{\left(0.57 \times 10^{-6}\right)}=34.87 \times 10^{-20} \mathrm{~J}$

$\left.E=34.87 \times 10^{-20}\right]$

Rate of emission of quanta per second

$=\frac{25}{34.87 \times 10^{-20}}=7.169 \times 10^{19} \mathrm{~s}^{-1}$