A 25 watt bulb emits monochromatic yellow light of wavelength 0.57μm. Calculate the rate of emission of quanta per second.

The energy of one photon is
E=hc/λ=6.626×10⁻³⁴Js×3×10⁸m/s/0.57×10⁻⁶m=3.487×10⁻¹⁹J.
25 watt corresponds to 25 J per second.
The number of photons emitted per second is
25/3.487×10⁻¹⁹=7.17×10¹⁹/s.