A modern 200 watt sodium street lamp emits yellow light of wavelength 0.6 μm. Assuming it to be 25% efficient in converting electrical energy to light, number of photons of yellow light it emits per second is -

$1.5×10^{20}$

$\frac{n}{t}=\frac{IA}{hv}=\frac{IAλ}{hc}=\frac{Wλ}{hc}$

$=\frac{50×6×10^{-7}}{6.6×10^{-34}×3×10^8}$

$=\frac{300}{20}×10^{19}$

$=1.5×10^{20}$