A 200 W sodium street lamp emits yellow light of wavelength

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

Give that, only 25% of 200 W converter electrical energy into light of yellow colour

		hc			× N = 200 ×	25
		λ				100

Where N is the No. of photons emitted per second, h = plank’s constant, c, speed of light.

N =	200 × 25	×	λ
	100		hc

=	200 × 25 × 0.6 × 10^-6	1.5 × 10²⁰
	100 × 6.2 × 10^-34 × 3 × 10⁸