A modern 200W sodium street lamp emits yellow light of wavelength 0.6mm. Assuming it to be 25% efficient …..

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

(a) 62 × 10²⁰

(b) 3 × 10¹⁹

(c) 1.5 × 10²⁰

(d) 6 × 10¹⁸

Ans: (c)

Sol: Energy of photon , E= hc/λ

E = 6.6×10^-34 × 3× 10⁸/(0.6× 10^-6)

E = 3.3× 10^-19 J

Efficiency , η = P₀/P_I

25/100 = P₀/200

P₀ = 50 W

Number of photons emitted per sec

N = P₀/E = 50/3.3× 10^-19 = 1.5× 10²⁰