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 × 10^{20}

(b) 3 × 10^{19}

(c) 1.5 × 10^{20}

(d) 6 × 10^{18}

**Ans: (c)**

Sol: Energy of photon , E= hc/λ

E = 6.6×10^{-34 }× 3× 10^{8}/(0.6× 10^{-6})

E = 3.3× 10^{-19} J

Efficiency , η = P_{0}/P_{I}

25/100 = P_{0}/200

P_{0} = 50 W

Number of photons emitted per sec

N = P_{0}/E = 50/3.3× 10^{-19} = 1.5× 10^{20}