A 200 W sodium street lamp emits yellow light of wavelength 0.6 μm. If 25% of electrical input becomes light, photons emitted per second are:
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Question

A 200 W sodium street lamp emits yellow light of wavelength 0.6 μm. If 25% of electrical input becomes light, photons emitted per second are:

G1 Physics · Accepted Answer

Optical power Pₒ = 0.25 × 200 = 50 W. Photon energy E = hc/λ = (6.626 × 10⁻³⁴ × 3 × 10⁸)/(0.6 × 10⁻⁶) ≈ 3.31 × 10⁻¹⁹ J. N = 50/(3.31 × 10⁻¹⁹) ≈ 1.51 × 10²⁰ photons/s. Answer: A.