A low-pressure sodium street lamp draws W of electrical power and converts of it into…
PHYSICS · Subjective · Fermi University Physics · Optics & Photonics · Quantum Mechanics · Origins of quantum theory
Quantum Mechanics: From the Photon to the Hydrogen Atom, Ch Light in lumps: blackbody radiation, the photoelectric effect and Compton scattering
Problem
A low-pressure sodium street lamp draws 90 W of electrical power and converts 30% of it into light of wavelength 589 nm, radiated equally in all directions. An observer stands 25 m from the lamp; the pupil of the eye is a circle of diameter 6.0 mm. Absorption and scattering by the air are ignored, and the lamp is treated as a point source. Estimate the number of photons entering the pupil each second. Use hc=1240 eV nm and 1 eV =1.602×10−19 J, and give the answer to one significant figure: it is an order-of-magnitude estimate.