Three separate estimates about photons

PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Photodetection, photonic sources and quantum optics · Photodetection, photonic sources and quantum optics

Optics and Photonics, Ch The photon

Problem

Three separate estimates about photons, each using only the numbers given. ((a)) A weak lamp delivers an irradiance of 2.0×10−42.0\times10^{-4} W m−2^{-2} to a metal surface whose work function is 2.32.3 eV. Treating the light classically, so that an atom of absorbing area 1.0×10−191.0\times10^{-19} m2^2 gathers energy continuously from the wave, estimate the time needed for one atom to accumulate enough energy to eject an electron. Compare this with the observed emission time of about 10−910^{-9} s. ((b)) A star delivers a photon flux density of 5.0×1055.0\times10^{5} photons s−1^{-1} m−2^{-2} at the entrance pupil of a telescope of diameter 200200 mm. Estimate the number of photons collected per second. ((c)) A photograph is taken under conditions in which a typical pixel collects a mean of 25002500 photons during the exposure. Estimate the fractional photon-counting noise of that pixel.

Browse the problem library