A caesium vapour cell is probed by a laser tuned just to the high-frequency side of…
PHYSICS · Multi-part · Fermi University Physics · Electricity, Magnetism & Electromagnetism · Optics & Photonics · Wave propagation and light in media
Optics and Photonics, Ch Light as an electromagnetic wave in matter
Problem
A caesium vapour cell is probed by a laser tuned just to the high-frequency side of the caesium resonance near 852 nm. Right there, inside the width of the resonance, the measured refractive index is n=0.99950.
((a)) Compute the phase velocity v=c/n of the light inside the cell.
((b)) A classmate, told that fine water spray looks faintly blue when lit from behind, argues: "the sky is blue because air itself is a pale-blue substance that lets more blue through than red, the way this blue-tinted spray does." Using this blue-air account of the sky as a coloured filter, what should happen to the colour of the Sun as it sets and the path length through the air grows to many times the overhead value? State the prediction, say whether it matches what is actually seen, and give the one-line reason from Rayleigh scattering for the true behaviour.