Gas molecules in our atmosphere have sizes (~ 10 À) much smaller than the wave- length of optical photons. As a result, bound electrons within the molecules preferentially scatter shorter wavelength photons (Rayleigh scattering). The photon cross-section o for Rayleigh scattering goes as A-4, where A is the photon wavelength. Obtain an expression for the typical travel time of a red photon (A = 8000 Ä) across a layer of atmosphere with thick- 4000 Å) has a mean-free-path length lmfp = d/3 in the same atmosphere. Express your answer using d and c, where c is the speed of light. ness d, knowing that a blue photon (A

Gas molecules in our atmosphere have sizes (~ 10 À) much smaller than the wave- length of optical photons. As a result, bound electrons within the molecules preferentially scatter shorter wavelength photons (Rayleigh scattering). The photon cross-section o for Rayleigh scattering goes as A-4, where A is the photon wavelength. Obtain an expression for the typical travel time of a red photon (A = 8000 Ä) across a layer of atmosphere with thick- 4000 Å) has a mean-free-path length lmfp = d/3 in the same atmosphere. Express your answer using d and c, where c is the speed of light. ness d, knowing that a blue photon (A