Atmospheric Transmission Beer's Law

3/31/10Optical depthThe optical depth expresses the quantity of light removed from a beam by absorptionor scattering during its path through a medium.If τ = 0.5, transmissivity = ~60%, if τ = 1, t = ~37% and if τ = 2.5, t = ~8%.If τ > 1, the medium is optically thickOptical depthIf a gas is optically thick, a photon will interact many,many times with particles before it finally escapesfrom the cloud. Any photon entering the cloud willhave its direction changed many times by scattering –so its ‘output’ direction has nothing to do with its‘input’ direction. We can't see anything at all throughthe cloud: it is opaque.You can't see through an optically thick medium; youcan only see light emitted by the very outermostlayers. But there is one convenient feature of opticallythick materials: the spectrum of the light they emit is ablackbody spectrum, or very close to it.9