Elastic scattering

For optical photons elastic scattering is usually unimportant. For $\lambda =.200\mu m$ we have $\sigma$_Rayleigh≈.2b for $N$₂

or $O$₂ which gives a mean free path of $\approx 1.7$ km in air and $\approx 1$ m in quartz. An important exception to this is found in aerogel, which is used as a Cerenkov radiator for some special applications. Because of the spectral properties of this material, Rayleigh scattering is extremely strong and this limits its usefulness as a RICH radiator. At present, elastic scattering of optical photons is not simulated in GEANT.