Atmospheric extinction

Atmospheric extinction attenuates starlight through absorption and scattering. It depends on wavelength and path length, and is described by an optical depth or an extinction coefficient in astronomical magnitudes per airmass.

The total extinction cross-section per unit volume of particles is a length-inverse coefficient. Its path integral is optical depth. It combines scattering and absorption and differs from the dimensionless imaginary part of a complex refractive index. For liquid droplets with size-independent $Q_{\mathrm{ext}}$, ${\beta }_{\mathrm{ext}}=3Q_{\mathrm{ext}}w/(4{\rho }_w{r}_e)$.