Optical thickness

The optical depth is a dimensionless quantity, generally noted τ, that measures how absorbing – or  scattering – a slab in a medium is. Depending on whether τ < 1 or τ > 1, a slab of medium is said to be optically thin or optically thick. In a medium of constant density, the optical depth is proportional to the thickness of the slab.

In an absorbing medium, the intensity of inward radiation decreases as exp(−τ). In an emitting medium, such as a stellar photosphere, or a planetary atmosphere in the infrared, the outward radiation is well approximated by assuming that it comes entirely from the layer of optical depth τ = 1.

 Extinction, Interstellar or Atmospheric

 Opacity

 Radiative Transfer

 Scattering