Editing Optical depth (section)

=== Atomic physics ===
In [[atomic physics]], the spectral optical depth of a cloud of atoms can be calculated from the quantum-mechanical properties of the atoms. It is given by<math display="block">\tau_\nu = \frac{d^2 n\nu} {2\mathrm{c} \hbar \varepsilon_0 \sigma \gamma} </math>where
* ''d'' is the [[transition dipole moment]];
* ''n'' is the number of atoms;
* ''ν'' is the frequency of the beam;
* ''c'' is the [[speed of light]];
* ''ħ'' is the [[reduced Planck constant]];
* ''ε''<sub>0</sub> is the [[vacuum permittivity]];
* ''σ'' is the cross section of the beam;
* ''γ'' is the [[natural linewidth]] of the transition.