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 bywhere
*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 Planck's constant;
*ε is the vacuum permittivity;
*σ the cross section of the beam;
*γ the natural linewidth of the transition.