text: 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.