a law, set forth by G. G. Stokes in 1852, stating that the wavelength of photoluminescence is greater than the wavelength of the exciting light. Stokes’ law is not always obeyed. In many cases anti-Stokes lines, whose wavelengths are shorter than the wavelength of the exciting light, are observed in photoluminescence spectra.

Stokes’ law has a broader sphere of application in the formulation of the German physicist H. van Lommel: the maximum of a luminescence spectrum is shifted relative to the maximum of the absorption toward longer wavelengths. According to Stokes’ law, the energy of the luminescence photons is less than the energy of the photons of the exciting light (seeLUMINESCENCE).