Line broadening by focusing

Abstract

We point out that the spectral width of a quasimonochromatic light beam broadens when the beam is focused. A quantitative formula for this broadening is derived from classical wave theory. The effect is shown to explain some experiments on laser beams done by Panarella which he explained using the ad hoc hypothesis that the frequency of the photons changes along with the intensity of the light beam. The line broadening by focusing might also contribute to gas ionization by incident light when the ionization potential is well above the mean photon energy. Remarks are made on some direct applications of the Heisenberg relations in comparison with our treatment.