On the Spectrum of Optical Waves Propagated through the Atmosphere

It is well known from our day-to-day experience that distant objects appear to shimmer, especially on still, hot hays when relatiively large temperature and humidity gradients may exits. It is ture, however, that refractive index gradients are in the air to some degree at all times; that is, they may be present at night and also when the air is disturbed by the winds. It is not suprising therefore that the power received at some distance from an optical source fluctuates randomly and possesses a characteristic low-frequency power spectrum.