In this Letter we discuss the observation of a pattern of concentric annuli formed as a result of thermal effects upon a light beam. To our knowledge, this is the first time such patterns have been reported.
When a beam of light passes through an absorbing medium whose refractive index decreases as a function of temperature, the medium behaves as a negative lens. Gordon et al.1 observed this phenomenon by placing a cell containing an absorbing liquid between the mirrors of a helium‐neon laser. The absorption constants for several liquids with very low absorbencies have been determined using this effect.2 Rieckhoff3 measured the increase in the diameter of a light beam that passed through a thermal self‐defocusing liquid placed external to the laser inter‐ferometer. Recently Leite et al.4 demonstrated that this effect may be used as a power limiting device.