Focusing of light by a vertical temperature gradient in aerated water.
A laser beam of 1-mrad divergence in water focuses to a thin horizontal line when propagating down a horizontal plastic tube filled with aerated tap water of temperature less than ambient. Temperature differences between top and bottom of tube range from 4 degrees C soon after filling to 0.2 degrees C after standing several days. The position of focus varies with the time after filling. Measured values of curvature range from two to three times greater than those calculated from known variation of refractive index with temperature and the assumption that the temperature gradient is constant.
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