Light propagation through a moving gas

A light beam sufficiently intense to appreciably heat a gas it traverses will be deflected on account of the heat-induced gradients in the index of refraction. We compute these effects (bending into the wind, and self-defocusing due to nonuniformities in the beam) for a nonzero wind velocity in the framework of geometrical optics. For each of the initial intensity distributions considered, a bending into the wind occurs, with accumulation of intensity in those regions in which the intensity gradient is negative. The beam becomes very sharp at a definite range, resulting in sharp bending and much detailed structure there; beyond, bending gives way to near-rectilinear propagation with uniform continuing beam spread.