Coherent forward scattering of starlight by a cloud of atomic hydrogen

Theory predicts that a plane wave scattered by a thin slab of gas yields, in the forward direction and under specific circumstances, a larger irradiance than would be observed in the absence of the gas. This enhanced Rayleigh scattering depends on the size of the Fresnel zones at the slab location, as seen from the observer's position, and results from the coherence of the scattering. On astronomical scales the exceptional size of Fresnel zones (~1500km) has particular relevance when considering forward-scattered starlight by an interstellar cloud of atomic hydrogen.