explain Fresnel distance??????????????????

fresnel distance is defined as the minimum distance which a light wave has to travel  in a straight line before it can show diffraction

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In optics and radio com munications (indeed, in any situation involving the radiation of waves, which includes electrodynamics, acoustics, and gravitational radiation), a Fresnel zone, named for physicist Augustin -Jean Fresnel, is one of a (theoretically infinite) number of concentric ellipsoids which define volumes in the radiation pattern of a (usually) circular aperture. Fresnel zones result from diffraction by the circular aperture.[1]

The cross section of the first (innermost) Fresnel zone is circular. Subsequent Fresnel zones are annular (doughnut-shaped) in cross section, and concentric with the first.

 

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