Babinet's principle is a concept in wave optics that relates to the diffraction of waves, particularly light, when they encounter an obstacle or aperture. It states that the diffraction pattern created by a particular aperture is identical to the diffraction pattern produced by an obstacle of the same shape and size but with its area blocked instead of open.
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In linear scalar diffraction theory, complementary aperture transmissions and give complex fields , where is the unobstructed field under identical illumination. Away from the direct beam in Fraunhofer diffraction, their diffracted fields differ by a sign and have equal intensity. Intensities do not generally add: in particular, complementary on-axis Fresnel diffraction patterns need not have equal intensity. The ideal scalar statement does not incorporate vector boundary details at a real conducting edge.