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Francesco Grimaldi (1665)[1] observed bands of light within the shadow of a rod illuminated by a small source. In other words, he observed that some light had bent around the rod into the shadow region. Grimaldi gave the name diffraction to this phenomenon. Diffraction refers to the deviation from rectilinear propagation that occurs when waves advance beyond an obstruction, Diffracted sound waves allow sound to be heard around corners. If the length of the obstruction is equal to about one wavelength, then the amount of diffraction becomes so large that the notion of a shadow becomes almost meaningless.

  1. Grimaldi, F. M., 1665, Physicomathesis de lumine, coloribus, et iride, aliisque annexis: Bologna.