This Demonstration shows the interference pattern produced by light diffracted through a slit. You can adjust the wavelength of the light and the slit width. The resulting phenomenon cannot be explained by geometric optics, according to which the intensity of light behind an obstacle is zero. But, using wave optics, this can be described more realistically, taking into account the diffraction of light waves. This is a classic instance of Fraunhofer diffraction, which is a limiting case of Kirchhoff diffraction theory.

According to Babinet's principle, the interference pattern produced by light diffracted from a slit and its geometric complement, a thin string or a fiber occupying the same space, are identical, except for the direct (geometric) projection of the given object. This also implies that the sum of the two intensities at a given point is identical to the square of the amplitude of plane waves emitted by the source.