fjärrdiffraktionsmönster

Fjärrdiffraktion, often translated as far-field diffraction or Fraunhofer diffraction, describes the diffraction pattern produced by a wave when the source of the wave and the observation screen are effectively infinitely far apart. This condition is practically achieved when both the source and the screen are placed in the focal planes of a lens, or by considering the limit as the distances become very large. In this regime, the rays originating from the source and passing through the aperture can be considered parallel when they reach the observation screen.

The mathematical description of far-field diffraction involves a Fourier transform of the aperture function. This means that the diffraction pattern observed on the screen is directly proportional to the Fourier transform of the shape of the object causing the diffraction. For instance, if a plane wave passes through a rectangular slit, the far-field diffraction pattern will be a sinc-squared function in one dimension. For a circular aperture, the pattern is the Airy disk.

Far-field diffraction is a fundamental concept in optics and is crucial for understanding phenomena such as the resolving power of telescopes and microscopes. It explains why even a perfect optical instrument cannot resolve details smaller than a certain limit, dictated by the wavelength of light and the instrument's aperture. Applications of far-field diffraction include optical signal processing, spectroscopy, and the design of diffractive optical elements. The principles also extend to other types of waves, such as sound waves and matter waves.