What do you mean by Fresnel and Fraunhofer class of diffraction?

Diffraction phenomena can conveniently divided into two groups.

  1. Fresnel diffraction phenomena: In the Fresnel class of diffraction, the source or the screen or the both are at finite distances from the aperture or obstacle causing diffraction. In this case, the effect at a specific point on the screen due to the exposed incident wavefront is considered and no modification is made by lenses and mirrors.
  2. Fraunhofer diffraction phenomena: In the Frauhofer class of diffraction phenomena, the source and the screen on which the pattern is observed are at infinite distances from the aperture or the obstacle causing diffraction. The incoming light is rendered parallel with a lens and the diffracted beam is focused on the screen with another lens.

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