ABSTRACT
There are many theories that explain diffraction phenomena, but the simplest one is the Huygens-Fresnel theory, which is surprisingly accurate in most cases. This theory is sufficient to explain all diffraction effects appearing in optical systems, with the exception of the value of the resulting phase. If any of the two distances, the distance from the light source to the diffracting aperture or the distance from the diffracting aperture to the observing screen, or both are finite, we have the so-called Fresnel diffraction theory. If both distances are infinite, then we have a Fraunhofer diffraction configuration. If we have two close point objects, their images will also be close to each other and their diffraction images may overlap. Even if this wavefront is spherical, the image is not a point but a diffraction image with some rings around, known as the Airy disk. This diffraction image's finite size limits the resolution of perfect optical systems.
