Diffraction is a certain set of phenomena that are observed when light waves move through a medium with pronounced inhomogeneities (small holes, opaque screens, etc.). They are associated with a deviation from the principles of geometric optics. The essence of this concept is the ability of electromagnetic radiation to go around any obstacles. Fraunhofer diffraction in optics has a much greater applied value than the Frenkel diffraction phenomena.
Due to this property, electromagnetic waves of light, enveloping obstacles, fall into the region of the so-called geometric shadow and penetrate holes of small diameter. Passing through them, the waves break up into components, which opens up wide possibilities for studying and spectral analysis of the nature of light studies. In this case, the Fraunhofer diffraction assumes that the size of the holes corresponds to the wavelength.
This phenomenon is also explained using the Huygens-Frenkel principle. It is used to solve some problems associated with research in the field of the distribution of the intensity of light radiation and their propagation in various environments with obstacles. The discovery of the diffraction phenomenon at one time served as key evidence of the wave nature of light. The first quantitative theory of this phenomenon belongs to the French physicist Frenkel. However, his postulates still have mainly theoretical significance.
And Fraunhofer diffraction has a practical implementation, in which the source of light radiation is placed in the focus of the lens. It is not required if the source is a laser, because it emits a parallel beam of radiation. Fraunhofer diffraction occurs on an obstacle placed in the path of light rays passing through the lens. The picture of this optical phenomenon is observed in the focal plane of another lens placed behind the obstacle.
But the simplest case for calculations and extremely important for practical optics is the Fraunhofer diffraction on one slit of a large length of a rectangular cross section. In this case, a monochromatic plane wave is incident on the hole. The light field behind the gap is constructed according to the Huygens principle as a result of interference of a coherent secondary wave. All waves of this type emanate from different sections of the wave front. The main feature of the diffraction pattern is that the secondary waves emitted by the wavefront strip, folding after passing through an obstacle, form one cylindrical wave, the axis of which is this strip.
The diffraction pattern in this case is an alternation of bright and dark areas and is characterized by the presence of a bright spot in the center where the geometric shadow zone is located. Characteristically, the picture will be colored for white light. In this case, the central strip will be light, and the rest will alternate in color from purple to red. Fraunhofer diffraction on the slit creates a blurry image of the light source, which is divided by dark stripes. The diffraction pattern at small holes gives a system of concentric circles with a dark Poisson spot in the center. Light bands appearing on the screen are called diffraction maxima, and dark ones are called minima.
The diffraction phenomenon is used to accurately measure light wavelengths. It is also taken into account in the design of various optical devices, since this phenomenon imposes some restrictions on their resolution.