The diffraction pattern occurs as a result of the interference of secondary light waves when the light beams around obstacles or their passage through multiple openings.

The idea of ​​the wave nature of light (cm. Spectrum of electromagnetic radiation) received serious confirmation as a result of the discovery and study at the beginning of the 19th century of the phenomena interference and diffraction Sveta. Traditional since Newton’s time and because of his indisputable authority, the idea of ​​light as a stream of particles, which has long remained unchanged, is the so-called corpuscular theory of light – turned out to be seriously questioned after the discovery of interference. And soon the corpuscular theory was completely forgotten – for almost a whole century – as a result of the discovery and study of the phenomena of diffraction, as a result of which the wave theory of light became a new orthodox and unshakable idea of ​​it. Only after the explanation from the corpuscular point of view of the photoelectric effect and the inception of quantum mechanics, the corpuscular concepts of light received a second birth within the framework of the complementarity principle.

The fundamentals of the phenomenon of diffraction can be understood if we turn to the Huygens principle, according to which each point along the path of propagation of a light beam can be considered as a new independent source secondary waves, and the further diffraction pattern turns out to be due to the interference of these secondary waves. When a light wave interacts with an obstacle, some of the secondary Huygens waves are blocked. For example, when a light wave falls from above at an acute angle onto the razor, secondary Huygens waves will be formed on the upper plane of the razor, but not on the lower one. However, as a result of constructive interference, the secondary waves will still go around the razor, and we will see a continuous light beam there, as if nothing stood in the way of its propagation. A similar wave “bending” around an obstacle can be observed in the seaport during a storm: ships anchored behind a breakwater, which, it would seem, should completely damp the waves, nevertheless “walk” up and down due to secondary waves.

If the light source and the observation point are at a small distance from the obstacle, the initial and resulting light rays are not parallel to each other – and we observe Fresnel diffraction (near-field diffraction)… If the source and the observation point are at a considerable distance from the obstacle (diffraction point), the rays are practically parallel, and we observe Fraunhofer diffraction (far-field diffraction). Fraunhofer, incidentally, invented a number of important precision optical instruments, including diffraction grating. It is a system of microscopic lines located at a small distance from each other, reflecting light. It was originally a darkened glass plate with carefully applied parallel strokes. Each such stroke reflects light, and it can be considered a secondary source of Huygens waves, which interfere and mutually amplify at certain angles after being scattered by the grating.

Since the middle of the 19th century, the diffraction grating has become the most important instrument of spectroscopy – with its help, scientists study the emission spectra of luminous objects and the absorption spectra of various substances and determine their chemical composition from them. One of Fraunhofer’s most important discoveries was the detection of dark lines in the solar spectrum. Today we know that they arise as a result of the absorption of light waves of a certain length by the relatively cold matter of the solar corona, and thanks to this we can judge the chemical composition of our star.

See also:
Light reflection law
Brewster’s law
Rayleigh criterion
Bragg’s law
Joseph von Fraunhofer, 1787-1826

German physicist and optician, born in Straubing, son of a glass-blowing artisan. Orphaned early, he became an apprentice to a glazier. At the age of 14, he was seriously injured in the collapse of a new building, spent several days under the rubble and thanks to this accident gained some popularity. In particular, he received monetary compensation from the Bavarian authorities, for which he opened his own glass business, with which in 1806 he joined the famous Bavarian company Utzscheider, which in those days enjoyed the fame of the manufacturer of the world’s best optical instruments. Fraunhofer investigated the phenomenon of diffraction from a purely applied point of view: his life’s work was the invention of ideal achromatic lenses that would not give a rainbow halo around the image.

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