diffraction

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Diffraction is the deviation of waves (light, sound, water) from straight-line propagation when encountering an obstacle or opening. It becomes noticeable when the slit size is comparable to the wavelength. In the case of light, it produces an interference pattern — a set of bands of varying brightness.

History

The phenomenon was first described by Francesco Grimaldi in 1665, and explained as a wave property of light by Thomas Young in 1801.

How it works

When a wave meets the edge of an object or a slit, it bends and interferes with itself. Where the crests coincide, the light is amplified (bright band), where they don't align, it cancels out (dark band).

💡 If you look through a thin fabric at a street lamp, you can see rainbow streaks — that's also diffraction on the threads.

Diffraction is a fundamental wave phenomenon inherent to all types of waves: electromagnetic, sound, water waves. It occurs when a wave interacts with an inhomogeneity in the medium — for example, the edge of an opaque screen, a slit, an aperture, or a periodic structure. According to the Huygens-Fresnel principle, each point on the wavefront becomes a source of secondary spherical waves, and the resulting field at the observation point is the superposition of these secondary waves. The interference of secondary waves leads to a spatial redistribution of intensity. A distinction is made between Fresnel diffraction (in the near zone) and Fraunhofer diffraction (in the far zone, where parallel rays arrive). Mathematically, diffraction is described by the Fresnel-Kirchhoff integral. The phenomenon is widely used to study the structure of matter: X-ray diffraction on crystals (X-ray crystallography), electron diffraction, neutron diffraction. The resolution limit of optical instruments is also determined by diffraction.

History

The first systematic description of light diffraction was given by the Italian physicist Francesco Grimaldi in a posthumous work of 1665, introducing the term itself. The wave nature of light, capable of explaining diffraction, was championed by Christiaan Huygens (1690). In 1801, Thomas Young demonstrated interference from two slits, confirming the wave theory. Augustin-Jean Fresnel in 1818 presented a rigorous mathematical theory, solving the problem of diffraction at an edge. Later, in 1912, Max von Laue and colleagues discovered X-ray diffraction in crystals, which earned him the Nobel Prize. William Lawrence Bragg and his father William Henry Bragg developed a method for deciphering crystal structures from diffraction patterns. Joseph von Fraunhofer invented the diffraction grating and used it for spectroscopy.

How it works

Diffraction is a consequence of wave superposition. When part of the wavefront is blocked by an obstacle, secondary waves from the open areas interfere, creating a complex pattern. For periodic structures such as a diffraction grating or crystal lattice, the condition for reinforcement is described by Bragg's law: nλ = 2d sin θ, where n is the diffraction order, λ is the wavelength, d is the interplanar spacing, and θ is the grazing angle. This law underlies X-ray crystallography. For gratings with many grooves, the intensity is concentrated in narrow peaks, allowing diffraction to be used in spectrometers to measure wavelengths with high precision.

n\lambda = 2d \sin\theta

n is the diffraction order (integer), λ is the wavelength of the incident radiation, d is the interplanar spacing in the crystal lattice, θ is the grazing angle (between the incident beam and the crystal plane)

Open problems & fun facts

Accurate calculation of diffraction fields for complex three-dimensional objects without simplifications remains computationally intensive.; Development of X-ray diffraction methods for studying disordered structures (liquids, amorphous solids) with time resolution.

💡 The diffraction grating of hummingbird feathers creates an iridescent coloration that changes with viewing angle; in technology, similar structures (photonic crystals) are used in optical fibers.

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diffraction

History

How it works

History

How it works

History

How it works

Open problems & fun facts

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