# Scattering and diffraction

Corresponding Wikipedia article: Scattering, Diffraction

The elastic scattering is the particles interaction without varying of their momentum, but with change in their direction. It occurs in the interaction of the lightweight particles with the heavy particles or with a particle system. For example: photons and matter, particles and crystals.

The Rayleigh scattering of photons occurs at the wavelengths, which are insufficient to absorb their energy by the atoms. For example, only the violet light is absorbed by the atmosphere, but the blue and green light is scattered, so the daytime sky is blue, and the sun is yellow or orange.

The reflection occurs at a surface of the interface between mediums. According to the law of conservation of momentum, the polarization direction or the phase of the fully reflected wave is opposite to the incident wave.

The diffraction in solids occurs on a diffraction grating, and the frequency properties are determined by its period. The diffractive scattering decreases with increase in frequency, when the wavelength becomes comparable to the grating period.

The diffraction of material particles. The moving particles, which are surrounded by the atoms, emit the beams of de Broglie wavelength, which are directed along their motion. These beams are propagated mainly within the gaps between the atoms, and therefore, the interference pattern is diffractive and determined by the crystal lattice structure. The particles are much slower than the beams, which outrun them. But each particle occupies a large volume, which intersects with these beams. The particles mainly go to the interference maxima, where the beams push them by own pressure. A single particle motion causes its interference with itself.

The Kapitza-Dirac effect is a diffraction of the particles by a standing electromagnetic (light) wave of wavelength $$\lambda$$, as by a diffraction grating with period of $$\lambda/2$$. The aether theory explains this effect simply: The standing wave is the alternating areas of the low and high mass density with a period of $$\lambda/2$$, in which the particles are scattered. Also the photons can be scattered, if the energy density of the standing wave is much higher than the photons energy density.