Scattering

In physics, scattering is generally understood to be the deflection of an object by interaction with another local object (scattering centre), more concretely the deflection of particle or wave radiation. Examples are the scattering of light by atoms or fine dust, of electrons by other electrons or of neutrons by atomic nuclei.

The strength of a scattering is indicated by the scattering cross section. The name comes from the fact that the scattering cross-section in classical scattering of mass points on a hard sphere is just equal to the cross-section of the sphere.

A distinction is made between elastic and inelastic (or inelastic) scattering:

• in the case of elastic scattering (see also elastic collision), the sum of the kinetic energies after the collision is equal to the sum beforehand
• In the case of inelastic scattering, on the other hand, it changes, e.g. part of the kinetic energy present is converted into excitation energy of an atom or is used to break a bond, for example in ionization processes.

Inelastic scattering in the narrow sense means that the incident particle is still present after the impact, albeit with reduced energy; in a broader sense, absorption processes (processes in which the incident particle "disappears") are sometimes also counted as inelastic scattering processes.

In the case of scattering waves, a distinction is also made between coherent and incoherent scattering. In the case of coherent scattering there is a fixed phase relationship between the incoming and the scattered wave (see reflection), in the case of incoherent scattering there is not. If coherent beams are scattered coherently, the scattered beams can interfere with each other. This is exploited in particular in X-ray diffraction.

The theoretical description of scattering is the task of scattering theory. Experiments in high-energy physics are generally referred to as scattering experiments, even if, for example, new particles are created in the process (deep-inelastic scattering). They provide information about the form of the interaction potential. Ernest Rutherford used kinematic relationships in the scattering of alpha particles by atoms to show that these must contain a heavy nucleus.

In contrast to scattering, diffraction involves a deflection of radiation due to the property of a wave front to propagate in all directions at the edge of an obstacle. In refraction, the deflection of radiation is due to the change in propagation velocity when the density or composition of the propagation medium changes, most clearly at phase boundaries.

Scattering angle, forward scattering and backward scattering

The scattering angle θ is the angle by which the scattered particle is deflected. Forward scattering refers to scattering processes in which only a small deflection occurs (small scattering angle). Backscattering or backward scattering refers to scattering processes with a scattering angle between and (see also Kinematics (particle impact)).

If both collision partners have a mass different from zero, the scattering angle in the center-of-mass system is often considered in scattering experiments in nuclear and particle physics. This is more important for the theoretical consideration than the scattering angle in the laboratory system.

In many cases, forward scattering is much stronger than scattering in other directions, i.e. it has a comparatively large differential cross-section. A well-known example from everyday life is the scattering of light by dust particles in the air: If you look almost in the direction of the light source (for example, when sunlight falls into a dark room), the dust particles are clearly visible as bright points. Something similar happens to fine water droplets.

Scattering in the backward direction () is usually weaker than in all other directions within the framework of classical physics, but can be stronger than scattering in adjacent directions due to quantum mechanical effects or interference effects. Coherent backscattering is also responsible for the high brightness of the full moon.

Classical scattering

Classicalmechanics distinguishes collisions between rigid bodies from scattering at a potential. For orbital motion of a point mass in a potential that decreases linearly with distance, equations always result that describe a conic section: Hyperbola, Parabola, or Ellipse. A positive, i.e. repulsive potential always leads to hyperbolas. Attracting potentials lead to ellipses if the energy of the collision partner is not large enough. In this sense, the movement of a comet is also the scattering at the gravitational potential of the sun.