Generally in scattering theory and in particular in quantum mechanics, the Born approximation consists of taking the incident field in place of the total field as the driving field at each point in the scatterer.
The Born approximation is named after Max Born who proposed this approximation in the early days of quantum theory development.
[1] It is the perturbation method applied to scattering by an extended body.
For example, the scattering of radio waves by a light styrofoam column can be approximated by assuming that each part of the plastic is polarized by the same electric field that would be present at that point without the column, and then calculating the scattering as a radiation integral over that polarization distribution.
with a momentum p and out-going (+) or in-going (−) boundary conditions is where
is the corresponding free scattering solution sometimes called the incident field.
on the right hand side is sometimes called the driving field.
Within the Born approximation, the above equation is expressed as which is much easier to solve since the right hand side no longer depends on the unknown state
The obtained solution is the starting point of the Born series.
Using the outgoing free Green's function for a particle with mass
in coordinate space, one can extract the Born approximation to the scattering amplitude from the Born approximation to the Lippmann–Schwinger equation above, where
In the Born approximation for centrally symmetric field, the scattering amplitude and thus the cross section
The Born approximation is used in several different physical contexts.
In neutron scattering, the first-order Born approximation is almost always adequate, except for neutron optical phenomena like internal total reflection in a neutron guide, or grazing-incidence small-angle scattering.
is the same as the Fourier transform of the scattering potential [3] .
Using this concept, the electronic analogue of Fourier optics has been theoretically studied in monolayer graphene.
[4] The Born approximation has also been used to calculate conductivity in bilayer graphene[5] and to approximate the propagation of long-wavelength waves in elastic media.
[6] The same ideas have also been applied to studying the movements of seismic waves through the Earth.
[7] The Born approximation is simplest when the incident waves
That is, the scatterer is treated as a perturbation to free space or to a homogeneous medium.
In the distorted-wave Born approximation (DWBA), the incident waves are solutions
that is treated by some other method, either analytical or numerical.
For nuclear reactions, numerical optical model waves are used.
For scattering of charged particles by charged particles, analytic solutions for coulomb scattering are used.
This gives the non-Born preliminary equation and the Born approximation Other applications include bremsstrahlung and the photoelectric effect.
For a charged-particle-induced direct nuclear reaction, the procedure is used twice.
There are similar methods that do not use the Born approximations.
In condensed-matter research, DWBA is used to analyze grazing-incidence small-angle scattering.