Eikonal approximation

The eikonal approximation is a method of approximation useful in wave scattering equations within the realms of quantum mechanics, optics, quantum electrodynamics, and partial wave expansion

Informal description

The main advantage the eikonal approximation offers is that the equations reduce to a differential equation in a single variable. This reduction into a single variable is the result of the straight line approximation or the eikonal approximation which allows us to choose the straight line as a special direction.

Relation to the WKB approximation

The early steps involved in the eikonal approximation in quantum mechanics are very closely related to the WKB approximation. It, like the eikonal approximation, reduces the equations into a differential equation in a single variable. But the difficulty with the WKB approximation is that this variable is described by the trajectory of the particle which, in general, is complicated.

Formal description

Making use of WKB approximation we can write the wave function of the scattered system in term of action S

$\Psi =e^{iS/{\hbar }}$

Inserting $\Psi$ in the Schrödinger equation we obtain

$-{\frac {{\hbar }^{2}}{2m}}{\nabla }^{2}\Psi =(E-V)\Psi$

$-{\frac {{\hbar }^{2}}{2m}}{\nabla }^{2}{e^{iS/{\hbar }}}=(E-V)e^{iS/{\hbar }}$

${\frac {1}{2m}}{(\nabla S)}^{2}-{\frac {i\hbar }{2m}}{\nabla }^{2}S=E-V$

We write S as a $\hbar$ power series

$S=S_{0}+{\frac {\hbar }{i}}S_{1}+...$

For the zero-th order:

${(\nabla S_{0})}^{2}=E-V$

If we consider straight trajectories then ${\nabla }^{2}\rightarrow {\delta _{z}}^{2}$

We obtain a differential equation with the boundary condition:

${\frac {S(z=z_{0})}{\hbar }}=kz_{0}$ for V → 0 (z→ -∞)

${\frac {d}{dz}}{\frac {S_{0}}{\hbar }}={\sqrt {k^{2}-2mV/{\hbar }^{2}}}$

${\frac {S_{0}(z)}{\hbar }}=kz-{\frac {m}{{\hbar }^{2}k}}\int _{-\inf }^{Z}{Vdz'}$

See also

• Eikonal equation

References

• [1]Eikonal Approximation K. V. Shajesh Department of Physics and Astronomy, University of Oklahoma