“Kirchhoff integral theorem”的意思、由来-开放百科全书

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Kirchhoff integral theorem

释义

Equation
Monochromatic waves Non-monochromatic waves
See also
References
Further reading

Kirchhoff's integral theorem (sometimes referred to as the Fresnel–Kirchhoff integral theorem)^[1] uses Green's identities to derive the solution to the homogeneous wave equation at an arbitrary point P in terms of the values of the solution of the wave equation and its first-order derivative at all points on an arbitrary surface that encloses P.^[2]

Equation

Monochromatic waves

The integral has the following form for a monochromatic wave:^[2]^[3]

where the integration is performed over an arbitrary closed surface S (enclosing r), s is the distance from the surface element to the point r, and ∂/∂n denotes differentiation along the surface normal (a normal derivative). Note that in this equation the normal points to the inner of the enclosed volume; if the more usual outer-pointing normal is used, the integral will have the opposite sign.

Non-monochromatic waves

A more general form can be derived for non-monochromatic waves. The complex amplitude of the wave can be represented by a Fourier integral of the form

where, by Fourier inversion, we have

The integral theorem (above) is applied to each Fourier component , and the following expression is obtained:^[2]

where the square brackets on V terms denote retarded values, i.e. the values at time t − s/c.

Kirchhoff showed that the above equation can be approximated in many cases to a simpler form, known as the Kirchhoff, or Fresnel–Kirchhoff diffraction formula, which is equivalent to the Huygens–Fresnel equation, but provides a formula for the inclination factor, which is not defined in the latter. The diffraction integral can be applied to a wide range of problems in optics.

References

1. ^G. Kirchhoff, Ann. d. Physik. 1883, 2, 18, p. 663.
2. ^¹²Max Born and Emil Wolf, Principles of Optics, 1999, Cambridge University Press, Cambridge, pp. 417–420.
3. ^Introduction to Fourier Optics J. Goodman sec. 3.3.3

Equation

Monochromatic waves

Non-monochromatic waves

See also

References

Further reading