In mathematics, the HELMHOLTZ EQUATION, named for Hermann von Helmholtz , is the partial differential equation 2 A + k 2 A = 0 {displaystyle nabla ^{2}A+k^{2}A=0} where ∇2 is the Laplacian , k is the wavenumber , and A is the amplitude . CONTENTS * 1 Motivation and uses * 2 Solving the
* 2.1 Vibrating membrane * 2.2 Three-dimensional solutions * 3
MOTIVATION AND USES The
For example, consider the wave equation ( 2 1 c 2 2 t 2 ) u ( r , t ) = 0. {displaystyle left(nabla ^{2}-{frac {1}{c^{2}}}{frac {partial ^{2}}{partial t^{2}}}right)u(mathbf {r} ,t)=0.}
Substituting this form into the wave equation, and then simplifying, we obtain the following equation: 2 A A = 1 c 2 T d 2 T d t 2 . {displaystyle {nabla ^{2}A over A}={1 over c^{2}T}{d^{2}T over dt^{2}}.} Notice the expression on the left-hand side depends only on R, whereas the right-hand expression depends only on t. As a result, this equation is valid in the general case if and only if both sides of the equation are equal to a constant value. From this observation, we obtain two equations, one for A(R), the other for T(t): 2 A A = k 2 {displaystyle {nabla ^{2}A over A}=-k^{2}} and 1 c 2 T d 2 T d t 2 = k 2 {displaystyle {1 over c^{2}T}{d^{2}T over dt^{2}}=-k^{2}} where we have chosen, without loss of generality, the expression −k2 for the value of the constant. (It is equally valid to use any constant k as the separation constant; −k2 is chosen only for convenience in the resulting solutions.) Rearranging the first equation, we obtain the Helmholtz equation: 2 A + k 2 A = ( 2 + k 2 ) A = 0. {displaystyle nabla ^{2}A+k^{2}A=(nabla ^{2}+k^{2})A=0.} Likewise, after making the substitution = d e f k c {displaystyle omega {stackrel {mathrm {def} }{=}}kc} the second equation becomes d 2 T d t 2 + 2 T = ( d 2 d t 2 + 2 ) T = 0 , {displaystyle {frac {d^{2}T}{dt^{2}}}+omega ^{2}T=left({d^{2} over dt^{2}}+omega ^{2}right)T=0,} where k is the wave vector and ω is the angular frequency . We now have Helmholtz's equation for the spatial variable R and a
second-order ordinary differential equation in time. The solution in
time will be a linear combination of sine and cosine functions, with
angular frequency of ω, while the form of the solution in space will
depend on the boundary conditions . Alternatively, integral transforms
, such as the Laplace or
Because of its relationship to the wave equation, the Helmholtz equation arises in problems in such areas of physics as the study of electromagnetic radiation , seismology , and acoustics . SOLVING THE HELMHOLTZ EQUATION USING SEPARATION OF VARIABLES The solution to the spatial
can be obtained for simple geometries using separation of variables . VIBRATING MEMBRANE The two-dimensional analogue of the vibrating string is the vibrating
membrane, with the edges clamped to be motionless. The Helmholtz
equation was solved for many basic shapes in the 19th century: the
rectangular membrane by
If the edges of a shape are straight line segments, then a solution is integrable or knowable in closed-form only if it is expressible as a finite linear combination of plane waves that satisfy the boundary conditions (zero at the boundary, i.e., membrane clamped). An interesting situation happens with a shape where about half of the solutions are integrable, but the remainder are not. A simple shape where this happens is with the regular hexagon. If the wavepacket describing a quantum billiard ball is made up of only the closed-form solutions, its motion will not be chaotic, but if any amount of non-closed-form solutions are included, the quantum billiard motion becomes chaotic. Another simple shape where this happens is with an "L" shape made by reflecting a square down, then to the right. If the domain is a circle of radius a, then it is appropriate to
introduce polar coordinates r and θ. The
We may impose the boundary condition that A vanish if r = a; thus A ( a , ) = 0. {displaystyle A(a,theta )=0.,} The method of separation of variables leads to trial solutions of the form A ( r , ) = R ( r ) ( ) , {displaystyle A(r,theta )=R(r)Theta (theta ),,} where Θ must be periodic of period 2π. This leads to + n 2 = 0 , {displaystyle Theta ''+n^{2}Theta =0,,} and r 2 R + r R + r 2 k 2 R n 2 R = 0. {displaystyle r^{2}R''+rR'+r^{2}k^{2}R-n^{2}R=0.,} It follows from the periodicity condition that = cos n + sin n , {displaystyle Theta =alpha cos ntheta +beta sin ntheta ,,} and that n must be an integer. The radial component R has the form R ( r ) = J n ( ) , {displaystyle R(r)=gamma J_{n}(rho ),,} where the
and ρ = kr. The radial function Jn has infinitely many roots for each value of n, denoted by ρm,n. The boundary condition that A vanishes where r = a will be satisfied if the corresponding wavenumbers are given by k m , n = 1 a m , n . {displaystyle k_{m,n}={frac {1}{a}}rho _{m,n}.,} The general solution A then takes the form of a doubly infinite sum of terms involving products of sin ( n ) or cos ( n ) , and J n ( k m , n r ) . {displaystyle sin(ntheta ){text{ or }}cos(ntheta ),{text{ and }}J_{n}(k_{m,n}r).} These solutions are the modes of vibration of a circular drumhead . THREE-DIMENSIONAL SOLUTIONS In spherical coordinates, the solution is: A ( r , , ) = = 0 m = ( a m j ( k r ) + b m y ( k r ) ) Y m ( , ) . {displaystyle A(r,theta ,varphi )=sum _{ell =0}^{infty }sum _{m=-ell }^{ell }left(a_{ell m}j_{ell }(kr)+b_{ell m}y_{ell }(kr)right)Y_{ell }^{m}(theta ,varphi ).} This solution arises from the spatial solution of the wave equation and diffusion equation . Here j ( k r ) {displaystyle j_{ell }(kr)} and y ( k r ) {displaystyle y_{ell }(kr)} are the spherical Bessel functions , and Y m ( , ) {displaystyle Y_{ell }^{m}(theta ,varphi )} are the spherical harmonics (Abramowitz and Stegun, 1964). Note that these forms are general solutions, and require boundary conditions to be specified to be used in any specific case. For infinite exterior domains, a radiation condition may also be required (Sommerfeld, 1949). Writing r 0 = ( x , y , z ) {displaystyle mathbf {r_{0}} =(x,y,z)} function A ( r 0 ) {displaystyle A(r_{0})} has asymptotics A ( r 0 ) = e i k r 0 r 0 f ( r 0 r 0 , k , u 0 ) + o ( 1 r 0 ) as r 0 {displaystyle A(r_{0})={frac {e^{ikr_{0}}}{r_{0}}}fleft({frac {mathbf {r} _{0}}{r_{0}}},k,u_{0}right)+oleft({frac {1}{r_{0}}}right){text{ as }}r_{0}to infty } where function f is called scattering amplitude and u 0 ( r 0 ) {displaystyle u_{0}(r_{0})} is the value of A at each boundary point r 0 {displaystyle r_{0}} . PARAXIAL APPROXIMATION Further information:
In the paraxial approximation of the Helmholtz equation, the complex amplitude A is expressed as A ( r ) = u ( r ) e i k z {displaystyle A(mathbf {r} )=u(mathbf {r} )e^{ikz}} where u represents the complex-valued amplitude which modulates the sinusoidal plane wave represented by the exponential factor. Then under a suitable assumption, u approximately solves 2 u + 2 i k u z = 0 , {displaystyle nabla _{perp }^{2}u+2ik{frac {partial u}{partial z}}=0,} where 2 = d e f 2 x 2 + 2 y 2 {displaystyle textstyle nabla _{perp }^{2}{stackrel {mathrm {def} }{=}}{frac {partial ^{2}}{partial x^{2}}}+{frac {partial ^{2}}{partial y^{2}}}} is the transverse part of the Laplacian . This equation has important applications in the science of optics , where it provides solutions that describe the propagation of electromagnetic waves (light) in the form of either paraboloidal waves or Gaussian beams . Most lasers emit beams that take this form. The assumption under which the paraxial approximation is valid is that the z derivative of the amplitude function u is a slowly-varying function of z: 2 u z 2 k u z . {displaystyle {bigg }{partial ^{2}u over partial z^{2}}{bigg }ll {bigg }{k{partial u over partial z}}{bigg }.} This condition is equivalent to saying that the angle θ between the wave vector K and the optical axis z is small: 1 {displaystyle theta ll 1} . The paraxial form of the
Expansion and cancellation yields the following: ( 2 x 2 + 2 y 2 ) u ( x , y , z ) e i k z + ( 2 z 2 u ( x , y , z ) ) e i k z + 2 ( z u ( x , y , z ) ) i k e i k z = 0. {displaystyle left({frac {partial ^{2}}{partial x^{2}}}+{frac {partial ^{2}}{partial y^{2}}}right)u(x,y,z)e^{ikz}+left({frac {partial ^{2}}{partial z^{2}}}u(x,y,z)right)e^{ikz}+2left({frac {partial }{partial z}}u(x,y,z)right)ik{e^{ikz}}=0.} Because of the paraxial inequalitiy stated above, the ∂2u/∂z2 term is neglected in comparison with the k·∂u/∂z term. This yields the paraxial Helmholtz equation. Substituting u ( r ) = A ( r ) e i k z {displaystyle u(mathbf {r} )=A(mathbf {r} )e^{-ikz}} then gives the paraxial equation for the original complex amplitude A: 2 A + 2 i k A z + 2 k 2 A = 0. {displaystyle nabla _{perp }^{2}A+2ik{frac {partial A}{partial z}}+2k^{2}A=0.} The
There is even a subject named "Helmholtz optics" based on the equation, named in honour of Helmholtz. INHOMOGENEOUS HELMHOLTZ EQUATION The INHOMOGENEOUS HELMHOLTZ EQUATION is the equation 2 A ( x ) + k 2 A ( x ) = f ( x ) in R n {displaystyle nabla ^{2}A(x)+k^{2}A(x)=-f(x){text{ in }}mathbb {R} ^{n}} where ƒ : Rn → C is a given function with compact support , and n = 1, 2, 3. This equation is very similar to the screened Poisson equation , and would be identical if the plus sign (in front of the k term) is switched to a minus sign. In order to solve this equation uniquely, one needs to specify a boundary condition at infinity, which is typically the Sommerfeld radiation condition lim r r n 1 2 ( r i k ) A ( r x ) = 0 {displaystyle lim _{rto infty }r^{frac {n-1}{2}}left({frac {partial }{partial r}}-ikright)A(r{hat {x}})=0}
uniformly in x {displaystyle {hat {x}}} with
x = 1 {displaystyle {hat {x}}=1} , where the vertical
bars denote the
With this condition, the solution to the inhomogeneous Helmholtz equation is the convolution A ( x ) = ( G f ) ( x ) = R n G ( x y ) f ( y ) d y {displaystyle A(x)=(G*f)(x)=int limits _{mathbb {R} ^{n}}!G(x-y)f(y),dy} (notice this integral is actually over a finite region, since f
{displaystyle f} has compact support). Here, G {displaystyle
G} is the Green\'s function of this equation, that is, the solution
to the inhomogeneous
The expression for the
for n = 1, G ( x ) = i 4 H 0 ( 1 ) ( k x ) {displaystyle G(x)={frac {i}{4}}H_{0}^{(1)}(kx)} for n = 2, where H 0 ( 1 ) {displaystyle H_{0}^{(1)}} is a Hankel function , and G ( x ) = e i k x 4 x {displaystyle G(x)={frac {e^{ikx}}{4pi x}}} for n = 3. Note that we have chosen the boundary condition that the
SEE ALSO * Laplace\'s equation (a particular case of the Helmholtz equation) NOTES * ^ J. W. Goodman. Introduction to Fourier
* Riley, K. F.; Hobson, M. P.; Bence, S. J. (2002). "Chapter 19". Mathematical methods for physics and engineering. New York: Cambridge University Press. ISBN 0-521-89067-5 . * Riley, K. F. (2002). "Chapter 16". Mathematical Methods for Scientists and Engineers. Sausalito, California: University Science Books. ISBN 1-891389-24-6 . * Saleh, Bahaa E. A.; Teich, Malvin Carl (1991). "Chapter 3". Fundamentals of Photonics. Wiley Series in Pure and Applied Optics. New York: John Wiley & Sons. pp. 80–107. ISBN 0-471-83965-5 . * Sommerfeld, Arnold (1949). "Chapter 16". Partial Differential Equations in Physics. New York: Academic Press. ISBN 0126546568 . * Howe, M. S. (1998).
EXTERNAL LINKS * Helmholtz |