View source for Kirchhoff's diffraction formula

{{Short description|Physics formula}}
'''[[Gustav Kirchhoff|Kirchhoff]]'s diffraction formula'''<ref name = "Born and Wolf">{{cite book
|last1=Born |first1=Max |author-link=Max Born
|last2=Wolf |first2=Emil |author2-link=Emil Wolf
|title=[[Principles of Optics|Principles of optics: electromagnetic theory of propagation, interference and diffraction of light]]|date=1999|publisher=[[Cambridge University Press]]|location=Cambridge|isbn=9780521642224|pages=986}}</ref><ref>{{cite book|last=Longhurst|first=Richard Samuel|title=Geometrical And Physical Optics|date=1986|publisher=Orient BlackSwan|isbn=8125016236|pages=651}}</ref> (also called '''Fresnel–Kirchhoff diffraction formula''') approximates [[electromagnetic radiation| light]] intensity and phase in optical [[diffraction]]: light fields in the boundary regions of shadows. The approximation
can be used to model light [[Wave propagation|propagation]] in a wide range of configurations, either [[Analytical expression|analytically]] or using [[numerical analysis|numerical modelling]]. It gives an expression for the wave disturbance when a [[monochromatic]] [[Wave equation#Spherical waves|spherical wave]] is the incoming wave of a situation under consideration. This formula is derived by applying the [[Kirchhoff integral theorem]], which uses the [[Green's identities|Green's second identity]] to derive the solution to the homogeneous [[wave equation|scalar wave equation]], to a spherical wave with some approximations.
The [[Huygens–Fresnel principle]] is derived by the Fresnel–Kirchhoff diffraction formula.
== Derivation of Kirchhoff's diffraction formula ==
[[Kirchhoff integral theorem|Kirchhoff's integral theorem]], sometimes referred to as the Fresnel–Kirchhoff integral theorem,<ref>{{cite journal |last=Kirchhoff |first=G. |title=Zur Theorie der Lichtstrahlen |journal=Annalen der Physik | date=1883 |volume=254 |issue=4 | pages=663–695 | doi=10.1002/andp.18832540409 | publisher=Wiley | language=de | bibcode=1882AnP...254..663K | url=https://zenodo.org/record/1423792}}</ref> uses [[Green's identities|Green's second identity]] to derive the solution of the homogeneous [[wave equation|scalar wave equation]] at an arbitrary spatial position '''P''' in terms of the solution of the wave equation and its first order derivative at all points on an arbitrary closed surface <math>S</math> as the boundary of some volume including '''P'''.
The solution provided by the integral theorem for a [[monochromatic]] source is
<math display="block">U(\textbf{P}) = \frac{1}{4\pi} \int_{S} \left[ U \frac{\partial}{\partial \textbf{n}} \left( \frac{e^{iks}}{s} \right) - \frac{e^{iks}}{s} \frac{\partial U}{\partial \textbf{n}} \right]dS,</math>
where <math>U </math> is the spatial part of the solution of the homogeneous [[Wave equation|scalar wave equation]] (i.e., <math>V(\mathbf{r},t) = U(\mathbf{r}) e^{-i\omega t}</math> as the homogeneous scalar wave equation solution), ''k'' is the [[wavenumber]], and ''s'' is the distance from '''''P''''' to an (infinitesimally small) integral surface element, and <math>\frac{\partial}{\partial n}</math> denotes differentiation along the integral surface element normal unit vector <math>n </math> (i.e., a [[normal derivative]]), i.e., <math>\frac{\partial f}{\partial n} = \nabla f \cdot n </math>. Note that the surface normal or the direction of <math>n </math> is toward the inside of the enclosed volume in this integral; if the more usual [[Normal (geometry)#Uniqueness of the normal|outer-pointing normal]] is used, the integral will have the opposite sign. And also note that, in the integral theorem shown here, <math>n </math> and '''''P '''''are [[Vector field|vector]] quantities while other terms are [[Scalar (mathematics)|scalar]] quantities.
For the below cases, the following basic assumptions are made.
* The distance between a point source of waves and an integral area, the distance between the integral area and an observation point '''P''', and the dimension of opening ''S'' are much greater than the wave wavelength <math>\lambda</math>.
*<math>U </math> and <math>\frac{\partial U}{\partial n} = \nabla U \cdot n </math> are discontinuous at the boundaries of the aperture, called ''Kirchhoff's boundary conditions''. This may be related with another assumption that waves on an aperture (or an open area) is same to the waves that would be present if there was no obstacle for the waves.
=== Point source ===
[[File:kirchhoff 1.svg|thumb|300px|right|A geometrical arrangement used in deriving the Kirchhoff's diffraction formula. The area designated by A<sub>1</sub> is the aperture (opening), the areas marked by A<sub>2</sub> are opaque areas, and A<sub>3</sub> is the hemisphere as a part of the closed integral surface (consisted of the areas A<sub>1</sub>, A<sub>2</sub>, and A<sub>3</sub>) for the [[Kirchhoff integral theorem|Kirchhoff's integral theorem]].]] Consider a monochromatic point source at '''P'''<sub>0</sub>, which illuminates an aperture in a screen. The [[Intensity (physics)|intensity]] of the wave emitted by a point source falls off as the inverse square of the distance traveled, so the amplitude falls off as the inverse of the distance. The complex amplitude of the disturbance at a distance <math>r </math> is given by
<math display="block">U(r) = \frac{a e^{ikr}}{r},</math>
where <math>a</math> represents the [[Magnitude (mathematics)#Complex numbers|magnitude]] of the disturbance at the point source.
The disturbance at a spatial position '''P''' can be found by applying the [[Kirchhoff integral theorem|Kirchhoff's integral theorem]] to the closed surface formed by the intersection of a sphere of radius '''R''' with the screen. The integration is performed over the areas ''A''<sub>1</sub>, ''A''<sub>2</sub> and ''A''<sub>3</sub>, giving
<math display="block">U(P) = \frac{1}{4\pi} \left[\int_{A_1} + \int_{A_2} + \int_{A_3} \right]\left( U \frac{\partial}{\partial n} \left( \frac{e^{iks}}{s} \right) - \frac{e^{iks}}{s} \frac{\partial U}{\partial n} \right) dS.</math>
To solve the equation, it is assumed that the values of <math>U </math> and <math>\frac{\partial U}{\partial n} </math> in the aperture area ''A''<sub>1</sub> are the same as when the screen is not present, so at the position '''Q''',
<math display="block">U_{A_1} = \frac{a e^{ikr}}{r},</math>
<math display="block">\frac{\partial U_{A_1}}{\partial n} = \nabla U_{A_1} \cdot n = \frac{ae^{ikr}}{r} \left[ik - \frac{1}{r} \right] \cos(n,r),</math>
where <math>r </math> is the length of the straight line '''P<sub>0</sub>Q''', and <math>(n,r) </math> is the angle between a straightly extended version of '''P'''<sub>0</sub>'''Q''' and the (inward) normal to the aperture. Note that <math>0 < (n,r) < \frac{\pi}{2} </math> so <math>\cos(n,r)</math> is a positive real number on ''A''<sub>1</sub>.
At '''Q''', we also have
<math display="block"> \frac{\partial}{\partial n} \left( \frac{e^{iks}}{s} \right) = \frac{e^{iks}}{s} \left[ik - \frac{1}{s} \right] \cos(n,s), </math>
where <math>s </math> is the length of the straight line '''PQ''', and <math>(n,s) </math> is the angle between a straightly extended version of '''PQ''' and the (inward) normal to the aperture. Note that <math>\frac{\pi}{2} < (n,s) < \frac{3 \pi}{2} </math> so <math>\cos(n,s)</math> is a negative real number on ''A''<sub>1</sub>.
Two more following assumptions are made.
* In the above normal derivatives, the terms <math>\frac{1}{r}</math> and <math>\frac{1}{s}</math> in the both square brackets are assumed to be negligible compared with the [[wavenumber]] <math>k=\frac{2\pi}{\lambda}</math>, means <math>r </math> and <math>s </math> are much greater than the [[wavelength]] <math>\lambda</math>.
* Kirchhoff assumes that the values of <math>U </math> and <math>\frac{\partial U}{\partial n} </math> on the opaque areas marked by ''A''<sub>2</sub> are zero. This implies that <math>U </math> and <math>\frac{\partial U}{\partial n} </math> are discontinuous at the edge of the aperture ''A''<sub>1</sub>. This is not the case, and this is <u>one of the approximations</u> used in deriving the Kirchhoff's diffraction formula.<ref>J.Z. Buchwald & C.-P. Yeang, [http://jzbuchwald.caltech.edu/downloads/Kirchoff%20and%20diffraction.pdf "Kirchhoff's theory for optical diffraction, its predecessor and subsequent development: the resilience of an inconsistent theory"] {{Webarchive|url=https://web.archive.org/web/20210624204644/http://jzbuchwald.caltech.edu/downloads/Kirchoff%20and%20diffraction.pdf |date=2021-06-24 }}, ''Archive for History of Exact Sciences'', vol.{{nnbsp}}70, no.{{nnbsp}}5 (Sep.{{nnbsp}}2016), pp.{{nnbsp}}463–511; {{doi|10.1007/s00407-016-0176-1}}.</ref><ref>J. Saatsi & P. Vickers, "Miraculous success? Inconsistency and untruth in Kirchhoff’s diffraction theory", ''British J. for the Philosophy of Science'', vol.{{nnbsp}}62, no.{{nnbsp}}1 (March 2011), pp. 29–46; [https://www.jstor.org/stable/41241806 jstor.org/stable/41241806]. (Pre-publication version, with different pagination: [http://dro.dur.ac.uk/10523/1/10523.pdf dro.dur.ac.uk/10523/1/10523.pdf].)</ref> These assumptions are sometimes referred to as ''Kirchhoff's boundary conditions''.
The contribution from the hemisphere ''A''<sub>3</sub> to the integral is expected to be zero, and it can be justified by one of the following reasons.
# Make the assumption that the source starts to radiate at a particular time, and then make ''R'' large enough, so that when the disturbance at '''P''' is being considered, no contributions from ''A''<sub>3</sub> will have arrived there.<ref name="Born and Wolf" /> Such a wave is no longer [[monochromatic]], since a monochromatic wave must exist at all times, but that assumption is not necessary, and a more formal argument avoiding its use has been derived.<ref>M. Born, ''Optik: ein Lehrbuch der elektromagnetischen Lichttheorie''. Berlin, Springer, 1933, reprinted 1965, p. 149.</ref>
# A wave emanated from the aperture ''A''<sub>1</sub> is expected to evolve toward a spherical wave as it propagates (Water wave examples of this can be found in many pictures showing a water wave passing through a relatively narrow opening.). So, if ''R'' is large enough, then the integral on ''A''<sub>3</sub> becomes <math display="block">U(P) \sim -\frac{ia}{2\lambda} \int_{A_3} \frac{e^{2ik(r')}}{r'^2} [\cos(n,r') - \cos(n,r')] d{A_3} = -\frac{ia}{2\lambda} \int_{A_3} e^{2ik(r')} [\cos(n,r') - \cos(n,r')] d{\Omega} = 0</math> where <math>r'</math> and <math>d\Omega</math> are the distance from the center of the aperture ''A''<sub>1</sub> to an integral surface element and [[Spherical coordinate system#Integration and differentiation in spherical coordinates|the differential solid angle in the spherical coordinate system]] respectively.
As a result, finally, the integral above, which represents the complex amplitude at '''P''', becomes
<math display="block">U(P) = -\frac{ia}{2\lambda} \int_{A_1} \frac{e^{ik(r+s)}}{rs} [\cos(n,r) - \cos(n,s)] d{A_1}.</math>
This is the ''Kirchhoff'' or ''Fresnel–Kirchhoff diffraction formula''.
==== Equivalence to Huygens–Fresnel principle ====
[[File:kirchhoff 2.svg|thumb|300px|right|Geometric arrangement used to express Kirchhoff's formula in a form similar to Huygens–Fresnel]]
The [[Huygens–Fresnel principle]] can be derived by integrating over a different closed surface (the boundary of some volume having an observation point ''P''). The area ''A''<sub>1</sub> above is replaced by a part of a wavefront (emitted from a ''P''<sub>0</sub>) at ''r''<sub>0</sub>, which is the closest to the aperture, and a portion of a cone with a vertex at ''P''<sub>0</sub>, which is labeled ''A''<sub>4</sub> in the right diagram. If the wavefront is positioned such that the wavefront is very close to the edges of the aperture, then the contribution from ''A''<sub>4</sub> can be neglected (assumed here). On this new ''A<sub>1</sub>'', the inward (toward the volume enclosed by the closed integral surface, so toward the right side in the diagram) normal <math>n </math> to ''A<sub>1</sub>'' is along the radial direction from ''P''<sub>0</sub>, i.e., the direction perpendicular to the wavefront. As a result, the angle <math>(n,r)=0 </math> and the angle <math>(n,s) </math> is related with the angle <math>\chi</math> (the angle as defined in [[Huygens–Fresnel principle]]) as
<math display="block">(n,s) = \pi - \chi.</math>
The complex amplitude of the wavefront at ''r''<sub>0</sub> is given by
<math display="block">U(r_0) = \frac{ae^{ikr_0}}{r_0}.</math>
So, the diffraction formula becomes
<math display="block">U(P) = -\frac{i}{2\lambda} \frac{ae^{ikr_0}}{r_0} \int_{S} \frac{e^{iks}}{s} (1 + \cos\chi) dS,</math>
where the integral is done over the part of the wavefront at ''r''<sub>0</sub> which is the closest to the aperture in the diagram. This integral leads to the [[Huygens–Fresnel principle|Huygens–Fresnel]] principle (with the obliquity factor <math display="inline">\frac {1 + \cos\chi}{2}</math>).
In the derivation of this integral, instead of the geometry depicted in the right diagram, double spheres centered at ''P''<sub>0</sub> with the inner sphere radius ''r''<sub>0</sub> and an infinite outer sphere radius can be used.<ref>{{Cite book| last=Hecht|first=Eugene| title=Optics| publisher=Pearson| year=2017| isbn=978-1-292-09693-3| edition=5th (Global) | pages=532–535| chapter=10.4 Kirchhoff’s Scalar Diffraction Theory}}</ref> In this geometry, the observation point ''P'' is located in the volume enclosed by the two spheres so the Fresnel-Kirchhoff diffraction formula is applied on the two spheres. (The surface normal on these integral surfaces are, say again, toward the enclosed volume in the diffraction formula above.) In the formula application, the integral on the outer sphere is zero by a similar reason of the integral on the hemisphere as zero above.
=== Extended source ===
Assume that the aperture is illuminated by an extended source wave.<ref>M. V. Klein & T. E. Furtak, 1986, ''Optics''; 2nd ed. John Wiley & Sons, New York {{ISBN|0-471-87297-0}}.</ref> The complex amplitude at the aperture is given by ''U''<sub>0</sub>(''r'').
It is assumed, as before, that the values of <math>U </math> and <math>\frac{\partial U}{\partial n} </math> in the area ''A''<sub>1</sub> are the same as when the screen is not present, that the values of <math>U </math> and <math>\frac{\partial U}{\partial n} </math> in ''A''<sub>2</sub> are zero (Kirchhoff's boundary conditions) and that the contribution from ''A''<sub>3</sub> to the integral are also zero. It is also assumed that 1/''s'' is negligible compared with ''k''. We then have
<math display="block">U(P) = \frac{1}{4\pi} \int_{A_1} \frac{e^{iks}}{s} \left[ ik U_0(r) \cos(n,s) - \frac{\partial U_0(r)}{\partial n} \right]\, dS. </math>
This is the most general form of the Kirchhoff diffraction formula. To solve this equation for an extended source, an additional integration would be required to sum the contributions made by the individual points in the source. If, however, we assume that the light from the source at each point in the aperture has a well-defined direction, which is the case if the distance between the source and the aperture is significantly greater than the wavelength, then we can write
<math display="block"> U_0(r) \approx a(r) e^{ikr}, </math>
where ''a''(''r'') is the magnitude of the disturbance at the point ''r'' in the aperture.  We then have
<math display="block"> \frac{\partial {U_0(r)}}{\partial n} = ik a(r)e^{ikr} \cos(n,r) </math>
and thus
<math display="block">U(P) = -\frac{i}{2\lambda} \int_S a(r) \frac{e^{ik(s+r)}}{s} [ \cos\chi + \cos(n,r)]\, dS. </math>
== Fraunhofer and Fresnel diffraction equations ==
In spite of the various approximations that were made in arriving at the formula, it is adequate to describe the majority of problems in instrumental optics. This is mainly because the wavelength of light is much smaller than the dimensions of any obstacles encountered. Analytical solutions are not possible for most configurations, but the [[Fresnel diffraction]] equation and [[Fraunhofer diffraction]] equation, which are approximations of Kirchhoff's formula for the [[near and far field#Near field|near field]] and [[near and far field#Far field|far field]], can be applied to a very wide range of optical systems.
One of the important assumptions made in arriving at the Kirchhoff diffraction formula is that ''r'' and ''s'' are significantly greater than λ.  Another approximation can be made, which significantly simplifies the equation further: this is that the distances '''P'''<sub>0</sub>'''Q''' and '''QP''' are much greater than the dimensions of the aperture.  This allows one to make two further approximations:
* cos(''n, r'') − cos(''n, s'') is replaced with 2cos β, where β is the angle between '''P'''<sub>0</sub>'''P''' and the normal to the aperture. The factor 1/''rs'' is replaced with 1/''r''{{'}}''s''{{'}}, where ''r''{{'}} and ''s''{{'}} are the distances from '''P'''<sub>0</sub> and '''P''' to the origin, which is located in the aperture.  The complex amplitude then becomes:<math display="block">U(P) = -\frac{ia \cos\beta}{\lambda r's'} \int_S e^{ik(r+s)}\,ds. </math>
* Assume that the aperture lies in the ''xy'' plane, and the coordinates of '''P'''<sub>0</sub>, '''P''' and '''Q''' (a general point in the aperture) are (''x''<sub>0</sub>, ''y''<sub>0</sub>, ''z''<sub>0</sub>), (''x'', ''y'', ''z'') and (''x''{{'}}, ''y''{{'}}, 0) respectively.  We then have:
*:<math> ~r^2 = (x_0 - x')^2 + (y_0 - y')^2 + z_0^2, </math>
*:<math> ~s^2 = (x - x')^2 + (y - y')^2 + z^2, </math>
*:<math> ~r'^2 = x_0^2 + y_0^2 + z_0^2, </math>
*:<math> ~s'^2 = x^2 + y^2 + z^2. </math>
We can express ''r'' and ''s'' as follows:
<math display="block">r = r'\left[1 - \frac{2(x_0x' + y_0y')}{r'^2} + \frac{x'^2 + y'^2}{r'^2}\right]^{1/2},</math>
<math display="block">s = s'\left[1 - \frac{2(xx' + yy')}{s'^2} + \frac{x'^2 + y'^2}{s'^2}\right]^{1/2}.</math>
These can be expanded as [[Taylor series#Binomial_series|power series]]:
<math display="block">r = r'\left[1 - \frac{1}{2r'^2}\left[2(x_0x' + y_0y') - (x'^2 + y'^2)\right] + \frac{1}{8r'^4}\left[2(x_0x' + y_0y') - (x'^2 + y'^2)\right]^2 + \cdots\right],</math>
<math display="block">s = s'\left[1 - \frac{1}{2s'^2}\left[2(xx' + yy') - (x'^2 + y'^2)\right] + \frac{1}{8s'^4}\left[2(xx' + yy') - (x'^2 + y'^2)\right]^2 + \cdots\right].</math>
The complex amplitude at '''P''' can now be expressed as
<math display="block">U(P) = -\frac{i\cos\beta}{\lambda} \frac{ae^{ik(r' + s')}}{r's'} \int_S e^{ikf(x',y')}\, dx' dy', </math>
where ''f''(''x''{{'}}, ''y''{{'}}) includes all the terms in the expressions above for ''s'' and ''r'' apart from the first term in each expression and can be written in the form
<math display="block">f(x',y') = c_1 x' + c_2 y' + c_3 x'^2 + c_4 y'^2 + c_5x'y' \cdots,</math>
where the ''c<sub>i</sub>'' are constants.
=== Fraunhofer diffraction ===
{{main|Fraunhofer diffraction}}
If all the terms in ''f''(''x''{{'}}, ''y''{{'}}) can be neglected except for the terms in ''x''{{'}} and ''y''{{'}}, we have the [[Fraunhofer diffraction]] equation.  If the direction cosines of '''P'''<sub>0</sub>'''Q''' and '''PQ''' are
<math display="block">\begin{align}
l_0 &= -x_0/r', \\
m_0 &= -y_0/r', \\
l &= x/s', \\
m &= y/s'.
\end{align}</math>
The Fraunhofer diffraction equation is then
<math display="block">U(P) = C \int_S e^{ik[(l_0 - l)x' + (m_0 - m)y']}\, dx' dy', </math>
where ''C'' is a constant.  This can also be written in the form
<math display="block">U(P) = C\int_S e^{i(\mathbf k_0 - \mathbf k) \cdot \mathbf r'}\, dr', </math>
where '''k'''<sub>0</sub> and '''k''' are the [[wave vector]]s of the waves traveling from '''P'''<sub>0</sub> to the aperture and from the aperture to '''P''' respectively, and '''r'''{{'}} is a point in the aperture.
If the point source is replaced by an extended source whose complex amplitude at the aperture is given by ''U''<sub>0</sub>('''r' '''), then the [[Fraunhofer diffraction]] equation is:
<math display="block">U(P) \propto \int_S a_0 (\mathbf r') e^{i(\mathbf k_0 - \mathbf k) \cdot \mathbf r'}\, dr',</math>
where ''a''<sub>0</sub>('''r'''') is, as before, the magnitude of the disturbance at the aperture.
In addition to the approximations made in deriving the Kirchhoff equation, it is assumed that
* ''r'' and ''s'' are significantly greater than the size of the aperture,
* second- and higher-order terms in the expression ''f''(''x''{{'}}, ''y''{{'}}) can be neglected.
=== Fresnel diffraction ===
{{main|Fresnel diffraction}}
When the quadratic terms cannot be neglected but all higher order terms can, the equation becomes the [[Fresnel diffraction]] equation. The approximations for the Kirchhoff equation are used, and additional assumptions are:
* ''r'' and ''s'' are significantly greater than the size of the aperture,
* third- and higher-order terms in the expression ''f''(''x''{{'}}, ''y''{{'}}) can be neglected.
== References ==
{{Reflist}}
== Further reading ==
* Baker, B.B.; Copson, E.T. (1939, 1950). ''The Mathematical Theory of Huygens' Principle''. Oxford.
* {{Cite book | isbn = 9780521575072 | title = The Cambridge Handbook of Physics Formulas | last1 = Woan | first1 = Graham | year = 2000 | publisher = Cambridge University Press | url-access = registration | url = https://archive.org/details/cambridgehandboo0000woan }}
* {{cite book|author=J. Goodman|year=2005|title=Introduction to Fourier Optics|edition=3rd|publisher=Roberts & Co Publishers|isbn=978-0-9747077-2-3|url= https://books.google.com/books?id=ow5xs_Rtt9AC}}
* {{Cite book | isbn = 978-0-321-85656-2 | title = Introduction to Electrodynamics | last1 = Griffiths | first1 = David J. | year = 2012 | publisher = Pearson Education, Limited | url = https://books.google.com/books?id=t-Q6LgEACAAJ  }}
* {{Cite book | isbn = 978-0-471-89931-0 | title = Light and Matter: Electromagnetism, Optics, Spectroscopy and Lasers | last1 = Band | first1 = Yehuda B. | year = 2006 | publisher = [[John Wiley & Sons]] | url = https://books.google.com/books?id=0Rar9yGfQfgC  }}
* {{Cite book | isbn = 978-0-19-856646-5 | title = The Light Fantastic: A Modern Introduction to Classical and Quantum Optics | last1 = Kenyon | first1 = Ian | year = 2008 | publisher = [[Oxford University Press]] | url = https://archive.org/details/lightfantasticmo0000keny  | url-access = registration }}
* {{Cite book | isbn = 978-0-89573-752-6 | title = Encyclopedia of physics | last1 = Lerner | first1 = Rita G.|author1-link=Rita G. Lerner | year = 1991 | publisher = VCH | url = https://archive.org/details/encyclopediaofph00lern | url-access = registration | last2 = George L. | first2 = Trigg  }}
* {{Cite book | isbn = 978-0-07-051400-3 | title = MacGraw-Hill Encyclopedia of Physics | last1 = Sybil P. | first1 = Parker | year = 1993 | publisher = McGraw-Hill Ryerson, Limited | url = https://books.google.com/books?id=q-H4MQEACAAJ  }}
{{DEFAULTSORT:Kirchhoffs diffraction formula}}
[[Category:Waves]]
[[Category:Physical optics]]
[[Category:Diffraction]]
[[Category:Gustav Kirchhoff]]