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Diffraction

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(Redirected fromDiffraction pattern)

Phenomenon of the motion of waves

Not to be confused withrefraction, the change in direction of a wave passing from one medium to another.

Adiffraction pattern of a redlaser beam projected onto a plate after passing through a small circularaperture in another plate

Diffraction is the deviation ofwaves from straight-line propagation without any change in their energy due to an obstacle or through anaperture. The diffracting object or aperture effectively becomes a secondary source of thepropagating wave. Diffraction is the same physical effect asinterference, but interference is typically applied to superposition of a few waves and the term diffraction is used when many waves are superposed.^[1]^: 433

Italian scientistFrancesco Maria Grimaldi coined the worddiffraction and was the first to record accurate observations of the phenomenon in1660.

Infinitely many points (three shown) along length $d {\displaystyle d}$ project phase contributions from thewavefront, producing a continuously varying intensity $\theta$ on the registering plate

{\displaystyle d} — Infinitely many points (three shown) along length $d {\displaystyle d}$ project phase contributions from thewavefront, producing a continuously varying intensity $\theta$ on the registering plate

Inclassical physics, the diffraction phenomenon is described by theHuygens–Fresnel principle that treats each point in a propagatingwavefront as a collection of individual sphericalwavelets.^[2] The characteristic pattern is most pronounced when a wave from acoherent source (such as a laser) encounters a slit/aperture that is comparable in size to itswavelength, as shown in the inserted image. This is due to the addition, orinterference, of different points on the wavefront (or, equivalently, each wavelet) that travel by paths of different lengths to the registering surface. If there are multipleclosely spaced openings, a complex pattern of varying intensity can result.

These effects also occur when alight wave travels through a medium with a varyingrefractive index, or when asound wave travels through a medium with varyingacoustic impedance – all waves diffract,^[3] includinggravitational waves,^[4]water waves, and otherelectromagnetic waves such asX-rays andradio waves. Furthermore,quantum mechanics also demonstrates that matter possesseswave-like properties and, therefore, undergoes diffraction (which is measurable at subatomic to molecular levels).^[5]

History

Thomas Young's sketch of two-slit diffraction for waterripple tank from his1807Lectures^[6]^: 139

The effects of diffraction of light were first carefully observed and characterized byFrancesco Maria Grimaldi, who also coined the termdiffraction, from theLatindiffringere, 'to break into pieces', referring to light breaking up into different directions.^[7] The results of Grimaldi's observations were published posthumously in1665.^[8]^[9]Isaac Newton studied these effects and attributed them toinflexion of light rays.James Gregory (1638–1675) observed the diffraction patterns caused by a bird feather, which was effectively the firstdiffraction grating to be discovered.^[10]Thomas Young performed acelebrated experiment in1803 demonstrating interference from two closely spaced slits.^[11] Explaining his results by interference of the waves emanating from the two different slits, he deduced that light must propagate as waves.

In 1818, supporters of thecorpuscular theory of light proposed that theParis Academy prize question address diffraction, expecting to see the wave theory defeated. However,Augustin-Jean Fresnel took the prize with his new theory wave propagation,^[12] combining the ideas^[13] ofChristiaan Huygens with Young's interference concept.Siméon Denis Poisson challenged the Fresnel theory by showing that it predicted light in the shadow behind a circular obstruction;Dominique-François-Jean Arago proceeded to demonstrate experimentally that suchlight is visible, confirming Fresnel's diffraction model.^[14]^: xxiii^[15]

Mechanism

Single-slit diffraction in a circularripple tank

Inclassical physics diffraction arises because of howwaves propagate; this is described by theHuygens–Fresnel principle and theprinciple of superposition of waves. The propagation of a wave can be visualized by considering every particle of the transmitted medium on a wavefront as apoint source for a secondaryspherical wave. The wave displacement at any subsequent point is the sum of these secondary waves. When waves are added together, their sum is determined by the relativephases as well as theamplitudes of the individual waves so that the summed amplitude of the waves can have any value between zero and the sum of the individual amplitudes. Hence, diffraction patterns usually have a series of maxima and minima.

In themodern quantum mechanical understanding of light propagation through a slit (or slits) everyphoton is described by itswavefunction that determines theprobability distribution for the photon: the light and dark bands are the areas where the photons are more or less likely to be detected. The wavefunction is determined by the physical surroundings such as slit geometry, screen distance, and initial conditions when the photon is created. The wave nature of individual photons (as opposed to wave properties only arising from the interactions between multitudes of photons) was implied by a low-intensitydouble-slit experiment first performed byG. I. Taylor in1909. The quantum approach has some striking similarities to theHuygens-Fresnel principle; based on that principle, as light travels through slits and boundaries, secondary point light sources are created near or along these obstacles, and the resulting diffraction pattern is going to be the intensity profile based on the collective interference of all these light sources that have different optical paths. In the quantum formalism, that is similar to considering the limited regions around the slits and boundaries from which photons are more likely to originate, and calculating the probability distribution (that is proportional to the resulting intensity of classical formalism).

There are various analytical models for photons which allow the diffracted field to be calculated, including theKirchhoff diffraction equation (derived from thewave equation),^[16] theFraunhofer diffraction approximation of the Kirchhoff equation (applicable to thefar field), theFresnel diffraction approximation (applicable to thenear field) and the Feynmanpath integral formulation. Most configurations cannot be solved analytically, but can yield numerical solutions throughfinite element andboundary element methods. In many cases it is assumed that there is only one scattering event, what is calledkinematical diffraction, with anEwald's sphere construction used to represent that there is no change in energy during the diffraction process. For matter waves a similar but slightly different approach is used based upon a relativistically corrected form of theSchrödinger equation,^[17] as first detailed byHans Bethe.^[18] The Fraunhofer and Fresnel limits exist for these as well, although they correspond more to approximations for the matter waveGreen's function (propagator)^[19] for the Schrödinger equation.^[20]^[21] More common is full multiple scattering models particular inelectron diffraction;^[22] in some cases similardynamical diffraction models are also used for X-rays.^[23]

It is possible to obtain a qualitative understanding of many diffraction phenomena by considering how the relative phases of the individual secondary wave sources vary, and, in particular, the conditions in which the phase difference equals half a cycle in which case waves will cancel one another out.

The simplest descriptions of diffraction are those in which the situation can be reduced to a two-dimensional problem. Forwater waves, this is already the case; water waves propagate only on the surface of the water. For light, we can often neglect one direction if the diffracting object extends in that direction over a distance far greater than the wavelength. In the case of light shining through small circular holes, we will have to take into account the full three-dimensional nature of the problem.

$Computer-generated intensity pattern formed on a screen by diffraction from a square aperture$
Computer-generated intensity pattern formed on a screen by diffraction from a square aperture
$Generation of an interference pattern from two-slit diffraction$
Generation of an interference pattern from two-slit diffraction
$Computational model of an interference pattern from two-slit diffraction$
Computational model of an interference pattern from two-slit diffraction
$Optical diffraction pattern (laser, analogous to X-ray diffraction)$
Optical diffraction pattern (laser, analogous to X-ray diffraction)
$Colors seen in a spider web are partially due to diffraction, according to some analyses.[24]$
Colors seen in aspider web are partially due to diffraction, according to some analyses.^[24]

Examples

The effects of diffraction are often seen in everyday life. The most striking examples of diffraction are those that involve light; for example, the closely spaced tracks on a CD or DVD act as adiffraction grating to form the familiar rainbow pattern seen when looking at a disc.

Pixels on smart phone screen acting as diffraction grating

Data is written on CDs as pits and lands; the pits on the surface act as diffracting elements.

This principle can be extended to engineer a grating with a structure such that it will produce any diffraction pattern desired; thehologram on a credit card is an example.

Diffraction in the atmosphere by small particles can cause acorona - a bright disc and rings around a bright light source like the sun or the moon. At the opposite point one may also observeglory - bright rings around the shadow of the observer. In contrast to the corona, glory requires the particles to be transparent spheres (like fog droplets), since thebackscattering of the light that forms the glory involvesrefraction and internal reflection within the droplet.

Lunarcorona.

A solarglory, as seen from a plane on the underlying clouds.

A shadow of a solid object, using light from a compact source, shows small fringes near its edges.

The bright spot (Arago spot) seen in the center of the shadow of a circular obstacle is due to diffraction

Diffraction spikes are diffraction patterns caused due to non-circularaperture in camera or support struts in telescope; In normal vision, diffraction through eyelashes may produce such spikes.

View from the end of Millennium Bridge; Moon rising above the Southwark Bridge. Street lights are reflecting in the Thames.

Simulated diffraction spikes in hexagonal telescope mirrors

Thespeckle pattern which is observed when laser light falls on an optically rough surface is also a diffraction phenomenon. Whendeli meat appears to beiridescent, that is diffraction off the meat fibers.^[25] All these effects are a consequence of the fact that light propagates as awave.

Diffraction can occur with any kind of wave. Ocean waves diffract aroundjetties and other obstacles.

Circular waves generated by diffraction from the narrow entrance of a flooded coastal quarry

Sound waves can diffract around objects, which is why one can still hear someone calling even when hiding behind a tree.^[26]

Diffraction can also be a concern in some technical applications; it sets afundamental limit to the resolution of a camera, telescope, or microscope.

Other examples of diffraction are considered below.

Single-slit diffraction

Main article:Diffraction formalism

2D Single-slit diffraction with width changing animation

Numerical approximation of diffraction pattern from a slit of width four wavelengths with an incident plane wave. The main central beam, nulls, and phase reversals are apparent.

Graph and image of single-slit diffraction

A long slit of infinitesimal width which is illuminated by light diffracts the light into a series of circular waves and the wavefront which emerges from the slit is a cylindrical wave of uniform intensity, in accordance with theHuygens–Fresnel principle.

An illuminated slit that is wider than a wavelength produces interference effects in the space downstream of the slit. Assuming that the slit behaves as though it has a large number of point sources spaced evenly across the width of the slit interference effects can be calculated. The analysis of this system is simplified if we consider light of a single wavelength. If the incident light iscoherent, these sources all have the same phase. Light incident at a given point in the space downstream of the slit is made up of contributions from each of these point sources and if the relative phases of these contributions vary by $2\pi$ or more, we may expect to find minima and maxima in the diffracted light. Such phase differences are caused by differences in the path lengths over which contributing rays reach the point from the slit.

We can find the angle at which a first minimum is obtained in the diffracted light by the following reasoning. The light from a source located at the top edge of the slit interferes destructively with a source located at the middle of the slit, when the path difference between them is equal to $\lambda /2$ . Similarly, the source just below the top of the slit will interfere destructively with the source located just below the middle of the slit at the same angle. We can continue this reasoning along the entire height of the slit to conclude that the condition for destructive interference for the entire slit is the same as the condition for destructive interference between two narrow slits a distance apart that is half the width of the slit. The path difference is approximately ${\frac {d\sin(\theta )}{2}}$ so that the minimum intensity occurs at an angle $\theta _{\text{min}}$ given by $d\,\sin \theta _{\text{min}}=\lambda ,$ where $d {\displaystyle d}$ is the width of the slit, $\theta _{\text{min}}$ is theangle of incidence at which the minimum intensity occurs, and $\lambda$ is the wavelength of the light.

A similar argument can be used to show that if we imagine the slit to be divided into four, six, eight parts, etc., minima are obtained at angles $\theta _{n}$ given by $d\,\sin \theta _{n}=n\lambda ,$ where $n {\displaystyle n}$ is an integer other than zero.

There is no such simple argument to enable us to find the maxima of the diffraction pattern. Theintensity profile can be calculated using theFraunhofer diffraction equation as $I(\theta )=I_{0}\,\operatorname {sinc} ^{2}\left({\frac {d\pi }{\lambda }}\sin \theta \right),$ where $I(\theta )$ is the intensity at a given angle, $I_{0}$ is the intensity at the central maximum( $\theta =0$ ), which is also a normalization factor of the intensity profile that can be determined by an integration from ${\textstyle \theta =-{\frac {\pi }{2}}}$ to ${\textstyle \theta ={\frac {\pi }{2}}}$ and conservation of energy, and $\operatorname {sinc} x={\frac {\sin x}{x}}$ , which is theunnormalized sinc function.

This analysis applies only to thefar field (Fraunhofer diffraction), that is, at a distance much larger than the width of the slit.

From theintensity profile above, if $d\ll \lambda$ , the intensity will have little dependency on $\theta$ , hence the wavefront emerging from the slit would resemble a cylindrical wave with azimuthal symmetry; If $d\gg \lambda$ , only $\theta \approx 0$ would have appreciable intensity, hence the wavefront emerging from the slit would resemble that ofgeometrical optics.

When the incident angle $\theta _{\text{i}}$ of the light onto the slit is non-zero (which causes a change in thepath length), the intensity profile in the Fraunhofer regime (i.e. far field) becomes: $I(\theta )=I_{0}\,\operatorname {sinc} ^{2}\left[{\frac {d\pi }{\lambda }}(\sin \theta \pm \sin \theta _{\text{i}})\right]$

The choice of plus/minus sign depends on the definition of the incident angle $\theta _{\text{i}}$ .

2-slit (top) and 5-slit diffraction of red laser light

Diffraction of a red laser using a diffraction grating

A diffraction pattern of a 633 nm laser through a grid of 150 slits

Diffraction grating

Main article:Diffraction grating

Diffraction grating

A diffraction grating is an optical component with a regular pattern. The form of the light diffracted by a grating depends on the structure of the elements and the number of elements present, but all gratings have intensity maxima at anglesθ_m which are given by the grating equation $d\left(\sin {\theta _{m}}\pm \sin {\theta _{i}}\right)=m\lambda ,$ where $\theta _{i}$ is the angle at which the light is incident, $d {\displaystyle d}$ is the separation of grating elements, and $m {\displaystyle m}$ is an integer which can be positive or negative.

The light diffracted by a grating is found by summing the light diffracted from each of the elements, and is essentially aconvolution of diffraction and interference patterns.

The figure shows the light diffracted by 2-element and 5-element gratings where the grating spacings are the same; it can be seen that the maxima are in the same position, but the detailed structures of the intensities are different.

Circular aperture

Main article:Airy disk

A computer-generated image of anAiry disk

Diffraction pattern from a circular aperture at various distances

The far-field diffraction of a plane wave incident on a circular aperture is often referred to as theAiry disk. Thevariation in intensity with angle is given by $I(\theta )=I_{0}\left({\frac {2J_{1}(ka\sin \theta )}{ka\sin \theta }}\right)^{2},$ where $a {\displaystyle a}$ is the radius of the circular aperture, $k {\displaystyle k}$ is equal to $2\pi /\lambda$ and $J_{1}$ is aBessel function. The smaller the aperture, the larger the spot size at a given distance, and the greater the divergence of the diffracted beams.

General aperture

The wave that emerges from a point source has amplitude $\psi$ at location $\mathbf {r}$ that is given by the solution of thefrequency domain wave equation for a point source (theHelmholtz equation), $\nabla ^{2}\psi +k^{2}\psi =\delta (\mathbf {r} ),$ where $\delta (\mathbf {r} )$ is the 3-dimensional delta function. The delta function has only radial dependence, so theLaplace operator (a.k.a. scalar Laplacian) in thespherical coordinate system simplifies to $\nabla ^{2}\psi ={\frac {1}{r}}{\frac {\partial ^{2}}{\partial r^{2}}}(r\psi ).$

(Seedel in cylindrical and spherical coordinates.) By direct substitution, the solution to this equation can be readily shown to be the scalarGreen's function, which in thespherical coordinate system (and using the physics time convention $e^{-i\omega t}$ ) is $\psi (r)={\frac {e^{ikr}}{4\pi r}}.$

This solution assumes that the delta function source is located at the origin. If the source is located at an arbitrary source point, denoted by the vector $\mathbf {r} '$ and the field point is located at the point $\mathbf {r}$ , then we may represent the scalarGreen's function (for arbitrary source location) as $\psi (\mathbf {r} |\mathbf {r} ')={\frac {e^{ik|\mathbf {r} -\mathbf {r} '|}}{4\pi |\mathbf {r} -\mathbf {r} '|}}.$

Therefore, if an electric field $E_{\mathrm {inc} }(x,y)$ is incident on the aperture, the field produced by this aperture distribution is given by thesurface integral $\Psi (r)\propto \iint \limits _{\mathrm {aperture} }\!\!E_{\mathrm {inc} }(x',y')~{\frac {e^{ik|\mathbf {r} -\mathbf {r} '|}}{4\pi |\mathbf {r} -\mathbf {r} '|}}\,dx'\,dy',$

On the calculation of Fraunhofer region fields

where the source point in the aperture is given by the vector $\mathbf {r} '=x'\mathbf {\hat {x}} +y'\mathbf {\hat {y}} .$

In the far field, wherein the parallel rays approximation can be employed, the Green's function, $\psi (\mathbf {r} |\mathbf {r} ')={\frac {e^{ik|\mathbf {r} -\mathbf {r} '|}}{4\pi |\mathbf {r} -\mathbf {r} '|}},$ simplifies to $\psi (\mathbf {r} |\mathbf {r} ')={\frac {e^{ikr}}{4\pi r}}e^{-ik(\mathbf {r} '\cdot \mathbf {\hat {r}} )}$ as can be seen in the adjacent figure.

The expression for the far-zone (Fraunhofer region) field becomes $\Psi (r)\propto {\frac {e^{ikr}}{4\pi r}}\iint \limits _{\mathrm {aperture} }\!\!E_{\mathrm {inc} }(x',y')e^{-ik(\mathbf {r} '\cdot \mathbf {\hat {r}} )}\,dx'\,dy'.$

Now, since $\mathbf {r} '=x'\mathbf {\hat {x}} +y'\mathbf {\hat {y}}$ and $\mathbf {\hat {r}} =\sin \theta \cos \phi \mathbf {\hat {x}} +\sin \theta ~\sin \phi ~\mathbf {\hat {y}} +\cos \theta \mathbf {\hat {z}} ,$ the expression for the Fraunhofer region field from a planar aperture now becomes $\Psi (r)\propto {\frac {e^{ikr}}{4\pi r}}\iint \limits _{\mathrm {aperture} }\!\!E_{\mathrm {inc} }(x',y')e^{-ik\sin \theta (\cos \phi x'+\sin \phi y')}\,dx'\,dy'.$

Letting $k_{x}=k\sin \theta \cos \phi$ and $k_{y}=k\sin \theta \sin \phi \,,$ the Fraunhofer region field of the planar aperture assumes the form of aFourier transform $\Psi (r)\propto {\frac {e^{ikr}}{4\pi r}}\iint \limits _{\mathrm {aperture} }\!\!E_{\mathrm {inc} }(x',y')e^{-i(k_{x}x'+k_{y}y')}\,dx'\,dy',$

In the far-field / Fraunhofer region, this becomes the spatialFourier transform of the aperture distribution. Huygens' principle when applied to an aperture simply says that thefar-field diffraction pattern is the spatial Fourier transform of the aperture shape, and this is a direct by-product of using the parallel-rays approximation, which is identical to doing a plane wave decomposition of the aperture plane fields (seeFourier optics).

Propagation of a laser beam

The way in which the beam profile of alaser beam changes as it propagates is determined by diffraction. When the entire emitted beam has a planar, spatiallycoherent wave front, it approximatesGaussian beam profile and has the lowest divergence for a given diameter. The smaller the output beam, the quicker it diverges. It is possible to reduce the divergence of a laser beam by first expanding it with oneconvex lens, and then collimating it with a second convex lens whose focal point is coincident with that of the first lens. The resulting beam has a larger diameter, and hence a lower divergence. Divergence of a laser beam may be reduced below the diffraction of a Gaussian beam or even reversed to convergence if the refractive index of the propagation media increases with the light intensity.^[27] This may result in aself-focusing effect.

When the wave front of the emitted beam has perturbations, only the transverse coherence length (where the wave front perturbation is less than 1/4 of the wavelength) should be considered as a Gaussian beam diameter when determining the divergence of the laser beam. If the transverse coherence length in the vertical direction is higher than in horizontal, the laser beam divergence will be lower in the vertical direction than in the horizontal.

Diffraction-limited imaging

Main article:Diffraction-limited system

The Airy disk around each of the stars from the 2.56 m telescope aperture can be seen in thislucky image of thebinary star zeta Boötis.

The ability of an imaging system to resolve detail is ultimately limited bydiffraction. This is because a plane wave incident on a circular lens or mirror is diffracted as described above. The light is not focused to a point but forms an Airy disk having a central spot in the focal plane whose radius (as measured to the first null) is $\Delta x=1.22\lambda N,$ where $\lambda$ is the wavelength of the light and $N {\displaystyle N}$ is thef-number (focal length $f {\displaystyle f}$ divided by aperture diameter $D {\displaystyle D}$ ) of the imaging optics; this is strictly accurate for $N\gg 1$ (paraxial case). In object space, the correspondingangular resolution is $\theta \approx \sin \theta =1.22{\frac {\lambda }{D}},$ where $D {\displaystyle D}$ is the diameter of theentrance pupil of the imaging lens (e.g., of a telescope's main mirror).

Two point sources will each produce an Airy pattern – see the photo of a binary star. As the point sources move closer together, the patterns will start to overlap, and ultimately they will merge to form a single pattern, in which case the two point sources cannot be resolved in the image. TheRayleigh criterion specifies that two point sources are considered "resolved" if the separation of the two images is at least the radius of the Airy disk, i.e. if the first minimum of one coincides with the maximum of the other.

Thus, the larger the aperture of the lens compared to the wavelength, the finer the resolution of an imaging system. This is one reason astronomical telescopes require large objectives, and whymicroscope objectives require a largenumerical aperture (large aperture diameter compared to working distance) in order to obtain the highest possible resolution.

Speckle patterns

Main article:Speckle pattern

Thespeckle pattern seen when using alaser pointer is another diffraction phenomenon. It is a result of the superposition of many waves with different phases, which are produced when a laser beam illuminates a rough surface. They add together to give a resultant wave whose amplitude, and therefore intensity, varies randomly.

Babinet's principle

Main article:Babinet's principle

Babinet's principle is a useful theorem stating that the diffraction pattern from an opaque body is identical to that from a hole of the same size and shape, but with differing intensities. This means that the interference conditions of a single obstruction would be the same as that of a single slit.

"Knife edge"

Theknife-edge effect orknife-edge diffraction is a truncation of a portion of the incidentradiation that strikes a sharp well-defined obstacle, such as a mountain range or the wall of a building.The knife-edge effect is explained by theHuygens–Fresnel principle, which states that a well-defined obstruction to an electromagnetic wave acts as a secondary source, and creates a newwavefront. This new wavefront propagates into the geometric shadow area of the obstacle.

Knife-edge diffraction is an outgrowth of the "half-plane problem", originally solved byArnold Sommerfeld using a plane wave spectrum formulation. A generalization of the half-plane problem is the "wedge problem", solvable as a boundary value problem in cylindrical coordinates. The solution in cylindrical coordinates was then extended to the optical regime byJoseph B. Keller, who introduced the notion of diffraction coefficients through hisgeometrical theory of diffraction (GTD). In 1974, Prabhakar Pathak andRobert Kouyoumjian extended the (singular) Keller coefficients via theuniform theory of diffraction (UTD).^[28]^[29]

$Diffraction on a sharp metallic edge$
Diffraction on a sharp metallic edge
$Diffraction on a soft aperture, with a gradient of conductivity over the image width$
Diffraction on a soft aperture, with a gradient of conductivity over the image width

Patterns

The upper half of this image shows a diffraction pattern of He-Ne laser beam on an elliptic aperture. The lower half is its 2D Fourier transform approximately reconstructing the shape of the aperture.

Several qualitative observations can be made of diffraction in general:

The angular spacing of the features in the diffraction pattern is inversely proportional to the dimensions of the object causing the diffraction. In other words: The smaller the diffracting object, the 'wider' the resulting diffraction pattern, and vice versa. (More precisely, this is true of thesines of the angles.)
The diffraction angles are invariant under scaling; that is, they depend only on the ratio of the wavelength to the size of the diffracting object.
When the diffracting object has a periodic structure, for example in a diffraction grating, the features generally become sharper. The third figure, for example, shows a comparison of adouble-slit pattern with a pattern formed by five slits, both sets of slits having the same spacing, between the center of one slit and the next.

Matter wave diffraction

See also:Matter wave,Neutron diffraction, andElectron diffraction

According to quantum theory every particle exhibits wave properties and can therefore diffract. Diffraction of electrons and neutrons is one of the powerful arguments in favor of quantum mechanics. The wavelength associated with a non-relativistic particle is thede Broglie wavelength $\lambda ={\frac {h}{p}}\,,$ where $h {\displaystyle h}$ is thePlanck constant and $p {\displaystyle p}$ is themomentum of the particle (mass × velocity for slow-moving particles). For example, a sodium atom traveling at about 300 m/s would have a de Broglie wavelength of about 50 picometres.

Diffraction ofmatter waves has been observed for small particles, like electrons, neutrons, atoms, and even large molecules. The short wavelength of these matter waves makes them ideally suited to study the atomic structure of solids, molecules and proteins.

Bragg diffraction

Further information:Bragg diffraction

FollowingBragg's law, each dot (orreflection) in this diffraction pattern forms from the constructive interference of X-rays passing through a crystal. The data can be used to determine the crystal's atomic structure.

Diffraction from a large three-dimensional periodic structure such as many thousands of atoms in a crystal is calledBragg diffraction.It is similar to what occurs when waves are scattered from adiffraction grating. Bragg diffraction is a consequence of interference between waves reflecting from many different crystal planes.The condition of constructive interference is given byBragg's law: $m\lambda =2d\sin \theta ,$ where $\lambda$ is the wavelength, $d {\displaystyle d}$ is the distance between crystal planes, $\theta$ is the angle of the diffracted wave, and $m {\displaystyle m}$ is an integer known as theorder of the diffracted beam.

Bragg diffraction may be carried out using either electromagnetic radiation of very short wavelength likeX-rays or matter waves likeneutrons (andelectrons) whose wavelength is on the order of (or much smaller than) the atomic spacing.^[30] The pattern produced gives information of the separations of crystallographic planes $d {\displaystyle d}$ , allowing one to deduce the crystal structure.

For completeness, Bragg diffraction is a limit for a large number of atoms with X-rays or neutrons, and is rarely valid forelectron diffraction or with solid particles in the size range of less than 50 nanometers.^[30]

Coherence

Main article:Coherence (physics)

The description of diffraction relies on the interference of waves emanating from the same source taking different paths to the same point on a screen. In this description, the difference in phase between waves that took different paths is only dependent on the effective path length. This does not take into account the fact that waves that arrive at the screen at the same time were emitted by the source at different times. The initial phase with which the source emits waves can change over time in an unpredictable way. This means that waves emitted by the source at times that are too far apart can no longer form a constant interference pattern since the relation between their phases is no longer time independent.^[31]^: 919

The length over which the phase in a beam of light is correlated is called thecoherence length. In order for interference to occur, the path length difference must be smaller than the coherence length. This is sometimes referred to as spectral coherence, as it is related to the presence of different frequency components in the wave. In the case of light emitted by anatomic transition, the coherence length is related to the lifetime of the excited state from which the atom made its transition.^[32]^: 71–74^[1]^{: 314–316}

If waves are emitted from an extended source, this can lead to incoherence in the transversal direction. When looking at a cross section of a beam of light, the length over which the phase is correlated is called the transverse coherence length. In the case of Young's double-slit experiment, this would mean that if the transverse coherence length is smaller than the spacing between the two slits, the resulting pattern on a screen would look like two single-slit diffraction patterns.^[32]^: 74–79

In the case of particles like electrons, neutrons, and atoms, the coherence length is related to the spatial extent of the wave function that describes the particle.^[33]^: 107

Applications

Diffraction before destruction

A new way to image single biological particles has emerged since the 2010s, utilising the bright X-rays generated by X-rayfree-electron lasers. These femtosecond-duration pulses will allow for the (potential) imaging of single biological macromolecules. Due to these short pulses, radiation damage can be outrun, and diffraction patterns of single biological macromolecules will be able to be obtained.^[34]^[35]

See also

References

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Original : Nobis alius quartus modus illuxit, quem nunc proponimus, vocamusque; diffractionem, quia advertimus lumen aliquando diffringi, hoc est partes eius multiplici dissectione separatas per idem tamen medium in diversa ulterius procedere, eo modo, quem mox declarabimus.
Translation : It has illuminated for us another, fourth way, which we now make known and call "diffraction" [i.e., shattering], because we sometimes observe light break up; that is, that parts of the compound [i.e., the beam of light], separated by division, advance farther through the medium but in different [directions], as we will soon show.
^Cajori, Florian"A History of Physics in its Elementary Branches, including the evolution of physical laboratories."Archived 2016-12-01 at theWayback Machine MacMillan Company, New York 1899
^Letter from James Gregory to John Collins, dated 13 May 1673. Reprinted in:Correspondence of Scientific Men of the Seventeenth Century …, ed. Stephen Jordan Rigaud (Oxford, England:Oxford University Press, 1841), vol. 2, pp. 251–255, especiallyp. 254 Archived 2016-12-01 at theWayback Machine.
^Thomas Young (1 January 1804)."The Bakerian Lecture: Experiments and calculations relative to physical optics".Philosophical Transactions of the Royal Society of London.94:1–16.Bibcode:1804RSPT...94....1Y.doi:10.1098/rstl.1804.0001.S2CID 110408369.. (Note: This lecture was presented before the Royal Society on 24 November 1803.)
^Fresnel, Augustin-Jean (1818), "Mémoire sur la diffraction de la lumière" ("Memoir on the diffraction of light"), deposited 29 July 1818, "crowned" 15 March 1819, published inMémoires de l'Académie Royale des Sciences de l'Institut de France, vol. V (for 1821 & 1822, printed 1826),pp. 339–475; reprinted inOeuvres complètes d'Augustin Fresnel, vol. 1 (Paris: Imprimerie Impériale, 1866),pp. 247–364; partly translated as"Fresnel's prize memoir on the diffraction of light", in H. Crew (ed.),The Wave Theory of Light: Memoirs by Huygens, Young and Fresnel, American Book Company, 1900, pp. 81–144. (First published, as extracts only, inAnnales de Chimie et de Physique, vol. 11 (1819), pp. 246–96,337–78.)
^Christiaan Huygens,Traité de la lumiere …Archived 2016-06-16 at theWayback Machine (Leiden, Netherlands: Pieter van der Aa, 1690), Chapter 1. Fromp. 15 Archived 2016-12-01 at theWayback Machine:"J'ay donc monstré de quelle façon l'on peut concevoir que la lumiere s'etend successivement par des ondes spheriques, … " (I have thus shown in what manner one can imagine that light propagates successively by spherical waves, … ) (Note: Huygens published hisTraité in 1690; however, in the preface to his book, Huygens states that in 1678 he first communicated his book to the French Royal Academy of Sciences.)
^Born, Max; Wolf, Emil (1980).Principles of optics: electromagnetic theory of propagation, interference and diffraction of light (6 ed.). Oxford New York: Pergamon Press.ISBN 978-0-08-026482-0.
^Sir David Brewster (1831).A Treatise on Optics. London: Longman, Rees, Orme, Brown & Green and John Taylor. pp. 95.
^Baker, B.B. & Copson, E.T. (1939),The Mathematical Theory of Huygens' Principle, Oxford, pp. 36–40.
^Schrödinger, E. (1926)."An Undulatory Theory of the Mechanics of Atoms and Molecules".Physical Review.28 (6):1049–1070.Bibcode:1926PhRv...28.1049S.doi:10.1103/PhysRev.28.1049.
^Bethe, H. (1928)."Theorie der Beugung von Elektronen an Kristallen".Annalen der Physik.392 (17):55–129.Bibcode:1928AnP...392...55B.doi:10.1002/andp.19283921704.ISSN 1521-3889.
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^Li, Kenan; Wojcik, Michael; Jacobsen, Chris (6 February 2017)."Multislice does it all—calculating the performance of nanofocusing X-ray optics".Optics Express.25 (3):1831–1846.Bibcode:2017OExpr..25.1831L.doi:10.1364/OE.25.001831.ISSN 1094-4087.PMID 29519036.
^Dietrich Zawischa."Optical effects on spider webs". Retrieved21 September 2007.
^Arumugam, Nadia (9 September 2013)."Food Explainer: Why Is Some Deli Meat Iridescent?".Slate.The Slate Group.Archived from the original on 10 September 2013. Retrieved9 September 2013.
^Andrew Norton (2000).Dynamic fields and waves of physics. CRC Press. p. 102.ISBN 978-0-7503-0719-2.
^Chiao, R. Y.; Garmire, E.; Townes, C. H. (1964)."Self-Trapping of Optical Beams".Physical Review Letters.13 (15):479–482.Bibcode:1964PhRvL..13..479C.doi:10.1103/PhysRevLett.13.479.
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