Wave optics plays an important role in physics both in theory and application.Molecules constituting macro materials in the light field or otherAlternating electric fieldUnder the action ofdipole, sending the same frequencySecondary wave("New Light").This model is used to explain the absorption, dispersionscattering, as well as magneto-optical, electro-optic and other phenomena, and even the emission of light is also the content of general wave optics.
Wave optics is a very important part of optics, includingInterference of light, opticaldiffraction, opticalpolarizationBoth theory and application play an important role in physics.
The objects around us (glass, mirror, transparent material, etc.) are composed of molecules, which in turn are composed of atoms.For example, water is composed of water molecules, while a water molecule is composed of one oxygen atom and two hydrogen atoms.Atoms are composed of positively charged nuclei and negatively charged extranuclear electrons. Charged particles will move under the force of electric field in the electric field.Since molecule is a system composed of several charged particles, and light is an electromagnetic wave of a specific frequency band (see the picture), when we incident a beam of light into the medium, its oscillating electric and magnetic fields will affect the movement of charged particles in the medium molecule, causing the charge in the molecule to vibrate under the action of electromagnetic waves,This vibration will cause electrons to radiate new electromagnetic waves of the same frequency, forming oscillating electric dipole.Generally speaking, when the medium interacts with light, it is equivalent to a secondary light source, generating new light with the same frequency as the original light, and overlapping with the original light.
Molecule is a system composed of several charged particles
Light is an electromagnetic wave with a specific frequency range
The above is the explanation of optical phenomena in classical physics.Use such a model to illustratePhotogenicdispersion、absorb, scattering, andMagneto-opticalEffectlightningEffect, even lightlaunchIt is also the content of general wave optics.Electromagnetic wave theoryThe discipline applied to crystals is calledCrystal optics。light waveThe wavelength in vacuum is about (3.9~7.6) × 10-5cm，GeneralobstacleOr the porosity is much larger than this, so it usually showsLinear propagation of lightPhenomenon.During this period, people also found someVolatilityRelevant optical phenomena, such as FM.GrimaldiFirst, he found that the light will deviate from the straight line when encountering obstacles. He named this phenomenon“diffraction”。Hooke and RBoyle It is observed thatNewtonian ringOfInterference phenomenon。These discoveries became the starting point of the history of wave optics.Over a hundred years after the 17th century,Corpuscular theory of light(SeeDuality of light）Has always dominated,Fluctuation theoryThe wave theory of light was not accepted by most people until the 19th century.[1]
Crystal optics
Detailed introduction
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eighteenth century
In 1800, T. Yang put forward several arguments against the particle theory, first proposed the term interference, and analyzed water waves andacoustic waveGenerated after stackingInterference phenomenon。In 1801, Yang was the first to demonstrate with double stitchesInterference of lightPhenomenon (see Young's experiment), first proposed the concept of wavelength, and successfully measuredlight waveWavelength.He also used the interference principle to explain the color of the film under white light irradiation.In 1809, E. LMariusFound the reflectionpolarizationPhenomenon (seeBrewster's law）, followed by A-J.fresnelAnd DF. J.Arago Using Yang's experimental deviceLinearly polarized lightYang and Fresnel used light totransverse waveThe experiment was successfully explained by the hypothesis of.In 1815, Fresnel establishedHuygens Fresnel principle, he used this principle to calculate thediffractionThe pattern convincingly explains the diffraction phenomenon.In 1818, the argument about the Aragor spot (see Fresnel diffraction) intensifiedFresnel diffraction The position of theory.So far, the wave theory of light is used to explainInterference of light, diffraction and polarization, which firmly established thewave theory Status.[2]
nineteenth century
In the 1860s, J. CmaxwellUnifiedElectromagnetic field theory, predicted the existence of electromagnetic waves and gave thewave velocityFormula.Then HR.hertzElectromagnetic waves are generated by experiment.Light andElectromagnetic phenomenonIt is believed that light is a kind of electromagnetic waveElectromagnetic theoryIt was fused into one body and produced the electromagnetic theory of light.The electromagnetic theory is applied to crystal, and the propagation law of light in crystal is explained strictly and satisfactorily.At the end of the 19th century, H. ALorentz FoundedElectronic theoryHe attributed the macroscopic properties of matter to the electronicCollective behavior(Since the atomic mass is mainly distributed on the atomic nucleus, the interaction between electromagnetic waves and electrons is mainly considered).The action of electromagnetic wave makes the internal charge of material moleculeparticlehappenForced vibrationAnd generate secondary electromagnetic waves of the same frequency.According to this theory, he explainedAbsorption of light、dispersionAnd scattering, etcMolecular opticsPhenomenon.This classical electromagnetic theory is not perfect, because the problem about the interaction between light and matter involves the behavior of microscopic particles, which must beQuantum TheoryCan be completely solved.
Polarizer
The research results of wave optics have deepened people's understanding of the nature of light.In the application field, interferometry based on the interference principle providesprecision measurement And means of inspection (seeInterferometer）, its precision has been improved to an unprecedented degree;diffractionThe theory points out that improving optical instrumentsDiscriminative ability(see Fraunhofer diffraction);diffraction grating Has become detachedSpectral lineTo proceedspectral analysisThe importance ofDispersive element；variouspolarizationDevices and instruments are used to inspect and measure rock and mineral crystals, etc.All these constitute the main content of applied optics.[2]
Twentieth Century
Since the 1950s, especially after the advent of lasers, wave optics has been derivedFourier optics、Fiber opticsandnonlinear optics And other new branches, greatly expanding the research and application range of wave optics.[2]
History
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seventeenth century
Newton
Since the 17th century, it has been found thatLinear propagation of lightA fact that does not fully correspond.ItalianNicholas Grimald (1618 ～ 1663)Diffraction of lightPhenomenon, he found thatPoint sourceUnder the irradiation of, the shadow formed by a straight rod is slightly wider than the width of the light assumed to travel in a straight line, that is, the light does not travel strictly in a straight line, but will bypassobstacleforward.Then, between 1672 and 1675, Hooke (1635 to 1703) also observed thatDiffraction phenomenon, andBoyle (1627~1691) independently studied the color interference fringes produced by the film, all of which are the seeds of the wave theory of light.In the second half of the seventeenth century,Newton(1642 ～ 1727) andHuygens(1629~1695) and so on led the research of light to the road of further development.Newton's white light experiment and the discovery of Newton's circle made the opticsgeometrical optics Enter wave optics.Huygens first put forwardWave theory of light。stayOn Light(1690), he believes that the movement of light is not the movement of material particles butmediumThe movement of is wave motionFluctuation theoryHe explained it wellReflection of light, refraction andcalciteOfBirefringencePhenomenon.[2]
nineteenth century
Optics in the 19th century was developed by British doctorsThomas YoungTo reviveFluctuation theoryThis is the prelude to our paper.In 1801, Yang read a paper on the color of the sheet to the Royal Society, in which the interference principle was formally introduced into optics and used to explain the color and fringe surface on the sheetdiffraction。In this paper, Yang also systematically proposed the basic principle of wave opticslight waveThe concept of "long" and the results of determination are given.Because the light wavelength is too shortobstacleThe ability to turn is not great, which is also the reason why it is difficult for people to observe this kind of phenomenon.In 1803, he also published the experiment and calculation of physical opticsDouble slit interferenceThe phenomenon is further explained.Published in 1807Lectures on Natural PhilosophyYang systematically expounded the basic principle of wave optics he proposed.
French physicist Fresnel (1788-1827) was almost independent in proposing the wave theory.In 1815, he submitted his first optical paper to the Academy of Sciences, in which he carefully studiedDiffraction of lightPhenomenon, and proposedInterference of lightPrinciple.Later, Fresnel and Yang worked together to make a large number of optical experiments based on wave theory, which made great progress in wave optics in the 19th century.[2]
Refraction phenomenon
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Refraction of light
Diffraction of lightIt's lightVolatilityOne of the important signs of lightDiffraction phenomenon, further revealing the wave nature of light.meanwhilediffractionIt is also the basis for discussing modern optical problems.The wave shows diffraction phenomenon in its propagation, which does not propagate along a straight line but in all directionsdiffractionPhenomenon.People inside and outside the window can hear each other's voice even though they don't see each other, which meansacoustic wave（Mechanical waveIt can spread around the edge of the window.Water waves can also bypassobstacleCommunication.Radio waves can go around mountain obstacles, so that the mountain can also receive radio broadcasts.These phenomena indicate that when a wave encounters an obstacle, it will deviate from the straight line. This phenomenon is calledWave diffraction。
The phenomenon of light passing from one medium to another and deviating from the straight line is calledRefraction of light。[3]
Propagation of light
Propagation of lightIt seems to be carried out along a straight line, and encountered opaqueobstacleIt will project a clear shadow, rough look,diffractionAnd linear propagation seem to contradict each other.
Interference of light
Diffraction of polychromatic light
Interference of lightThe phenomenon is the result of the superposition of several beams of light.In fact, even if a single beam of light is projected on the screen, after careful observation, there are light and dark stripes.For example, putYoung's interference experimentOne of the two small holes on the diaphragm in the device is shielded, so thatPoint sourceThe emitted light shines on the screen through a single hole. When carefully observed, it can be seen that the bright area on the screenLinear propagation of lightIt is estimated to be much larger, and the light and shade also appearuniform distributionIlluminance.Light passing through a slit, or even passing through the edge of any object, is similar to each other in varying degrees.Take a thin metal wire (as lightobstacle）It is placed in the front of the screen. It should be the darkest place in the middle of the shadow, but actually it is bright. This light bypasses the obstacles and deviates from the straight line to enter the geometric shadow, and the uneven distribution of light intensity on the screen is calledDiffraction of light。
The discovery of light diffraction, andLinear propagation of lightThe phenomenon is contradictory. If we can't explain these two points uniformly from the viewpoint of fluctuation, it is difficult to establish the lightVolatilityConcept.in fact,Mechanical waveThere is also the phenomenon of linear propagation.exceedacoustic waveHas obvious directionality.When ordinary sound waves encounter huge obstacles, they will also project clear shadows. For example, the sound in front cannot be heard behind tall walls.At the seaportbreakwaterInside, huge waves cannot reach.Microwave generally also travels in a straight line.Diffraction phenomenonThe appearance of is mainly determined byobstacledimensionAnd the size of the wavelength.Only when the linearity and wavelength of the obstacle are comparable, the diffraction phenomenon will be obvious.The wavelength of sound waves can reach tens of meters, and the wavelength of radio waves can reach hundreds of meters. The obstacles they encounter are usually much smaller than the wavelength, so they can bypass these obstacles to reach different angles during the transmission.Once encountering huge obstacles, linear propagation is more obvious.exceedacoustic waveWavelength ofOrder of magnitudeThe small ones are only a few millimeters, and the order of magnitude of the microwave wavelength is similar to this. The obstacles usually encountered are far larger than this, so they can generally be regarded as straight line propagation.[3]
Wavelength of light wave
light waveThe wavelength in vacuum is about (3.9~7.6) × 10-5cm ，GeneralobstacleOr the porosity is much larger than this, so it usually shows the phenomenon of linear propagation of light.Once it is similar to the wavelengthOrder of magnitudeWhen there are obstacles or pores,Diffraction phenomenonIt becomes obvious.[3]
Diffraction phenomenon
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PhotogenicdiffractionIt is one of the important signs of the wave nature of light. The diffraction phenomenon of light in the process of propagation further reveals the wave nature of light.At the same time, diffraction is also the basis for discussing modern optical problems.
The wave shows the diffraction phenomenon in the propagation, which does not propagate along a straight line but diffracts in all directions.People inside and outside the window can hear each other's voice even though they don't see each other, which meansacoustic wave(mechanical waves) can travel around the edge of a window.Water waves can also travel around obstacles on the water.Radio waves can go around mountain obstacles, so that the mountain can also receive radio broadcasts.These phenomena show that when a wave encounters an obstacle, it will deviate from the straight line, which is called wave diffraction.
The propagation of light seems to be along a straight line. When encountering an opaque obstacle, it will project a clear shadow. Roughly, diffraction and linear propagation seem to be contradictory phenomena.
PhotogenicInterference phenomenonIt is the result of superposition of several beams of light.In fact, even if a single beam of light is projected on the screen, after careful observation, there are light and dark stripes.For example, mask one of the two small holes on the diaphragm in the Young's interference experimental device, so thatlight sourceThe emitted light shines on the screen through a single hole. When carefully observed, it can be seen that the bright area on the screen is much larger than estimated based on the linear propagation of light, and there is also an uneven distribution of illuminance.Light passing through a slit, or even passing through the edge of any object, is similar to each other in varying degrees.Put oneMetalThe thin line (as an obstacle to light) is placed in front of the screen. It should be the darkest place in the middle of the shadow, but actually it is bright. This kind of light bypasses the obstacle and deviates from the straight line to enter the geometric shadow, and the phenomenon of uneven distribution of light intensity on the screen is called light diffraction.
The discovery of light diffraction phenomenon is contradictory to the phenomenon of straight line propagation of light. If these two points cannot be explained uniformly from the viewpoint of wave, it is difficult to establish the concept of wave of light.in fact,Mechanical waveThere is also the phenomenon of linear propagation.ultrasonicHas obvious directionality.When ordinary sound waves encounter huge obstacles, they will also project clear shadows, for example, when they are tallwallYou can't hear the sound from the front.In the harbor breakwater, huge waves cannot reach.microwaveIt also spreads in a straight line.Diffraction phenomenonThe appearance or not mainly depends on the contrast between the obstacle linearity and wavelength.Only when the obstacle linearity andwavelengthWhen it can be compared, the diffraction phenomenon is obvious.The wavelength of sound waves can reach tens of meters, and the wavelength of radio waves can reach hundreds of meters. The obstacles they encounter are usually much smaller than the wavelength, so they canspreadYou can bypass these obstacles and reach different angles on the way.Once encountering huge obstacles, linear propagation is more obvious.The wavelength of ultrasonic waves is only a few millimeters in order of magnitude, and the wavelength of microwave waves is also similar to this. The obstacles usually encountered are far larger than this, so they can generally be regarded as straight line propagation.[3]
Relationship with geometrical optics
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Refraction of light
Andvisible lightPropagate associatedelectromagnetic field, characterized by very fast vibration (frequencyOrder of magnitude10s), or the wavelength is very short (order of magnitude: 10-15cm）。Therefore, it can be expected that in this case, a good first order approximation of the light propagation law can be obtained by completely ignoring the finite size of the wavelength.It has been found that for many optical problems, such treatment is completely suitable.In optics, wavelength can be ignored, that is, equivalent toλzeroThis branch of the → 0 limit case is commonly referred to asgeometrical optics , because under this approximation, optical laws can be expressed in the language of geometry.Diffraction phenomenonOne of the simplest typical examples ofslitFraunhofer diffraction.It contains many main characteristics of diffraction phenomenon.From light sourceSA beam expander consisting of a telescope systemLoneThe beam expansion is projected directly onto a slit.Place one behind the slitlensLtwo, then in the lensLtwoOffocal planeScreen placed onF'FWill produce alternating light and darkdiffractionpattern.It is characterized by a particularly bright bright stripe in the center, and some less intense bright stripes arranged on both sides.There is a dark stripe between adjacent bright stripes.If the spacing between adjacent dark stripes is taken as the width of the light stripes, the light stripes on both sides are equal in width, and the width of the central light stripe is twice that of other stripes.The angle from the bright stripe to the center of the lens is called the angular width.The angular width of the central bright stripe is not equal to that of other bright stripes.The angle of the central bright stripe is equal to 2λ/b（bIs the width of the seam), that is, it is equal to 2 times the width of the angle of other bright stripes.Then the half width Δ of the central bright stripeθ=λ/b, just equal to other bright linesCorner width。
Since the central bright spot concentrates most of the light energy, its half width can be taken asDiffraction effectA measure of strength.Formula Δθ=λ/bTell us that for a given wavelength, ΔθAnd seam widthbInversely, that is, inwavefrontUpper pairlight beamThe greater the limit,diffractionThe more diffuse the field, the wider the spread of the diffraction pattern;On the contrary, when the slit width is large and the beam almost propagates freely, Δθ→ 0, which indicates that the diffraction field is basically concentrated in the direction of straight line propagationlensfocal planeThe upper diffraction spot shrinks togeometrical optics Like a dot.Formula Δθ=λ/bIt also tells us thatSeam widthUnder constant conditions, ΔθAndλIs proportional, the longer the wavelength,Diffraction effectThe more significant;The shorter the wavelength, the more negligible the diffraction effect.So geometric optics isb＞＞λAn approximation of time, orλ→ 0 approximation.In addition to the linear propagation law, there are two other laws as the basis of geometric optics-Reflection lawandLaw of refraction, which is only approximately true under very small conditions, so the scope of application of geometric optical principles is limited, and it needs to be replaced by more rigorous wave theory when necessary.But becausegeometrical optics The method to deal with problems is much simpler, and it is accurate enough for many practical problems encountered in various optical instruments, so geometric optics is an important theoretical basis for various optical instruments.[3]
