Photogenicdispersion(separation of light)Polychromatic lightExploded intoMonochromatic lightPhenomenon of;The phenomenon that polychromatic light is decomposed into monochromatic light through prism;Different frequencies of light source spectral components in optical fibergroup velocityLight causedPulse broadeningPhenomenon.Dispersion is also a description of the relationship between a propagation parameter of optical fiber and frequency.Newton first used it in 1666prismObserve the dispersion of light, and decompose white light into colorLight band(Spectrum).The dispersion phenomenon indicates the speed of light in the mediumv=c/n(orRefractive indexn）Follow light frequencyfAnd change.The dispersion of light can be achieved by using a prism,diffraction grating ，InterferometerAnd so on.
After the light passes through the prism, the white light forms a visible spectrum due to dispersion
The dispersion of light requires a medium that can refract light, and the refractive index of the medium varies with the frequency of light wave or the frequency in vacuum.When polychromatic light refracts on the medium interface, the medium has different refractive index for light of different frequenciesRefraction angleDifferent but separate from each other.In 1666, Newton usedprismtakesunlightDecompose into colorLight bandThis is the first dispersion experiment.Refractive index of ordinary mediumnOr the relationship between dispersion rate and frequency.The dispersion of any medium can be separatedNormal dispersionandAnomalous dispersionTwo.
light wave
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Dispersion chromatography
Of course, the dispersion of light also includes light waves.Light waves have a certain frequency. The color of light is determined by the frequency of light wavesvisible lightIn the region, the frequency of red light is the smallest, and the frequency of purple light is the largest. The propagation speed of light of various frequencies in vacuum is the same,Approximately equal to3.0×10eightm/s。But different frequenciesMonochromatic lightWhen propagating in the medium, due to the interaction with the medium, the propagation speed is smaller than that in the vacuum, and the speed is different from each other.Medium to redRefractive indexSmall, the refractive index of purple light is large.When different colored lights have the sameAngle of incidenceRed light deflects the least when it hits the prism, and it is at the end near the top angle in the spectrum.Purple light has a large frequency and a large refractive index in the medium. It is also arranged at the end closest to the bottom edge of the prism in the spectrum.
Fundamentals
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In optics, the process of decomposing polychromatic light into monochromatic light is called dispersion of light.
Consisting of two or moreMonochromatic lightThe light composed of two or more frequencies is called polychromatic light.Light that can no longer be decomposed (with only one frequency) is called monochromatic light.
The refractive index of light with different frequencies to the same medium is not the same.
Note:EyefulColor visionCells receivevisible lightWhen the color is different, the color is generated by the stimulation of light of different frequencies on color perception cells.
Generally let white light (polychromatic light) pass throughprismCan produce dispersion of light.For the same medium, the higher the frequency of light, the higher theRefractive indexThe bigger.In visible light, purple light has the highest frequency, while red light has the lowest frequency.When the white light passes through the prism, the prism has the largest refractive index of purple light. After the light passes through the prism, the deflection degree of purple light is the largest, and the deflection degree of red light is the smallest.In this way, the triangular prism separates the light of different frequencies, resulting in the dispersion of light.
The phenomenon that polychromatic light decomposes into monochromatic light to form a spectrum is called light dispersion.(After the white light disperses, the monochrome light from top to bottom is "red, orange, yellow, green, blue, indigo and purple".)
Visible spectrum
dispersionAvailableprismOr grating as the instrument of "dispersion system".Press a certain colorSequential arrangementForm a spectrum.Spectra are polychromatic light passing through dispersion systems (such as prisms and gratings)SpectroscopicAfter that, the pattern of the monochromatic light that is dispersed and arranged in order according to the frequency (or wavelength) is called the optical spectrum.The largest part of the spectrumVisible spectrumyesElectromagnetic spectrumThe part visible to the human eye in thisfrequency range Internalelectromagnetic radiationIt is called visible light.The spectrum does not contain all the colors that human brain vision can distinguish, such as brown and pink.
Solar spectrum
French mathematician Cauchy found that the relationship between refractive index and light frequency can be expressed in a series:
n(f)=a+bftwo+cffour。
amonga，b，cIs the three Cauchy dispersion coefficients, which are different for different substances.It is only necessary to measure the refractive index at three different frequenciesn(f）, substitutingCauchy dispersion formulaThree can be obtained fromSimultaneous equationBy solving these simultaneous equations, we can get the three Cauchy dispersion coefficients of the material.With three Cauchy dispersion coefficients, we can calculate the refractive index at other frequencies without further measurement.
In addition to Cauchy dispersion formula, there are other dispersion formulas.Such as Hartmann dispersion formula, Conrady dispersion formula, Hetzberger dispersion formula, etc.
The phenomenon that polychromatic light decomposes into monochromatic light is called lightdispersion。Newton was the first to use a prism to observe the dispersion of light in 1666 and decompose white light into colorsLight band(Spectrum).The dispersion phenomenon indicates the speed of light in the mediumv(or dispersion index of lightn=c/v）Follow light frequencyfAnd change.The dispersion of light can be achieved by using a prismdiffraction grating 、InterferometerAnd so on.
White light consists of red, orange, yellow, green, blue, indigo, purple, etcChromatic lightThe light formed by mixing monochromatic light is called polychromatic light.Colored light that can no longer be decomposed is called monochromatic light.
Dispersion can be realized by using prism or grating as the instrument of "dispersion system".
In addition, we watch TVPhosphorThis is the same combination. You can go to the color TV and look at the CRT. But don't look at the computer in front of youmonitorHis pixels are too small to be distinguished by the naked eye.RGBThe combination of these three colors forms almost all colors.
Red, green and blue are called light“Tricolor”Because the natural red, green and blue colors cannot be usedColor BlendingThe other colors can be obtained by proper mixing of red, green and blue light, so the three colors of red, green and blue are called the "three primary colors" of light.
When polychromatic light refracts on the medium interface, the medium has different refractive index for light of different frequencies, and each color light willRefraction angleDifferent but separate from each other.In 1672, Newton used a prism to reduce the dispersionsunlightThis is the first dispersion experiment.Usually, the refractive index of the mediumnorDispersion rateAnd frequency.The dispersion of any medium can be divided into normal dispersion and abnormal dispersion.
The phenomenon that polychromatic light decomposes into monochromatic light to form a spectrum.Let a white light shine on the glass prism. After the light is refracted by the prism, it forms a colored light band on the white paper screen on the other side. The color arrangement is red near the top corner of the prismthe baseOne end is purple, and the middle is orange yellow green indigo. Such light band is called spectrum.Each colored light in the spectrum can no longer be decomposed into other colored lights, which is called monochromatic light.The light mixed by monochromatic light is called polychromatic light.The sunlight, incandescent electric lamps and fluorescent lamps in nature are all polychromatic light.When the light shines on the object, part of the light is reflected by the object and part of the light is absorbed by the object.If the object is transparent, there is still a part that penetrates the object.Different objects reflect, absorb and transmit different colors differently, so they present different colors.
Light waves have a certain frequency,Color of lightIt is determined by the frequency of light wavesvisible lightThe frequency of red light is the smallest, and the frequency of purple light is the largest. The propagation speed of light of various frequencies in vacuum is the same, equal to 3.0 × 10eightm/s。But different frequenciesMonochromatic light, due to the interaction with the medium,propagation velocityThe speed is smaller than that in vacuum, and the speed is different from each other.The medium has a small refractive index of red light and a large refractive index of purple light.When differentChromatic lightWith the sameAngle of incidenceShoot toprismThe red light has the least deflection, and it is at the end near the top angle in the spectrum.The frequency of purple light is large, and its refractive index in the medium is large. In the spectrum, it is also arranged closest to the prismthe baseOne end of.
After rain in summer, colored arcs often appear in the sky facing the sun, which is called rainbow.The reason for the formation of rainbow is that after the rain, there are many tiny water drops suspended in the sky. The sunlight shines at a certain angle, and the dispersion occurs in these small water drops. Looking at the small water drops, a colorful rainbow will appear.The colors of the rainbow are red outside and purple inside.
Historical Studies
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Ancient China
Light oriented in ancient ChinadispersionThe understanding of the phenomenon originated from the understanding of the natural dispersion phenomenon - rainbow.Rainbow is a kind of dispersion phenomenon caused by refraction and reflection, which is caused by the complicated water drops emitted by sunlight into the air at a certain angle.
As early as in ChinaYin DynastyOracleThere are records about the rainbow.At that time, the word "Hong" was written as "Jiang".Warring States Period《Poetry of the South》There is a record that the color of rainbow is divided into "five colors".Eastern Han DynastyCai Yong(132-192) described the formation conditions and location of the rainbow in Yueling Zhangju.Early Tang DynastyKong Yingda(574~648) in the Notes to the Book of Rites, he roughly revealed the optical cause of the rainbow: "If the clouds are thin and the sun leaks, the sun shines on the raindrops and the rainbow grows", which means that the rainbow is a kind of rainbow produced by the sunlight shining on the raindropsnatural phenomena。In the middle of the eighth century,Zhang Zhihe(744~773) The first experiment was conducted in Xuanzhenzi · Spirit of Taomethod studyLiao Hong, and this is the first conscious white light dispersion experiment: "BackDaily sprayingShou ShuichengRainbowThe shape is not straight, and the shadow is even.After the Tang Dynasty, similar experiments were repeated, such asSouthern Song DynastytowardsCai BianAn experiment simulating "sunshine and raindrop" was carried out to link the rainbow with the phenomenon of the sun and moon halo, which meant that the generation of rainbow was a dispersion process, and the relationship between the rainbow and the position of sunlight was pointed out.Southern Song DynastyCheng Dachang(1123-1195)《Play Fanlu》Dewdrops are described inSpectroscopicIt also points out that sunlight can be changed into many colors through a liquid drop, which is actually dispersion, and this color is not possessed by the water drop itself, but by the color of sunlight, which clearly points out that sunlight contains several colors, which appear through the role of water drops. It can be said that he has come into contact with the essence of dispersion.
Since the Jin Dynasty, many ancient books and records have recordedcrystalDispersion phenomenon.As recordedpeacockMao and someinsectThe skin changes color continuously in the sun,Mica sheetLight of various colors can be observed when you lift it toward the sun.Li ShizhenIt was also pointed out that larger hexagonal crystals and smaller crystal beads can form dispersion.By the end of the Ming Dynasty,Fang Yizhi(1611-1671) in《Physical knowledge》Based on previous research results, he made a wonderful summary of the dispersion phenomenonTrigonopsisThe crystal divides the white light into five colors, which is related to the five color artificial rainbow formed by spraying water towards the sun, the five color phenomenon caused by sunlight shining on the fountain, and the natural phenomena such as rainbow color, halo of the sun and the moon, and the five color cloud. It is believed that "the same principle" means the dispersion of white light.All these indicate that China had a comprehensive understanding of the essence of dispersion phenomenon before the Ming Dynasty, but also reflect that the knowledge of ancient Chinese physics is mostly scattered and empirical.
Before Newton
In the early days of optical development, the interpretation of color was particularly difficult.Before Newton, Europeans' understanding of color was popularAristotleThe view of.Aristotle believed that color is not an objective property of an object, but a subjective feeling of people. The formation of all colors is the result of the proportional mixing of light and darkness, white and black.In 1663, Boyle also studied the color of objects. He believed that the color of objects did not belong to the band of objectsSubstantiveIt is caused by the variation of light on the surface of the irradiated object.Can be completelyReflected lightObjects are white,Complete absorptionLight objects are black.In addition, there are many scientists, such asDescartesHooke and others also discussed the problem of white light dispersing or aggregating into colors, but they all advocated a complicated and disordered theory that red light is greatly condensed light and purple light is greatly diluted light.So before Newton, the refraction produced by prism was supposed to actually produce color, rather than just separating existing colors.
Exploratory research
(1) Design and carry out prism experiment
When white light passes through colorless glass and variousgemstoneThe fact that a variety of bright colors of light will form when the fragments ofexperimental study This problem has been solved.The experiment was rated as one of the "most beautiful experiments in physics".
Newton first made a famous prism experiment. He wrote in his book: "At the beginning of 1666, I made atriangleGlass edge ofCylindrical mirror, use it to study the color of light.For this reason, I made the room into paint and ink, and made a small hole in the window to let the right amount of sunlight come in.I put the prism at the entrance of the light so that the refracted light can reach the opposite wall. When I first saw the bright and strong light color generated by it, I felt very happy. "Through this experiment, we got a color on the wallfaculaThe colors are red, orange, yellow, green, blue, indigo and purple.NewtonThis color spot is called spectrum.
(2) Further design the experiment to obtain pure spectrum
Newton's spectrum obtained in the above experiment is impure. He believes that the reason why the spectrum is impure is that it is composed of a series of overlapping circlesMottleIs formed by the image of.In order to obtain a very pure spectrum, Newton designed a set of optical instruments for experiments.
Illuminate with white light after passing through a lensslitS, slitBackConverging lens（Convex lens）To form a slitSOfS‘。Then place a prism on the light path of the lens.As a result, the light passing through the prism is separated due to different deflection angles, so that it is in whiteLight screenFrom red to purpleSpectral band。This spectral band is composed of a series of adjacent slit color images.If the slit is very narrow, the overlap can be reduced toMinimumTherefore, the spectrum becomes very pure.
(3) Newton put forward a theory to explain spectrum
In order to explain the decomposition of white light in the prism experiment, Newton believed that white light was composed of light of different colors,Glass pairThe refractive index of various colored lights is different. When white light passes through the prism, the colored lights are refracted at different angles, and the results are divided into color spectra.When white light passes through the prism, it deflects towards the bottom edge of the prism, with purple light deflecting the most and red light deflecting the least.The phenomenon that a prism separates white light into various colored light is called dispersion.Strictly speaking, there are many fine lines of various colors in the spectrum, which are very smoothly blended into the adjacent fine lines, so that people can not perceive its boundaries.
(4) Design experiments to verify the correctness of the above theories
Newton
In order to further study the color of light and verify the correctness of the above theory, Newton did another experiment.Newton punched a hole in the screen DE for spectrum observation, and then placed a screen de with a hole behind it, so that the light passing through the hole is monochromatic light with a certain color.Newton put a second prism on the path of the beamabc, and put a new observation screen behind itV。Experiments show that the second prismabcIt just deflects the monochromatic beam at an angle without changing the color of the light.In the experiment, Newton rotates the first prismABC, allowing light of different colors in the spectrum to pass throughDEanddeThe holes on the screen, in all these cases, these monochromatic lights of different colors can not be decomposed again by the second prism. They only deflect a certain angle after passing through the second detectorOptical deflectionDifferent angles.
Through these experiments, Newton came to the conclusion that white light can be decomposed into light of different colors. These lights are monochromatic, and the prism can no longer decompose them.
(5) Experiment of monochromatic light compounding into white light
Since white light can be decomposed into monochromatic light, can monochromatic light also be compounded into white light.Form the spectrum into a row of small rectanglesPlane mirrorThe colored light of the spectrum can be recombined into white light.Adjust the included angle between each plane mirror and the incident light so thatreflected lightAll fall onLight screenAt the same position, you will get a white light class.
Newton pointed out that there was another way toChromatic lightRecompose to white light.Draw the spectrum on the disk to form a fan, and then rotate the disk at high speed, and the disk will appear white.This experimental effect is generally called“Visual persistenceEffects ".eyeretinaThe brain can retain the impression for a few tenths of a second after the image on the top disappears.Thus, the brain can combine rapidly changing color images to form a static white image.The reason why we can see continuous images on TV screens or movie screens is that we have taken advantage of people's "visual persistence effect".
(6) Newton's research achievements on dispersion of light
Newton summarized the results into several items through a series of dispersion experiments and theoretical studies, and the main points are as follows:
① Light varies in color with its refractive index.Color is not a change of light, but the inherent nature of light.
② The same color belongs to the same refractive index, and vice versa.
③ The type of color and the degree of refraction are inherent to light rays and do not change due to refraction, reflection or any other reason.
④ It is necessary to distinguish the original pure color from the color compounded from them.
⑤ There is no light that is white by itself.White is produced by properly mixing light of all colors.In fact, experiments can be carried out to synthesize the color of the spectrum to obtain white light.
⑥ According to the above items, we can explain the reason why the triangular prism makes light produce color and the principle of rainbow.
⑦ The color of a natural object is due to the fact that the material reflects more of one light and less of other light.
⑧ It can be seen from this that color is the quality of light (various rays), so light itself cannot be the quality.Because such qualities as color originate from light,
It can be seen from the above that Newton adopted the research method of typical physical laws of experimental induction - hypothesis theory - experimental verification in his study of light dispersion, and permeated with the method of analysis (decomposing white light intoMonochromatic lightResearch) and comprehensive methods (compounding monochromatic light into white light).
The dispersion of light indicates that light hasVolatility。Because dispersion is caused by the different refractive index of light components (different colored light), and the refractive index is determined by the wave frequency.
photoelectric effect
The most typical example of the particle nature of light isphotoelectric effect。
Phenomenon examples
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rainbow
rainbow
After rain in summer, colored arcs often appear in the direction opposite to the sun, which is called rainbow.We are also called rainbow.The reason for the formation of a rainbow is that after the rain, there are many tiny drops of water suspended in the sky,sunlightWhen it is shot at a certain angle, these small water drops will occurdispersion, looking at the small water drop, there will be colored rainbow.The colors are generally seven colors, red, orange, yellow, green, blue, indigo and purple from the outside to the inside.In China, there is often the saying "red, orange, yellow, green, blue and purple".(Actually, this is a phenomenon of light dispersion.) Mao Zedong once wrote it in the summer of 1933[1]The word describes the color of the rainbow: "red, orange, yellow, green, blue, purple,Who dances in the sky with colorful practice。After the rain, the sun sets again, and the mountains turn pale... "
Neon
Sometimes a second rainbow can be seen on the outside of the rainbow. Its luster is slightly lighter than the first rainbow, and its color sequence is outside purple and inside red.be calledAccessory rainbowOr neon.
The only difference between neon and rainbow is that light produces twice in raindropsinternal reflectionTherefore, when light comes to our eyes after passing through raindrops, the light arc color band is just the opposite of the rainbow.
appendix
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Let's suppose there is a columnplane wave(i.e. a bunchDirectional light）The equation of plane wave is when it strikes the surface of the medium along the z-axis.
Assume that this series of light waves will pass through a thickness of ΔzIf the speed of light in the medium isc/n, then light will spend some extra time to propagate in the medium.
Therefore, compared with the original light vector, the light vector behind the medium is equivalent to "delaying" a phase.That is, it is multiplied by a phase factor.
When light shines on a medium (such as glass) from vacuumcharged particleWill be affected and vibrated.of courseNucleusIt is impossible to oscillate under various constraints. It is only the electrons in the outer layer of the atom that oscillate.
If we set the media surface toz=0, then the light vector at the surface of the medium is.
The electrons in the medium are subjected to such aRestoring forceExercise.If we think that the electrons in the outer layer of an atom behave like aSpring vibrator(This assumption is taken for granted, but it is indeed a good approximation of the real situation), then a spring vibrator will behave as aSimple harmonic vibration，Equation of motionIs.
here,
Is the natural frequency of the spring vibrator.
This formula is used in college physics courseskinematicsSome have already learned.This is an undampedSimple harmonic vibrationAnd its solution is.
All the electrons are in theirBalance positionDo such simple harmonic vibration nearby.When all electrons do this simple harmonic vibration at the same time, a whole vibration will be generatedfield strength, resulting in a new light vector.(The derivation in this part is tedious, so it is omitted)
By comparison, we can see that.
Among them,
Is the "natural frequency" of electrons in the medium,
Is the frequency of light, inversely proportional to the wavelength of light.
At this point, we can see what the relationship between refractive index and frequency is.
Put all the irrelevant constants in the above formula together, and use the frequency
Indicates that the following results can be obtained:
in fact,Transparent material(such as glass) often has multiple different natural frequenciesForced vibrationIt is not simple harmonic vibration, butDamped vibration。Taking these two factors into account in the above derivation process, we can get the refractive index that is closer to the actual situationexpression。But for the derivation of Cauchy dispersion formula, here is enough.
