Differential frequency(English: beat note) comes from two words in physicsfrequencySimilar but differentwaveOfinterferenceOf the obtained interference signalfrequencyIt is the difference between the frequencies of the original two waves, so it is called the difference frequency.stayInternational System of UnitsIn, the unit of frequency - Hertz（English: HertzHz）German physicistHeinrich Hertz （EnglishHeinrich Rudolf Hertz, February 22, 1857 January 1, 1894).1 Hz means that the event occurs once every second.Other units used to express frequency include: the traditional measurement unit used in the field of rotating machinery and equipment isRPM(rpm), etc.In medicine,heart rateIn bpm.
Hertz.And becausecycleIt is defined as the minimum time interval for the occurrence of repeated events, so the frequency can also be expressed as periodicreciprocal：
Among them,
Represents a period.
For convenience, longer, slower waves, like those on the surface of the oceansurface wave, which is usually described in terms of period.Shorter and faster waves, such as microwaves and radio waves, are usually characterized by frequency and wavelength.
stayInternational System of UnitsIn, the unit of frequency - Hertz（English: HertzHz）German physicistHeinrich Hertz （EnglishHeinrich Rudolf Hertz, February 22, 1857 January 1, 1894).1 Hz means that the event occurs once every second.
Other units used to express frequency include: the traditional measurement unit used in the field of rotating machinery and equipment isRPM(rpm), etc.In medicine,heart rateIn bpm[1]。
physical optics
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radiant energy（English: radial energy) YesElectromagnetic wave propagationEnergy.The sun or electric light source is the source of radiant energy.The light waves that human optical sensors (eyes) can distinguish are calledvisible light, is composed of several colors (red, orange, yellow, green, blue, indigo, purple);Each of these colors has a specificfrequency band（English：frequency band）。Visible light in the wholeelectromagnetic radiationOfspectrumOnly a small part of.ultraviolet rays(UV) frequency is greater than visible light and cannot be seen with naked eyes;alsoinfraredThe frequency of (IR) is less than that of visible light, which must also be usedNight vision gogglesAnd othersThermal sensing equipmentCan be observed.Electromagnetic radiation greater than the ultraviolet frequency includesX-rayandGamma ray。Less than infrared frequencymicrowaveandradio waves, the frequency band isMegahertzandKilohertz, and the frequency band isMillihertzandMicrohertzNatural waves.A wave with a frequency of 2 millihertz has a wavelength approximately equal to the distance from the earth to the sun.The wavelength of micro Hebo is about 0.0317Light year。NahrThe wavelength of the wave is about 31.6881 light-years.
Electromagnetic waves can be classified aselectric energy、radio waves、microwave、infrared、visible light、ultraviolet rays、X-rayandGamma raywait.Used in general experimentsa spectrometerIt is enough to analyze electromagnetic waves with frequencies ranging from 120 to 150000 terahertz.With this instrument, detailed physical properties of objects, gases or even stars can be known.This isAstrophysicsThe necessary instrument of.For example, becauseHyperfine division（English：hyperfine splitting），hydrogen atomIt emits radio waves of 1.42 GHz.
The human eye can observe electromagnetic radiation with a frequency between about 300 THz and 750 THz, which is calledvisible light[2]。
For example, usingBeam splitterPut a bunchmonochromelight beamAfter splitting into two bundles, let them overlap in an area in the space, and you will find thatlight intensityIt is not evenly distributed: its brightness varies with its position in space. The lightest place exceeds the sum of the original two light intensities, while the darkest place may have zero light intensity. This redistribution of light intensity is called "interference fringe".In history, the interference phenomenon andRelated experimentsIt's proof of lightVolatilityHowever, this interference property of light was not gradually discovered until the early 19th century, mainly because coherent light sources are not easily available.
In order to obtain the visible light interferenceCoherent light sourcePeople invented and manufactured various optical devices and interferometers that generated coherent light. These interferometers had very high measurement accuracy at that time:Albert MichelsonWith the help ofMichelson interferometerCompleted the famousMichelson Morey experiment, gotEther windThe zero result of the observation.Michelson also measured with this interferometerStandard meter gaugeAnd thus obtained the exact length ofThe nobel prize in physics。After the 1960s,laserThis invention of high-intensity coherent light source makesOptical interferometryThe technology has been widely used as never before, and can be seen in various precision measurementsLaser interferometerThe figure of.As people know, two buncheselectromagnetic waveThe interference ofWave particle dualityThe interference of light is alsophotonSelfProbability amplitudeStacked result[3]。
Interference conditions
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Two waves in the samemediumWhen the propagation in the overlap occursparticleIt is affected by two waves at the same time.JovianamplitudeIt is not large. At this timeVibrationdisplacementEqual to the displacement caused by individual fluctuationsvectorAnd, which is calledSuperposition principle of wave。If two wavepeak(ortrough）When arriving at the same place at the same time, it is said that two waves are in phase at that point, and the interference wave will produce the maximum amplitude, which is called constructive interference (constructive interference);If one of the two wavespeakWith another wavetroughWhen arriving at the same place at the same time, it is said that two waves are antiphase at that point, and the interference wave will produce the smallest amplitude, which is called destructive interference (destructive interference).
Theoretically, the superposition of two lines of infinite monochromatic waves can always cause interference, but the actual physical modelWavetrainIt can't be infinitely long, and from the microscopic mechanism of wave generation, there are random fluctuations in the amplitude and phase of the wave, so there is no strictly monochromatic wave in reality.for examplesunlightThe light wave emitted comes fromphotosphereLayeredElectronicsAndhydrogen atomIf the interaction time of each interaction is on the order of 10 seconds, the wave trains generated by two events with a long time interval cannot interfere with each other.For this reason, it can be considered that the sun is an extended light source composed of many incoherent point light sources.As a result, sunlight has a very wide frequency domain, and its amplitude and phase have rapid random fluctuations. Ordinary physical instruments cannot track and detect such rapid fluctuations, so interference of light waves cannot be observed through sunlight.Similarly, for two trains of light waves from different light sources, if the amplitude and phase fluctuations of these two trains of waves are not related to each other, it is said that these two trains of waves do not havecoherence。On the contrary, if two trains of light waves come from the same point light source, the fluctuations of these two trains of waves are generally related to each other, and the two trains of waves are completely coherent at this time.
If two coherent waves are generated from a single incoherent wave source, two different methods can be used: one is calledWavefront segmentation, that is, for a wave source whose geometric size is small enough, let the wave train generated by it pass through the slit placed side by side, according toHuygens Fresnel principle, these are inwavefrontThe wavelet generated on the is coherent with each other;The other isWave amplitude segmentation method, halftransmission, halfreflexA semi silver plated mirror can divide the light wave into two parts to produce transmission wave and reflection wave.The reflection wave and transmission wave thus generated come from the same wave source and have high coherence. This method is also applicable to the extended wave source[2]。
