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1、1,第三章 光的干涉和干涉系统 31 光波的干涉条件,一、干涉现象,1、什么是干涉现象（Interference）,2、相干光波（Coherent wave）和相干光源 （Coherent light source） 能够产生干涉的光波，叫相干光波； 其光源称为相干光源。,2,干涉现象（Interference）,在两个光波叠加的区域形成稳定的光强分布的现象，称为光的干涉现象,The term Interference refers to the phenomenon that waves, under certain conditions, intensify or weaken each
2、other.,3,干涉现象实例（Interference Examples）,4,二、干涉条件,一般情况下，,5,对于两个平面简谐波,6,干涉条件（必要条件）：,7,当两光波振动方向有一定夹角时， ，即只有两个振动的平行分量能够产生干涉，而其垂直分量将在观察面上形成背景光，对干涉条纹的清晰程度产生影响。一般夹角值小时，这种影响可以忽略。,8,32 杨氏干涉实验 （Youngs double-slit experiment） 一、干涉图样的计算,1、P点的干涉条纹强度,光强 I 的强弱取决于光程差,9,2、光程差D的计算,光程差：,10,3、干涉条纹（Interference fringes）
3、及其意义,x,对于接收屏上相同的x值，光强I相等。条纹垂直于x轴。,11,用光程差表示：,结论： 1、干涉条纹代表着光程差的等值线。 2、相邻两个干涉条纹之间其光程差变化量为一个波长l，位相差变化2p。,在同一条纹上的任意一点到两个光源的光程差是恒定的。,12,13,4、干涉条纹的间隔,定义：两条相干光线的夹角为相干光束的会聚角，用w表示。,m+1,14,5、干涉条纹间隔的影响因素,1）相干波源到接收屏之间的距离D 2）两相干波源之间的距离d 3）波长,15,干涉条纹间隔与波长的关系,x,0,白条纹,白条纹,白光条纹,16,二、两个点源在空间形成的干涉场,17,在三维空间中，干涉结果：等光程差
4、面,局部位置条纹,18,Key words,1. Path difference 2. Phase difference 3. The order of interference 4. The light distribution 5. A maximum amount of light (maxima) 6. A minimum amount of light (minima),19,本课内容回顾,6、干涉条纹间隔与波长：多色光的干涉,7、两个点源在空间形成的干涉场：等光程差面,2、P点的干涉条纹强度：,3、光程差D的计算：,5、干涉条纹的间隔：,1、干涉现象和干涉条件,20,Homewor
5、k (3-1&2),1. Light passes through two narrow slits of d=0.8mm. On screen 1.6m away the distance between the two second-order maxima is 5mm. What is the wavelength of the light?,21,22,Thomas Young (1773-1829),A British physician and physicist. He could read at age 2, at 6 began studying Latin, and at
6、 13 had also mastered Greek, Hebrew, Italian and French. At 19 he entered medical school, correctly explained the accommodation of the eye and was elected Fellow of the Royal Society. In 1796, he graduated from the University of Gottingen Medical School, opened a practice in London, and 5 years late
7、r became professor of Natural Philosophy at the Royal Institution. That same year,1801, he read the first of several papers presenting the wave theory of light and the principle of interference, much to the opposition of Newtons followers. Young made noteworthy contributions also to acoustics, atmos
8、pheric refraction, elasticity, fluid dynamics and color vision.,23,The light of distribution resulting from a superposition of waves will consist of alternately bright and dark bands called interference fringes. Such fringes can be observed visually, projected on a screen, or recorded photoelectrica
9、lly.,Interference fringes,24,Interference fringes,Zeroth-order maximum,First-order minimum,First-order maximum,25,33 干涉条纹的可见度 the visibility (contrast) of interference fringes,可见度(Visibility, Contrast)定义：,26,式（11-14）,27,一、振幅比 对条纹可见度的影响,28,二、光源宽度 的影响和空间相干性,相干性（Coherence）,相干性与干涉（Coherence & interferen
10、ce）,29,1、光源宽度 对条纹可见度的影响(扩展光源),30,31,32,讨论：,1）光源的临界宽度：条纹可见度为0时的光源宽度,2）光源的允许宽度：能够清晰地观察到干涉条纹时，允许的光源宽度(K=0.9),33,2、空间相干性（Spatial Coherence ),若通过光波场横向两点的光在空间相遇时能够发生干涉，则称通过空间两点的光具有相干性。,34,当光源是点光源时，所考察的任意两点S1和S2的光场都是空间相干的，当光源是扩展光识时，光场平面上具有空间相干性的各点的范围与光源的大小成反比。对于一定的光波长和干涉装置，当光源宽度b较大，且满足,时，通过S1和S2两点的光将不发生干涉，
11、因而这两点的光场没有空间相干性。 我们从另一个角度考察光的空间相干性范围。对一定的光源宽度b，通常称光通过S1和S2恰好不发生干涉时所对应的这两点的距离为横向相干宽度，以么表示，,35,调节d的大小，观察什么时间条纹消失，此时 测量d，就可以计算出星体角直径。,36,三、光源非单色性 的影响和时间相干性,1、光源非单色性 对条纹可见度的影响,37,38,讨论：,相干长度(coherence length)：对于光谱宽度为（或k）的光源能够产生干涉的最大光程差。,39,2、时间相干性 (Temporal Coherence),时间相干性：若同一光源在相干时间内发出的光经过不同的路径在空间相遇时，
12、能够产生干涉，则称光具有时间相干性。 相干时间 t：光通过相干长度所需的时间。,公式：,40,Partial Coherence & contrast,It is incorrect to think of light as either coherent or incoherent. Light can have different degrees of coherence,which introduced the concept of partial coherence. Complete coherence is merely a theoretical limit.,41,Partia
13、l Coherence & contrast,Assume that two wavetrains of light, each of finite length s, overlap to their full extent. Such complete overlap will result in distinct maxima and minima of the highest degree of contrast. But even if the wavetrains overlap only in part, interference is possible. Although th
14、e degree of contrast of the fringes is less, depending on the degree of overlap.,42,本课内容回顾,2、振幅比与可见度的关系：,3、光源宽度与可见度的关系,4、光源单色性与可见度的关系,5、名词解释：空间相干性、时间相干性、相干长度、相干时间、干涉孔径角,1、可见度的定义,43,Homework,1. 比较说明空间相干性和时间相干性的联系和区别。,44,Visibility,It can be defined as K=(Imax-Imin)/(Imax+Imin) where Imax and Imin are
15、 the intensities at the maxima and minima of the fringe pattern.,45,Contrast,It can be defined as the ratio of the difference between maximum areance Emax, and mimimum areance, Emin, to the sum of such areances: K=(Emax-Emin)/(Emax+Emin) The amount of power incident per unit area is called areance (
16、illuminance).,46,Coherence,The property of light necessary to produce interference is called coherence.,47,Coherence & Interference,Coherence is a property of light. Interference is the process of interaction. Coherence means that two or more waves in a radiation field are in a fixed and predictable phase relationship to each other. We distinguish two classes of coherence, spatial coherence and temporal coherence.,48,Spatial Coherence,
