大學(xué)物理下課件35 Interference 課件

1、大學(xué)物理下課件大學(xué)物理下課件35 Interference35 InterferenceLight As A Wave -Huygens principle Huygens principle : All points on a wavefront serve as point sources of spherical secondary wavelets. After a time t, the new position of the wavefront will be that of a surface tangent to these secondary waveletsLight As

2、 A Wave Phase Difference11111 nLLNnnumber wavelengthConclusion: The phase difference between two light waves can change if the waves travel through different materials having different indexes of refraction22222 nLLNnnumber wavelength1212nnLNNNote: A phase difference of 45.6 wavelengths is equivalen

3、t to an effective phase difference of 0.6 wavelength.Youngs Interference Experiment (1801)Experiment set-upInterference patternYoungs Interference Experiment Locating the fringessindL :difference length Path, 2, 1, 0 sinmaxmmdL:position Maxima, 2, 1, 0 21sinminmmdL:position Minimadm1maxsindm21sin1mi

4、nSample Problem - Double-slit interference pattern Problem: What is the distance on screen C in Figure(a) between adjacent maxima near the center of the interference pattern? The wavelength of the light is 546 nm, the slit separation d is 0.12 mm, and the slitscreen separation D is 55 cm. Assume tha

5、t in the figure is small enough to permit use of the approximations sin = tan =, in which is expressed in radian measure.Solution: dDmydmDymmsin,tandDmym11m105 . 231dDyymmIntensity in Double-slit InterferencetEEtEEsin,sin0201sin2,21cos4:Intensity20dLkII0max42, 1, 0,2IImm02, 1, 0,12minImmInterference

6、 From Thin FilmsNo phase change in reflection when the incident pulse is in the denser string.There is a phase change in reflection when the incident pulse is in the lighter string. and of wavesebetween th2L differencelength path the021rrL :difference length PathL Phase change in reflection:Interfer

7、ence From Thin Films - Equations:maximafor difference length path The, 2, 1, 0,212122mnmmLnAssume the incident beam is from air ( n1 = 1.0) to film (n2 1.0) There is a phase change in reflection when the incident pulse is in air A phase change in reflection equivalent to the path length difference /

8、2, 2, 1, 0,22mnmmLn :mininafor difference length path TheInterference From Thin Films - Film Thickness Much Less Than , 2, 1, 0,22mnmmLn :mininaFor - The thickness increases from top to bottom because gravitation has caused the film to slump.Example: a vertical soap film- Bright white light illumina

9、tes the film.- The top portion is so thin L = 0 m = 0 there is only phase shift of in that potion because of reflection the fringe is dark .- In the (somewhat thicker) middle color of fringes depends primarily on the wavelength at which reflected light undergoes fully constructive interference for a

10、 particular thickness., 2, 1, 0,2122mnmL:maximaFor Sample Problem - Thin-film interference of a water film in airProblem: White light, with a uniform intensity across the visible wavelength range of 400 to 690 nm, is perpendicularly incident on a water film, of index of refraction n2 = 1.33 and thickness L = 320 nm, that is suspended in air. At what wavelength is the light reflected by the film brightest to an observer?Solution: , 2, 1, 0,2122mnmL:maximaFor 2122mLnseen benot caninfrarednm17000mlight green