光的干涉optics03.film interference.ppt

Amplitude-splitting Interferometers7.4.1 Dielectric Films-Double-Beam Interference (Thin-Film Interference)7.4.2 Mirrored InterferometersContentslThin-film interferencel1.Fringes of equal inclinationl2.Fringes of equal thickness:wedge thin-film and Newtons rings.lMirrored interferometersl1.Michelson interferometerl2.Mach-Zehnder interferometerl3.Sagnac interferometer.Teaching purpose:l知道各干涉仪的装置，会画出示意图；l会计算两束反射光（或折射光）之间的光程差，并讨论干涉花纹；l会利用干涉仪进行微小测量，波长测量等。New wordsl1.The index of refraction and the speed of lightlThe index of refraction of a medium is the ratio of the speed of light in vacuum to its speed in the transparent medium.lThe frequency of the light is the same in both medium.thenFringes of equal inclinationl2.Equivalent path difference and phase difference lEquivalent path=lgeometrical pathlindex of refractionFringes of equal inclinationlOptical path 光程=n rlOptical path difference 光程差l =n2r2-n1r1lPhase differencel=2/0 n1n2S2S1r2r1pFringes of equal inclinationl3.thin-film interference-amplitude divisionMedium n1,n3Thin-film n2,dIncident light,i,rReflect light 2 and 3is coherent.Transmission light 4and 5 is coherent.Fringes of equal inclinationlOptical path difference of reflect light 2,3lPhase differencePhase changes upon reflection must be done on a case by-case basisFringes of equal inclinationlOptical path difference of transmission light 4,5lPhase differenceFringes of equal inclinationlphase changes upon reflectionlif i=0,Phase difference=lPhase differenceReflection light has phase changeTransmission lighthas phase changeFringes of equal inclinationFringes of equal thicknesslIn our daily life the color fringes on an oil film or on a soap bubble are often seen.These are common examples of interference taking place in thin film.l1.wedge thin-filmlOptical path differenceFringes of equal thicknesslThe interference fringes trace out contours of constant thickness of the wedge thin-film.lThe thickness difference between adjacent bright(dark)fringeslThe fringes width(space between adjacent dark fringes)Fringes of equal thicknesslExample 1:A broad beam of red light,with wavelength=632.8 nm,incidences on am air wedge as shown in figure.An observer looking dowm on the block with the air wedge sees an interference pattern consisting of six dark fringes and five bright red fringes along the wedge.What is the change in thickness T(=TR-TL)along the wedge?Fringes of equal thicknesslSolution:lAt the left and right ends,dark fringe implies the destructive interferenceFringes of equal thicknessl5.Newtons ringslOptical path difference:lPhase differencelNewtons rings traced out contours of constant thickness of the air gap between the surfaces.Fringes of equal thicknesslThereforelThe radius of the Newtons rings:Fringes of equal thicknessFringes of equal thicknesslExample 2:A glass lens is coated on one side with a thin film of magnetium fluoride(MgF2)to reduce reflection from the lens surface.The index of refraction of MgF2 is 1.38;that of the glass is 1.50.What is the least costing thickness that eliminates the reflections at the middle of the visible spectrum(=550nm)?Assume that the light is approximately perpendicular to the lens surface.l.Fringes of equal thicknesslSolution:lOptical path difference:Fringes of equal thicknesslConstructive interferenceFringes of equal thicknessBlue-purple