《物理双语教学课件》Chapter 23 Interference 干涉理论_大学物理课件干涉

《物理双语教学课件》Chapter 23 Interference 干涉理论由刀豆文库小编整理，希望给你工作、学习、生活带来方便，猜你可能喜欢“大学物理课件干涉”。

Chapter 23 Interference

Sunlight, as the rainbow shows us, is a composite of all the colors of the visible spectrum.Soup bubbles and oil slicks can also show striking colors, produced not by refraction but by constructive and destructive interference of light.23.1 Young’s Interference Experiment 1.In 1801 Thomas Young experimentally proved that light is a wave, contrary what most other scientists then thought.He did so by demonstrating that light undergoes interference.2.Figure gives the basic arrangement of Young’s double-slit interference experiment.On

the viewing screen C, Points of interference maxima form visible bright rows-called bright bands, bright fringes, or maxima.Dark regions-called dark bands, dark fringes, or minima-result from fully destructive interference and are visible between adjacent pairs of bright fringes.The pattern of bright and dark fringes on the screen is called an interference pattern.Figure is a photograph of the interference pattern.3.To find

what

exactly determines the locations of the fringes in Young’s double-slit interference experiment, let us see the

figure.The

L

r2path-length-difference between rays be written as

r1 and can Ldsin, where d is the separation of the two slits.(1)For a bright fringe, number of wavelengths.It is dark fringe, It is L

L

must be zero or an integer

dsinmm0,1,2,.(2)For a

must be an odd multiple of half a wavelength.2m1,2,.(3)Using above two equations, dsin(2m1)we can find the angle  to any fringe and thus locate that fringe;further, we can use the values of m to label the fringes.4.We now wish to derive an expreion for the intensity I of the fringes as a function of .(1)Let us aume that the electric field components of the light waves arriving at point P in the figure from the two slits are not in phase and vary with time as

E1E0sint and

E2E0sin(t2dsin)respectively.(2)So we have where 2dsin.(3)Thus

11EE1E22E0cos()sin(kx),22the intensity is I(2E0cos1)2

214I0cos2.2

5.Combining more than two waves:

23.2 Interference from Thin Films 1.The colors we see when sunlight illuminates a soap bubble or an oil slick are caused by the interference of light waves reflected from the front and back surfaces of a thin transparent film.The thickne of the soap or oil film is typically the order of magnitude of the wavelength of the light involved.2.Figure shows a thin transparent film of uniform thickne L and index of refraction n2, illuminated by bright light of wavelength  from a distant point source.For now, we aume that air lies on both sides of the film and thus that

n1n3

in figure.For simplicity, we also aume that the light rays are almost perpendicular to the film.3.Reflection phase shifts: Refraction at an interface never causes a phase change.But reflection can, depending on the indices of refraction on two sides of the interface.When an incident wave travels in the medium of leer index of refraction(with greater speed), the wave that is reflected at the interface undergoes a phase shift of  rad, or half a wavelength.4.The optic path-length difference in the case of thin film is 2nL2.2nL2mm1,2,.5.For a bright film, we have 6.For a dark film, we have

2nL2(2m1)2m0,1,2,.23.3 Michelson’s Interferometer 1.An interferometer is a device that can be used to measure lengths or changes in length with great accuracy by means of interference fringes.2.Figure shows

Michelson’s interferometer.