課件文稿案例22-1diffraction of light

1、一 The phenomena of diffraction Diffraction Patterns of a narrow slit , a razor blade and a dish The Fresnel Bright SpotDiffraction by a Circular Aperture 1二The principle of Huygens-Fresnel (惠更斯菲涅爾原理) :element on the wavefront Fresnel pointed out that the diffraction intensity at point P is determine

2、d by the superposition of secondary wavefronts.The amplitude of waves at p depends on DS/r and q. :Wavefront at t *Two kinds of diffraction sPFresnel Diffraction: the distance from objects to light source and screen is limited.Fraunhofer Diffraction: the distance from objects to light source and scr

3、een is infinite. sPf1f2L12LThe condition to carry out Fraunhofer Diffraction三Diffraction by a single slitIntensityCentral fringe The intensity in single-slit diffractionFirst maximumFirst of the secondary maximasecond of the secondary maxima The intensity in double-slit interference PqlfaOThe distri

4、bution of the dark and bright fringes in the diffraction of a single-slit is caused by the interference of infinite secondary wavelets on the incident wavefront. Solution method1 integration 2 half-wavelength zones213213qll2l2aIf along q direction, the path difference between the secondary wavelets

5、at ends of slit a exactly equals one l. half-wavelength zoneshalf-wavelength zonesThe slit is just divided into two half-wavelength zones.dark fringe is produced At the correspondingpoints 1,1, 2,2, the path difference is l/2. thus the dark fringe will be produced.along the directionWidth of slitDar

6、k fringeBright fringeintermediate(k half-wavelength zones) HWZ HWZCentral bright Fringe(HWZ)Dark fringeBright fringeWhen q is very smallDark fringeBright fringediscuss(1) The distance from the first-order dark fringe to the center of the screen.The diffraction angle of the first-order dark fringe wh

7、en b remains constant, increase in l cause q increase. And the diffraction is more apparent. travels along straight line Wavelength l keeps constantLargest diffractionThe diffraction angle of the first dark fringeAngle range Line rangeThe width of the central fringe（2）central fringe（ dark fringes） I

8、f the width of the slit change, how does the width of the central fringe change？ If the incident wavelength change, how does the diffraction pattern change?If l increase, the q will also increase, and the diffraction pattern will be more apparent.（4）The dynamic change of the diffraction When the sli

9、t moves up and down, the diffraction doesnt change. The central fringe is still on the axis of the lens.（3）The width of fringeDark fringeBright fringeWidth of other dark or bright fringe except central fringe（5）When the light is not perpendicular to the slit.（the central fringe moves down ward）（the

10、central fringe moves upward）a monochromatic light is incident on a slit with angle a, the width of the slit is b, what is the diffraction angle of the all fringes? Solution:Based on condition to produce the dark fringe We produce a diffraction pattern on a viewingscreen by means of a long narrow sli

11、t illuminated by blue light. Does the pattern expand away from the bright center (the maxima and minima shift away from the center) or contract toward it if we (a) switch to yellow light or (b) decrease the slit width. Solution:CP. 1(p.894)(a) expand.(b) expand.Ch31-P727:5,7,8 IntensityCentral fring

12、e四 The intensity in single-slit diffractionConsider: What is the combination of the field components E1 and E2 at point P?Thus the intensity at point p is PqlfaODivide the slit into infinitesimal zones of width Dx, and each zone can acts as a source of Huygens wavelets. (1) Central Maximum atThe wav

13、elets all arrive have the same amplitude DE and are in phase. Because there is zero phase difference between the wavelets, there is zero angle betw-een each pair of adjac-ent phasors.(2)The first side minimumThe first side maximumIn the following we will proof that the intensity of the pattern as a

14、function of qAfter Dividing the slit into inf-initesimal zones of width Dx, the arc of the phasors approac-hes the arc of a circle with radius R, and the length of the arc must be Em Two wavelengths, 650nm and 430nm,are usedseparately in a single-slit diffraction experiment. The results asgraphs of

15、intensity I. versus angle for the two diffraction patterns. If both wavelengths are then used simultaneously, what color will be seen in the combined diffraction pattern at (a) angle A and (b) angle B?Solution:CP. 3(p.898)(a) l=430nm.(b) l=650nm The figures represent, in smoother form (withmore phasors) that Fig.37-6,the phasor diagrams for two pointsof a diffraction op