浙江大学大学物理甲下 chapter 42.ppt

phypzq@ Chapter 42Diffraction *Content of this Chapter Diffraction and the wave theory of light Single-slit diffraction Intensity in single-slit diffraction Diffraction at a circular aperture Double-slit interference and diffraction combined Chapter 42 Diffraction 42-1 Diffraction and the Wave Theory of Light When a light passes through a hole, or meets a disk etc., with dimensions comparable to the wavelength, we can see patterns on the screen behind them. Evidence for the wave nature of light* ? Fresnel’s theory of light (1788-1827) Makes it feasible for calculation of intensity at any point on screen Fraunhofer diffraction —infinite separation Fresnel diffraction—finite separation ? Two cases of diffraction 42-2 Single-Slit Diffraction ? Fraunhofer’s Single-slit Diffraction P0 All rays arriving at P0 are in phase, they interfere constructively. ? Half-wavelength strip The first minimum of the diffraction pattern:The whole slit is divided into N strips so that the adjacent rays have path difference of a half-wavelength: The angle ? (the position on the screen) for the maxima and minima can be determined as following: ? Diffraction pattern Secondary maxima Diffraction pattern of single-slit f Width of the bright fringe at the center: The other fringes: Sample problem 42-1,2 Quick quiz-1If a classroom door is open slightly, you can hear sounds coming from the hallway. Yet you cannot see what is happening in the hallway. Why is there this difference? (1) light waves do not diffract through the single slit of the open doorway. (2) Sound waves can pass through the walls, but light waves cannot. (3) The open door is a small slit for sound waves, but a large slit for light waves. (4) The open door is a large slit for sound waves, but a small slit for light waves. 42-3 Intensity in Single-Slit Diffraction The whole slit is divided into N strips with ?x = a/N, which can be regarded as Huygen’s wavelet, therefore, Optical path difference of ajacent ray