浙江大学大学物理甲下chapter41.ppt

Chapter 41 Interference * Content of this Chapter Two-source interference Double-slit interference Coherence Intensity in double-slit interference Interference from thin film Michelson’s interferometer Chapter 41 Interference Geometrical optics vs. Wave optics Geometrical optics: The light travel in straight line paths or rays. a/λ1 Wave optics (physical optics): interference; diffraction. a~λ 41-1 Two-source interference t +1 -1 t +1 -1 t +2 -2 + In Phase Constructive Interference t +1 -1 t +1 -1 t +2 -2 + Destructive Interference Out of Phase 180 degrees + Different f Interference Requirements Need two (or more) waves Must have same frequency and same polarizing direction Must be coherent (i.e. waves must have definite phase relation) Interference: When identical waves from two sources overlap at a point in space, the combined wave intensity at that point can be greater or less than the intensity of either of the two waves. Can’t produce coherent light from separate sources. (f ? 1014 Hz) Single source Two different paths Interference possible here Need two waves from single source taking two different paths One source, two slits Reflection (thin films) Diffraction* Superposition of Two Waves Two waves encountered at point P: Here we have assumptions: (1) Same frequencies, (2) Amplitude in same direction, and (3) initial phases are not change with time. Δr: path difference. the relation of phase difference and path difference. The result amplitude depends on Optical Path Length The optical path length in a medium: Therefore, difference of the optical path lengths: B L A n 41-2 Double-Slit Interference* Single source of monochromatic light ? d D Need Dd, λd Fringes distribution The angle ? is small, Does the distances on the screen between the adjacent maxima are equal all? ? Young’s double-slit experiment (1801) Quick quiz-1 In the Young double slit experiment, is it possible to see interferen