|MadSci Network: Physics|
1A) It has to do with constructive and destructive interference of the two wave fronts from the slits. A single coherent light source is used in the experiment which produces two in-phase wave fronts on exit from the slits. It is these two waves that interact causing the characteristic observed pattern - alternating light and dark bands. 2A) The classical double-slit experiment uses a single coherent light source that illuminates a plate with two slits cut into it. Therefore, the two waves exiting the slit will be in-phase. Coherent waves must not only have the same amplitude, but must also have a constant phase difference (zero). Goto http://micro.magnet.fsu.edu/primer/java/interference/doubleslit/ for a visual animation and description of the original experiment.
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