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Unformatted text preview: Physics 7C University of California at Berkeley Department of Physics Diffraction Gratings I. Important Background Information About Interference and Diffraction We’ve already learned about interference in a previous lab. To remind you: interference is the phenomenon of two waves combining their effects at the point where they intersect. What’s important to know about the two waves is their relative phase. Waves that are in phase combine constructively and their effects reinforce each other. Waves that are out of phase combine destructively and their effects counteract each other. Only waves that have the same frequency and are coherent (maintain their relative phase for a long time) display a simple, repeated pattern of interference. In this lab we are going to learn about a phenomenon known as diffraction. When a wave travels past a comer or sharp edge, it bends, or diffracts, around it. This happens for all waves, including light. The easiest way to understand diffraction is by using Huygens’ Principle, which deals with the cross— section of a traveling wave, known as a wavefront. The Principle states that every point along a wavefront acts like a source of spherical wavelets (waves) with the same frequency and phase as the wavefront, and that if you draw the line or curve tangent to all those spherical wavelets after some time, you get the shape and position of the real wavefront after that same amount of time. What’s important is the idea that you can treat each point on a wavefront as an independent wave source, with the same frequency and phase as all the other point sources. If you treat the points of a wavefront passing an edge as independent wave sources, and then trace the curve of tangency to those waves a little while later, you’ll ﬁnd that the wavefront has bent around the edge (see Figure 1). DIFFRACTION GRATINGS 1 41 ...
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