Diffraction A kind of interference occurs whenever waves pass through very small openings in a barrier. On the far side, they spread out in expanding, round fronts that are capable of interacting with each other, resulting in an interference phenomenon called diffraction . The simplest case to examine is the double slit experiment . In this, two vertical slices are made very close together in a piece of foil and monochromatic, coherent light (one color and all in phase, as from a laser) is shown through them. The screen that the interference pattern appears upon is so far away compared to the distance l between the slits that we can pretend any rays from either slit aimed at the same spot on the screen are parallel. Obviously they can’t be truly parallel or they would not hit the same spot on the screen, but the difference in angle is so small that we can safely ignore it. The figure to the right shows two rays from a pair of slits in a barrier departing toward a screen at the same angle. In reality, the angle shown
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