Single Slit Diffractio2

Single Slit Diffractio2 - source between the source and the...

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Single Slit Diffraction When a pattern created by an aperture is viewed on a nearby screen we see a clearly recognizable image of the aperture with accompanying fringes. This is called Frenel or near-field diffraction. At larger distances the pattern spreads out much more, such that the image of the aperture is likely to be unrecognizable; in this region, moving the screen changes only the size of the pattern and not the shape. This is called the far-field or Fraunhoffer diffraction. We will only treat the latter in the case of a single slit. Fraunhoffer diffraction is the (linear) limit in which the incoming and outgoing wavefronts are essentially planar. This usually occurs when L , the distance between the aperture and the screen is L > d 2 / λ , where d is the width of the aperture. The Fraunhoffer condition can be achieved in practice by placing a lens with its focus at the
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Unformatted text preview: source between the source and the aperture and another lens between the aperture and the screen, with its focus at the screen. Consider a single slit of width d . Assume that plane, monochromatic waves fall on the slit. Because the screen on which the diffraction pattern is to be observed is far away (compared to the width of the slit), light rays heading for any point can be considered essentially parallel. Clearly, all rays heading towards the center of the screen will arrive in phase, and produce a maximum. Consider waves heading off at some angle m such that the path difference between the point source at A and the point source at B is . Figure %: Rays through a single slit....
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