As an example of the latter, consider starlight entering a telescope. When both source and screen are far from the aperture, the term Fraunhofer diffraction is used. For example, an open window can cause sound waves to be diffracted through large angles.įresnel diffraction refers to the case when either the source or the screen are close to the aperture. With a large aperture most of the beam will pass straight through, with only the edges of the aperture causing diffraction, and there will be less "fuzziness." But if the size of the aperture is comparable to the wavelength, the diffraction pattern will widen. If both the source and the screen are far from the aperture the amount of "fuzziness" is determined by the wavelength of the source and the size of the aperture. The diffraction pattern will look something like the aperture (a slit, circle, square) but it will be surrounded by some diffracted waves that give it a "fuzzy" appearance. When a source of waves, such as a light bulb, sends a beam through an opening or aperture, a diffraction pattern will appear on a screen placed behind the aperture. Instead, there is a gray area along the edge that was created by light that was "bent" or diffracted at the side of the pole. But careful observation of the shadow's edge will reveal that the change from dark to light is not abrupt. From a distance the darkened zone of the shadow gives the impression that light traveling in a straight line from the Sun was blocked by the pole. Consider the shadow of a flagpole cast by the Sun on the ground. All waves are subject to diffraction when they encounter an obstacle in their path.
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