8. Electron Diffraction - Electron Microscopy and Diffraction
Diffraction PHYS261 fall 2006. Diffraction is a phenomenon when a wave that passes through an...
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Transcript of Diffraction PHYS261 fall 2006. Diffraction is a phenomenon when a wave that passes through an...
![Page 1: Diffraction PHYS261 fall 2006. Diffraction is a phenomenon when a wave that passes through an aperture or around an obstacle forms a pattern on a screen.](https://reader036.fdocuments.us/reader036/viewer/2022081516/56649f535503460f94c7867a/html5/thumbnails/1.jpg)
Diffraction
PHYS261 fall 2006
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• Diffraction is a phenomenon when a wave that passes through an aperture or around an obstacle forms a pattern on a screen.
• What causes diffraction is interference of an infinite number of waves that are emitted by the points of the aperture
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• Huygens principle says that a large hole can be approximated by many small holes where each are a point source.
• The point source generating spherical waves is the source of diffraction.
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• There are two different limiting types of diffraction observations
- Fresnel diffraction patterns- Fraunhofer diffraction patterns
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Fresnel cases: quadratic dependence on the distance from the normal (from the "axis" or "the edge")
Fraunhofer cases: large distance; the quadratic term becomes negligible, the linear is important
Formulas discussed later
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• For Fraunhofer diffraction pattern there is a large distance between aperture and the screen.
• For Fresnel diffraction the distance between the aperture and the screen is generally small.
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The field generated by the source is propa-
gating towards an aperture and there will be
diffraction through the aperture.
The figure shows a bending of light around
a circular aperture:
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Example of Fresnel diffraction
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Example of Fresnel diffraction
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theta and l the wavelength lambda) then the Airy disk has its first minimum.
D is the diameter of the aperture.
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Fraunhofer diffraction.
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Airy disk: This is a Fraunhofer diffraction.
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• At a circular aperture the intensity pattern is called an Airy disk.
• It is a ring system so that the plots are radial sections of a pattern possesing circular symmetry. When theta = 1.22 lambda/D ( theta should equal the angle theta and l the
wavelength lambda) then the Airy disk has its first minimum.)
Fraunhofer diffraction.
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Example of Fresnel diffraction at straight edge:
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• The intensity distribution shows that at the
edge the intensity of the light is reduced to
a quarter of the intensity and behind the edge it is falling monotonically to zero.
Outside the intensity is increasing and it oscillates with increasing frequency.
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Fresnel diffraction at straight edge: