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.

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Presentation transcript:

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. What causes diffraction is interference of an infinite number of waves that are emitted by the points of the aperture

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.

There are two different limiting types of diffraction observations -Fresnel diffraction patterns -Fraunhofer diffraction patterns

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

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.

The field generated by the source is propagating towards an aperture and there will be diffraction through the aperture. The figure shows a bending of light around a circular aperture:

Example of Fresnel diffraction

theta and l the wavelength lambda) then the Airy disk has its first minimum. D is the diameter of the aperture.

Fraunhofer diffraction.

Airy disk: This is a Fraunhofer diffraction.

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.

Example of Fresnel diffraction at straight edge:

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.

Fresnel diffraction at straight edge: