20-08-2013, 04:23 PM

Diffraction of light

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HISTORY

Diffraction effects were first carefully observed and characterized in 1665 by Francesco Maria Grimaldi, who also coined the term diffraction. Isaac Newton studied these effects and attributed them to inflexion of light rays. James Gregory (1638–1675) observed the diffraction patterns caused by a bird feather, effectively the first diffraction grating. Thomas Young observed two-slit diffraction in 1803 and deduced that light must propagate as waves. Fresnel did more definitive studies and calculations of diffraction, published in 1815 and 1818, and thereby gave great support to the wave theory of light that had been advanced by Christian Huygens and reinvigorated by Thomas Young, against Newton's theories.

General facts about diffraction

Several qualitative observations reveal that :

Light which passes through a narrow slit does not produce a sharp, geometrical shadow which images the slit, but a distributed pattern of light intensity produced by diffraction.

Variation of Diffraction with slit witdh

The angular spacing of the features in the diffraction pattern is inversely proportional to the dimensions of the object causing the diffraction, in other words: the smaller the diffracting object the 'wider' the resulting diffraction pattern and vice versa. (More precisely, this is true of the sines of the angles.)

The diffraction angles are invariant under scaling; that is, they depend only on the ratio of the wavelength to a dimension, a, of the diffracting object.

When the diffracting object is repeated, for example in a diffraction grating the effect is to create narrower maximum on the interference fringes, concentrating its energy within a narrower range of angles. The third figure, for example, shows a comparison of a double-slit pattern with a pattern formed by five slits, both sets of slits having the same spacing, a, between the center of one slit and the next.

Diffraction of particles

It is the diffraction of "particles," such as electrons, which stood as one of the powerful arguments in favor of quantum mechanics. It is possible to observe diffraction of particles such as neutrons or electrons and hence we are able to infer the existence of wave-particle duality. Indeed, this diffraction is a useful tool; the wavelengths of these particle-waves are small enough that they are used as probes of the atomic structure of crystals. See electron diffraction and neutron diffraction.

Fraunhofer Single Slit

This is an attempt to more clearly visualize the nature of single slit diffraction. The phenomenon of diffraction involves the spreading out of waves past openings which are on the order of the wavelength of the wave. The spreading of the waves into the area of the geometrical shadow can be modeled by considering small elements of the wavefront in the slit and treating them like point sources.