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DIFFRACTION
OF X-RAYS THROUGH CRYSTALS
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| The
nature of x-rays is electromagnetic i.e. they are electromagnetic waves.
X-rays have very short wavelength of the order of 10 x 10 -10 m.
Therefore it is not possible to produce interference fringes of x-rays by
Young's double slit experiment or by thin film method. The reason is that
the fringe spacing is D x = lL/d and unless the slits are separated by a distance of 10 x 10 -10 m, the fringes so obtained will be closed together that they can not be observed. How ever it is possible to obtain x-rays diffraction by making use of crystals such as rock salt in which the atoms are uniformly spaced in planes and separated by a distance of order of 2 A to 5A. Therefore, the diffraction of x-rays takes place when they incident on the surface of crystals. |
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BRAGG'S
EQUATION
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Consider
a set of parallel lattice planes having spacing 'd' between each
other as shown. Consider two rays 'a' and 'b' incident on the surface of crystal of NaCl. After reflection, these rays reflected and are in phase. After reflection they interfere each other. |
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| The path difference between the two reflected rays is given by: | ||||
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| Now
the X-rays will interfere constructively if the path difference is an integral
multiple of wavelength l. Thus, |
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| This relation is known as Bragg's Law. The spacing of the atomic layers of crystals can be found from the density and atomic weight. Both 'm' and 'q' can be measured and hence the wave length of x-rays can be measured by using Bragg's equation. | ||||
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