Von Laue Experiment
The diffraction of a wave is possible only when the width of the
diffraction transmission grating is of the order of its wavelength. A
transmission grating has 6000 lines per cm. The width of the slit between two
lines of the grating is of the order of wavelength of ordinary light. (10 m).
llence, diffraction of ordinary light can be observed with the help of such a
transmission grating However, the wavelength of X-ray is of the order of 10
12 m Hence the diffraction of X-rays can not be obtained by using the
diffraction transmission grating.
The only one way to confirm the wave nature of X-rays (or any other
rays) was to observe diffraction. Hence, Von Laue and his co-workers in
to observe diffraction of X-rays. The problem for them was to obtain a
suitable grating for X-rays diffraction. That is, a grating element whoe
width between two slits is of the order of wavelength of X-rays.
Von and his coworkers thought that a crystal, where the spacing
between two atoms is of the order of 10-10 m, could meet their purpose. Then
in 1913, they devised an experiment as in figure no. 4 (a). They passed a fine
beam of X-ray through a zinc sulphide (ZnS) crystal and allowed the
emerging beam to fall on a photographic plate (P). After the X-rays beam
falling on the plate for several hours, the photographic plate was developed
for image. On the developed plate, it was found that a large number of spots
were regularly arranged around a central spot as in figure no. 4 (b).
From this experiment, Von and his co-workers concluded that in a
crystal the atoms are arranged in a regular three dimensional lattice and the
regular arrangement of atom in the crystal is responsible for diffracting the X-
rays. The regular spots observed on the developed plate were the images of
the diffraction patterns produced by the X-rays on the photographic plate
Hence, Von Laue and his coworkers were able to confirm the wave nature a
X-rays.
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