When x-ray is passed through very thin slits, it is called x-ray diffraction. The nature and arrangement of molecule in a crystals is studied through x-ray diffraction. In 1913, W. Laue showed the diffraction of X-ray through a crystal. The arrangement for the diffraction is shown below:
The apparatus consists of x-ray source, two slits, a ZnS crystal and a photographic plate placed straight to one another. When X-ray is passed though two slits and ZnS crystals for a hour, a faint regular spot were formed in the photographic plate. These spots are known as Laue spot. From this experiment, it was concluded that X-ray is electromagnetic wave and atoms of crystals are arranged in three dimensional lattice.
Bragg studied the diffraction of x-ray in detail and introduced a method for determining the wavelength of X-rays.
Let us suppose a crystal with lattice spacing d. when X-rays are passed to the crystals at a glancing angle, there raises a path difference between various rays in the screen due to the reflection of X-rays from different planes. This gives rise to formation of diffraction pattern on screen. Bragg studied these patterns and found a relation called Bragg’s law.
In the above figure the rays PQR and P’Q’R’ are diffracted from two successive planes of a crystal. The path difference is given by
Path difference = MQ’ + NQ’
= dsinθ + dsinθ
If the path difference is integral multiple of λ then bright fringe is obtained. So,
2dsinθ = nλ
The above equation is known as Bragg’s Law.