Bragg's Law of Diffraction

Class XII - Physics 

Diffraction of a wave occurs only when the wavelength of the wave is nearly equal to the spacing between the line on the diffracting surface. X-ray connot be diffracted by a transmission gratting because the spacing in a transmission gratting is very larger then the wavelength of the X- ray. Hence, diffraction cannot take place. So, X-ray can only be diffracted by the use of natural gratting since the spacing is equal to the wavelength of X-ray.

A law was given by Bragg's on this basis. He subjected the X-ray beam over the crystal or natural gratting and X-ray was defracted.

Let us consider , the rays of X-ray ABC and DEFGH incident on the crystal gratting at an angle (glacing) Ѳ. Let 'd' be the difference between the layers of crystal. Then, the path difference between the two rays is given by,

Path difference                = nλ …… (i)

From the above figure,

                Pathdifference = EF + FG             (ii)

Then, by geometry,

                EF = FG = dsinѲ

Then,    Path difference = dsinѲ + dsinѲ

                Path difference(nλ) = 2dsinѲ

This is the required expression for Bragg's law.