*♦ Learn about : Moseley’s Law , Diffraction of X- Rays & Uses of X – Rays ♦*

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Moseley studied the characteristic X-ray spectrum of a number of a heavy elements, and observed a simple relationship between them. He found that the spectra of different elements are very similar, and with increasing atomic number Z , the spectral lines nearly shift towards shorter wavelength or higher frequencies.

He plotted a graph of the k-series between the square root of frequency (√ν)and atomic number (Z) and found it very approximately to be a straight line.

**He therefore concluded that the square- root of the frequency of a K-line is closely proportional to the atomic number of the element. This is called Moseley’s Law and may be expressed as :**

a = proportionality constant

b = screening constant

a and b does not depend on the nature of target.

**Diffraction of X- Rays :-**

(1) According to Laue the wavelength of X-Rays is very small and the atoms in crystals are arranged in form of three dimensional lattice.

(2) The diffraction of X-Rays is not possible by ordinary grating because the size of grating element is much larger than the wavelength of X-Rays.

(3) Diffraction of X-Rays is possible by the crystals. Because the inter atomic spacing in a crystal lattice is of the order of wavelength of X-Rays.

(4) Diffraction of X-Rays was first verified by laue spots.

(5) Diffraction of X-Rays takes place according to Bragg’s law

- (a) 2d sin θ = nλ
- (b) For maximum wavelength n
_{min}= 1

(sin θ)_{max} = 1

λ_{max} = 2d

__Uses of X – Rays__

(i) In Study of crystal structure

(ii) In Surgery

(iii) In Medicine

(iv) In Spying

(v) In Engineering

(vi) In Research in the laboratories

(vii) In Industries

(viii) In Radiography