W.L. Bragg and W.H. Bragg studied the diffraction of X-rays in detail and used a crystal of rock salt to diffract X-rays and succeeded in measuring the wavelength of X-rays.

*Bragg's
law for X-ray diffraction*

W.L. Bragg and W.H. Bragg studied the
diffraction of X-rays in detail and used a crystal of rock salt to diffract X-rays
and succeeded in measuring
the wavelength of X-rays.

Consider
homo-geneous X-rays of wave length λ incident on a crystal at a glancing angle
θ. The incident rays AB and DE after reflection from the lattice planes Y and Z
travel along BC and EF respectively as shown in Fig .

Let
the crystal lattice spacing between the planes be *d.* BP and
BQ are perpendiculars drawn from *B* on
*DE* and *EF *respectively. Therefore, the path difference between the two
waves* *ABC and DEF is equal to *PE + EQ*.

In the ∆*PBE*, sin θ = PE/BE (or) *PE* = *BE* sin θ = *d* sin θ

In the ∆*QBE*, sin θ = EQ/BE (or) *EQ = BE* sin θ = *d*
sin θ

Path difference = *PE* + *EQ* = *d* sin θ + *d*
sin θ = *2d* sin θ

If this path difference 2*d* sin θ is equal to integral multiple of wavelength of
X-ray i.e. *n**λ*, then constructive
interference will occur between the reflected beams and they will reinforce
with each other. Therefore the intensity of the reflected beam is maximum.

*2d *sin θ* *=* n**λ** *where, *n*
= 1, 2, 3

etc. This is known as Bragg's law.

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