Bragg's law A beam of X-rays directed at a crystal interacts with the atoms of the crystal. The electrons in the atoms oscillate under the influence of varying electromagnetic field of the X rays. They become new sources of electromagnetic radiation. The waves emitted by the electrons, have the same frequency as that of incident radiation. The emissions will be in phase and re-enforce one another only in certain specific direction of the incidence of the X-rays. Bragg visualized the process, considering the atomic planes as semi transparent. They allow a part of the X-rays to pass through and reflect the other part. The incident angle (called the Bragg angle) is equal to the reflected angle. There is a path difference between rays reflected from plane 1 and adjacent plane 2 in the crystal (see fig).. The two reflected rays will re-enforce each other only when this path difference is equal to integral multiple of wavelength. The Bragg's condition for reflection is,

n l = 2 d sin q

where n is an integer, and l is the wavelength of X-rays.

The Bragg equation is used for determining the lattice parameter of crystals

				d
	Fig. b7 X ray diffraction in a crystal