X-rays is a form of electromagnetic radiation having wavelength in the range of 0.01 to 10 nanometers corresponding to frequencies in range $(3 \times 10^{16} Hz$ to $3 \times10^{19} Hz)$. Energy of X-ray range from 100 eV to 100k eV.

X-rays with photon energies above 5-10k eV (below 0.2-0.1 nm wavelength) are called hard X-rays. While those with lower energy are called soft X-rays. Due to their penetrating ability, hard X-rays are widely used to image the inside object e.g. in medical radiography & airport security.

Application of X-rays in Crystallography –

It is a tool used for identifying the atomic and molecular structure of crystal in which the crystalline atom cause a beam of incident X-rays to diffract into specific direction. By measuring the angles and intensities of these diffracted beams a 3D picture of density of electrons within the crystal is produced. From this electron density the mean positions of atoms in crystal as well as their chemical bond, their disorder & various other information can be determined.