Jawaharlal Nehru Technological University Hyderabad B.Tech IV X-RAY METALLOGRAPHY Previous Question Paper (March - 2021 - R16)

When an X-ray beam strikes an electron, the oscillating electric field of the incident electromagnetic wave causes the electron to oscillate at the same frequency as the incident radiation. According to classical electrodynamics, an accelerating charge emits electromagnetic radiation. Consequently, the electron re-radiates or 'scatters' the X-rays in various directions. This process is known as Thomson scattering.

The intensity of the scattered radiation is highest in the forward and backward directions and lowest at 90 degrees to the incident beam. The scattered waves are coherent, meaning they maintain a constant phase relationship with the incident wave. If the incident beam is unpolarized, the intensity of the scattered beam depends on the angle of scattering, described by the polarization factor.

A simple sketch would show an incident X-ray wave vector hitting an electron at the origin, with scattered wave vectors radiating outward in different directions, representing the spherical wavefronts generated by the oscillating electron.

X-rays are high-energy electromagnetic radiations with wavelengths typically ranging from 0.01 to 10 nanometers. Their characteristic properties include:

• Penetrating power: Due to their short wavelengths, X-rays can penetrate through materials that are opaque to visible light, with the degree of penetration depending on the material's density and atomic number.
• Ionization: X-rays possess sufficient energy to eject electrons from atoms, causing ionization in gases and biological tissues.
• Fluorescence: They cause certain materials, such as zinc sulfide or calcium tungstate, to emit visible light when struck.
• Photographic effect: X-rays can darken photographic plates or films, which is the basis for traditional radiography.
• Diffraction: X-rays undergo diffraction when interacting with the periodic arrangement of atoms in a crystal lattice, following Bragg's Law, which is fundamental to X-ray crystallography.
• No charge: Being electromagnetic waves, they are not deflected by electric or magnetic fields.
• Speed: They travel at the speed of light in a vacuum.