Modern Physics
Thermion emission
Free electrons exist in all metals. Normally these electrons do not possess sufficient energy to escape from the metal surface, for, as they prepare to do so, positive charge is created in the metal which attracts backward the escaping electrons. If the metal is heated, the electrons get their kinetic energies increased and so can escape from the metal. These electrons are called thermions and their emission from a heated metallic surface is called thermion emission.
Properties of X-rays
(a) X-rays are electromagnetic waves like light rays, (b) Wavelengths of X-rays are smaller than those of visible light, for this reason, they cannot produce any sensation of vision, (c) X-rays affect photographic plates and create fluorescence on some fluorescent substances, like, zinc sulphide, barium platino-cyanide etc. (d) X-rays travel in vacuum with the speed equal to that of light.
Uses of X-rays
The penetrating property of x-rays through matters, finds many applications.
(1) In medical science: (a) To detect fractured or broken bones. X-rays easily pass through flesh but not through densely constituted bones. So, photographs of bones may be easily taken with the help of x-rays. From the photographs, any broken or fractured part of bone is located, (b) X-rays are sometimes used, to treat cancerous cells or tissues, since, x-rays can destroy living cells.
(2) In physical sciences: X-rays are used in studies of crystal structures and structures of metals. So, with the help of X-rays, crystalline diamond may be distinguished from non-crystalline glass.
(3) In customs departments: Arms or other illegal items if hidden in bags or boxes can be viewed with the help of x-rays.
(4) In industry: X-rays are used to detect cracks or defects produced during casting and-welding in metal structures.