Quick Facts
Born:
Oct. 6, 1893, Seoratali, near Dacca, India
Died:
Feb. 16, 1956, New Delhi (aged 62)
Subjects Of Study:
Saha equation

Meghnad N. Saha (born Oct. 6, 1893, Seoratali, near Dacca, India—died Feb. 16, 1956, New Delhi) was an Indian astrophysicist noted for his development in 1920 of the thermal ionization equation, which, in the form perfected by the British astrophysicist Edward A. Milne, has remained fundamental in all work on stellar atmospheres. This equation has been widely applied to the interpretation of stellar spectra, which are characteristic of the chemical composition of the light source. The Saha equation links the composition and appearance of the spectrum with the temperature of the light source and can thus be used to determine either the temperature of the star or the relative abundance of the chemical elements investigated.

Saha became professor of physics at the University of Allāhābād in 1923 and was elected a fellow of the Royal Society in 1927. He went to the University of Calcutta in 1938, where he was instrumental in the creation of the Calcutta Institute of Nuclear Physics, of which he became honorary director.

In his later years Saha increasingly turned his attention to the social relation of science and founded the outspoken journal Science and Culture in 1935. In 1951 he was elected to the Indian Parliament as an independent. He co-authored A Treatise on Heat (4th ed., 1958) and A Treatise on Modern Physics (1934). One of his most important papers is “Ionization in the Solar Chromosphere,” Phil. Mag. (vol. 40, 1920).

This article was most recently revised and updated by Encyclopaedia Britannica.

ionization, in chemistry and physics, any process by which electrically neutral atoms or molecules are converted to electrically charged atoms or molecules (ions) through gaining or losing electrons. Ionization is one of the principal ways that radiation, such as charged particles and X rays, transfers its energy to matter.

In chemistry, ionization often occurs in a liquid solution. For example, neutral molecules of hydrogen chloride gas, HCl, react with similarly polar water molecules, H2O, to produce positive hydronium ions, H3O+, and negative chloride ions, Cl-. At the surface of a piece of metallic zinc in contact with an acidic solution, zinc atoms, Zn, lose electrons to hydrogen ions and become colourless zinc ions, Zn2+.

Ionization by collision occurs in gases at low pressures when an electric current is passed through them. If the electrons constituting the current have sufficient energy (the ionization energy is different for each substance), they force other electrons out of the neutral gas molecules, producing ion pairs that individually consist of the resultant positive ion and detached negative electron. Negative ions are also formed as some of the electrons attach themselves to neutral gas molecules. Gases may also be ionized by intermolecular collisions at high temperatures.

The environmental effects of nuclear radiation
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radiation: Ionization phenomena

Ionization, in general, occurs whenever sufficiently energetic charged particles or radiant energy travel through gases, liquids, or solids. Charged particles, such as alpha particles and electrons from radioactive materials, cause extensive ionization along their paths. Energetic neutral particles, such as neutrons and neutrinos, are more penetrating and cause almost no ionization. Pulses of radiant energy, such as X-ray and gamma-ray photons, can eject electrons from atoms by the photoelectric effect to cause ionization. The energetic electrons resulting from the absorption of radiant energy and the passage of charged particles in turn may cause further ionization, called secondary ionization. A certain minimal level of ionization is present in Earth’s atmosphere because of continuous absorption of cosmic rays from space and ultraviolet radiation from the Sun.

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Erik Gregersen.