Saha , Meghnad N.


Saha , Meghnad N.
(1894–1956) Indian astrophysicist
Born the son of a small shopkeeper in Dacca (now in Bangladesh), Saha won a scholarship to the Government School there in 1905 but was expelled for participating in the boycott of a visit by the Governor of Bengal. He completed his education in Calcutta, at the Presidency College, where he obtained his MA in applied mathematics in 1915. After lecturing in mathematics and then physics at Calcutta's University College of Science (1916–19), he visited London and Berlin on a traveling scholarship. He returned to India in 1921 and from 1923 to 1938 taught at the University of Allahabad. In 1938 he moved to the chair of physics at Calcutta where he remained until his death.
Early in his career Saha became interested in both thermodynamics and astrophysics and this led to his work on the thermal ionization that occurs in the very hot atmospheres of stars. In 1920 he published a fundamental paper, On Ionization in the Solar Chromosphere, in which he stated his ionization equation. The absorption lines of stellar spectra differ widely, with some stars showing virtually nothing but hydrogen and helium lines while others show vast numbers of lines of different metals. Saha's great insight was to see that all these spectral lines could be represented as the result of ionization. He saw that the degree of ionization, i.e., the number of electrons stripped away from the nucleus, would depend primarily on temperature. As the temperature increases, so does the proportion of ionized atoms. The remaining neutral atoms will thus produce only weak absorption lines that, when the temperature gets high enough, will disappear entirely. But the singly, doubly, and even triply ionized atoms will absorb at different sets of wavelengths, and different sets of lines will appear in stellar spectra, becoming stronger as the proportions of these ions grow.
In later years Saha moved into nuclear physics and worked for the creation of an institute for its study in India, which was later named for him. He also devoted much time to social, economic, and political problems in India.

Scientists. . 2011.

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