Indian astrophysicist noted for his development in 1920 of the thermal ionization equation (Saha 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).