The Allahabad University Reorganization Act of 1921 transformed the University from an Affiliating and Examining Body into a Teaching and Residential University. The reorganized unitary University of Allahabad in 1922 started with the then Muir Central College converted into the science block of the University. Department of Physics, Mathematics, Chemistry, Botany and Zoology were established to constitute the Faculty of Science.
The existing Physics laboratories were extended and new laboratories constructed during 1922 - 23. On the advice of Einstein and Eddington, Saha was appointed as the first Professor and Head of the Department in 1923. When Prof. Saha joined the Department, there were about 120 B. Sc. And 20 M. Sc. Students, the laboratories were well stocked with demonstration and classroom apparatus suitable for undergraduate classes but ill equipped for M. Sc. teaching and research. But, soon Prof. Saha's love and labour bore fruit and the Department became famous world wide for its pioneering research work on Spectroscopic Analysis of Atomic Spectra, Ionospheric Studies, Acoustic & Ultrasonic Measurements, X-ray Spectroscopy. It will not be out of place to mention that the Photography class of the Department is the oldest such course being run by any Indian University. Nobel Laureates like Compton, Eddigton, Raman, and Summerfield have visited the Department.
Prof. Saha left Allahabad to join Calcutta University as Palit Professor in 1938 initially on leave but finally resigned with effect from March 31, 1940. Then the post of Professor and Head of the Department was offered to Nobel Laureate Prof. Erwin Schrodinger. Schrodinger accepted the offer but he could not come out to join the post due to the onset of World War II. Finally, R. S. Krishnan, F. R. S. joined the post in 1942.
He strengthened the existing infra-structure for post-graduate teaching and research, thoroughly revised and updated the syllabus of both B.Sc. and M.Sc., placed the Photography class on permanent footing by instituting a Diploma Course in Photography in addition to the existing Certificate Course in Photography. He also started Physics Colloquium for advanced students for reading original papers and discussion of special topics in Physics. After serving the Department for 5 years he left Allahabad in 1947 to join the newly established National Physical Laboratory at Delhi as its first Director.
Both Prof. Saha and Prof. Krishnan were honored by the University for their distinguished services with award of Honorary Doctorate Degree during the Diamond Jubilee Celebrations at a special convocation on December 13, 1947.
Dr. R. N. Ghosh who worked with Prof. Raman at Calcutta on musical instruments initiated the work on acoustical analysis. Work on absorbing materials was also carried on to improve the acoustic of the halls. He also started the work on ultrasonic studies with the design and fabrication of ultrasonic interferometer for measurements of ultrasonic velocity and absorption in liquids. Works of Prof. G. S. Verma and Prof. S. K. Kor were very well recognized. The group is currently engaged in studying the properties of liquid crystals in different frequency regions at different temperatures.
Prof. K. Banerjee who joined in 1952 initiated the work on X-ray studies of crystals. The growth of X-ray laboratory was rapid. Study of diffuse X-ray scattering for determining the elastic constants were undertaken. Dr. G. B. Deodhar started the work on X-ray spectroscopy. His group designed a fabricated curved spectrograph for recording extremely weak emission and satellite lines.
Prof. Singwi, who stayed for a short period was succeeded by Prof. S.N. Ghosh. Prof. Ghosh initiated the theoretical and experimental investigations of Earth and other planetary atmospheres. Extensive works on airglow and ionospheric structure, composition and chemistry were carried out. Later on, he became the first Head of the newly established J. K. Institute of Applied Physics of the University and Prof. Vachaspati became the Head of the department. Prof. Vachapati and his group did some very good works in the field of quantum optics and non-linear optics.
In late 40's Prof. Krishnaji initiated the work on microwave studies practically from the scratch. The group designed and fabricated all the necessary instruments including klystron power supplies and the frequency standard in the Departmental Workshop. Soon the group earned eminence for their work on dielectric studies of liquids, microwave spectroscopy and molecular interaction in gases. The research workers of this group established microwave laboratories in different institution of the country. Presently, this microwave group is led by Prof. S. L. Srivastava.
Prof. Bipin Kumar Agarwal besides working in experimental X-ray spectroscopy also stated work in theoretical particle physics in 1958 when very few Universities in India were active in this聽