To support his family, after leaving the University of Calcutta, Mitra secured an appointment as a lecturer at his former college, T. N. J. College. Following a brief period there, he was appointed a lecturer at Bankura Christian College. Frustrated by the lack of research opportunities at both institutions, Mitra channelled his energy into developing innovative experiments to demonstrate to his students and writing popular scientific articles in Bengali. In 1916, he was invited by Ashutosh Mukherjee to return to Calcutta University as a post-graduate physics scholar in the new University Science College. There he conducted research into the diffraction and interference of light under C. V. Raman, enhancing Raman's previous research on the diffraction of monochromatic light in an oblique single slit and also devising a better method for determining heliometer diffraction patterns. For this work, which Raman greatly appreciated, Mitra gained a D.Sc. degree in 1919. He also published three papers in the Philosophical Magazine, including one on "Asymmetry of the Illumination Curves in Oblique Diffraction", and another on Arnold Sommerfeld 's approaches to diffraction.