Yes, one can get admission into M.Sc. Physics in universities in Bihar after completing B.Tech in Mechanical Engineering. For applying to any other postgraduate course from a different field, there shouldn't be very strict limitations based on your undergraduate degree, but there might be some challenges that you'll need to think about-is

Eligibility

University-specific: Many universities will not have any undergraduate degree restrictions but may require specific courses offered in physics or mathematics as part of the B.Tech program. See the individual eligibility criteria for M.Sc. Physics programs offered at universities you are interested in.

Entrance Tests: Some colleges may require you to take entrance tests like JAM (Joint Admission for MSc) or might even be some private entrance test conducted by the college. The entrance tests check your surface knowledge of all basic Physics concepts.

Preparation:

As you would be doing M.Sc. in Mechanical engineering, your physics background may not be upto the mark. You may need to refresh your knowledge in Physics. You may enroll in some other courses or self-study the basic coursework of Physics to prepare you for an entrance exam or help fill in the gap while competing with other M.Sc. Physics aspirants.

https://engineering.careers360.com/colleges/list-of-be-btech-colleges-in-bihar