B.Sc

You can not get admission in B.Sc third-year after Diploma in Engineering.

Dear Kamlesh,In Indian education system, there is not any 'dual specialisation' exist. However, you can do double B.Sc in astronomy (from one institution in regular mode) and B.Sc in quantum physics from another college in distance learning mode. Actually, this is practically impossible but this is the only option you are left with if you are really want to do both at a single time. But my suggestion is that go for only one in which you are more interested.
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Good wishes.

Hi, if you want to become an astronaut then you must do astrophysics as it is the study of mostly space but in simple physics, you do not get the opportunity to gain more knowledge about space.

It is not a professional qualification. So, you have to do your PG and you must get a relevant research qualification. Then only, you will have a good career.

You can do M.Sc in medical itself as it is what you did your B.Sc with.