If you wish to go for the Defense Research, then you need to do the course of MSc in Physics/Applied Physics course. After completion of this course, you have to apply for the GATE and on qualifying the GATE examination/the DRDO examination (if conducted) then you can go for the DRDO as a scientist and you can serve our mother nation India as a Defense Scientist.

For more information regarding GATE consider the link below:

https://engineering.careers360.com/exams/gate

For more information regarding Physics as a career consider the link below:

https://www.careers360.com/courses/physics-course

I hope this helps. Wish you a great day!