There are institutes in India and abroad where you can do M.Sc and PhD after completing your under graduation in Engineering. The following are some of the institutes which are offering M.Sc Physica to engineers.
1.University of Duisburg-Essen, Essen, Germany
2.Central University of Haryana, Mahendragarh, India
3, University of Surrey, Surrey, England
4. Indian Institute of Space Science and Technology, Thiruvananthapuram, India
5. University of Pune, Pune, India
6.Lovely Professional Univesity, Phagwara, India
7.University of Delhi, New Delhi, India
8. Jawaharlal Nehru University, New Delhi, India
9. Tata Institute of Fundamental Research, Mumbai, India
10. Inter-University Center for Astronomy and Astrophysics, Pune, India
The admission procedure for each university will be different. So you have to go that particular university website and follow the procedures given there.
As you have not done your B.Sc in Physics, to get through the written test for admission may be a little difficult. So you have to concentrate on studying the basics of Physics. There are many coaching institutes where you can join and get trained in the basics of the subject and get ready for the examination
After completing your M.Sc you can do your Doctorate also. As on date after M.Sc you can appear for NET for becoming a lecturer. But UGC is contemplating to make Doctorate as a must for lecturership. So you should be ready to do PhD also to become a lecturer and teach Physics to undergraduate and PG students. That means you may have to spend a minimum of 5 years of getting qualified. Some Institutes mentioned above are offering integrated PhD course also. If you want you can try for getting admitted into that course also.
Alternatively, you can think of doing M.Tech in any related field and then can become a lecturer in any engineering college and while working there you can do your PhD also. You may not be able to teach directly Physics. But many engineering subjects are applications of Pure sciences only and hence you will get the satisfaction of teaching Physics. All Engineering students will have Physics in 1st year of their Engineering, you can easily teach that subject to engineering students. If you can convince the management they may allow you to teach one or two classes of that subject also. This will save a lot of your time and you will have plenty of chances to get a good job.
Alternatively, you can go for B.Ed after B.Tech. You can take Physics as one of the subjects in B.Ed. Once you complete the training course you can join as a teacher in any high school and start teaching Physics to students there. While working as a teacher you can for M.Sc also under Qualification improvement programme (QIP) and then you can become a junior lecturer to teach Physics to Inter students. While working as a Junior Lecturer you can work for your PhD and become a lecturer in Degree college and can teach B.Sc students.
You can think of all the above alternatives and select the best way you feel and go ahead.