How to decide whether a particular topic is studied in classical physics or modern physics?
What do you mean by it?
Why does it matter what classification is given to a topic?
Typically, they are studied in an order or grouping determined by the college's plan of study, and how to classify them isn't really a relevant issue.
I actually wanted to know what are the different branches of classical physics and modern physics.
Who says classical physics can't be modern?
The term "Modern" Physics may be specific to US and Canada.
It is more like an academic term (used as a course name in college).
Includes mainly relativity and "quantum physics" (which may not be the same as quantum mechanics proper but rather atomic and nuclear physics). I suppose includes the physics advances made after about 1900.
Modern physics is used as the title for the first year quantum mechanics and special relativity course at some universities in the UK also.
Ultimately, I think what the collective answers here are telling you is that these questions don't have a lot of meaning. The level of organization of topics you are suggesting you want to know about doesn't really exist.
Oh.. Okay! Thank you
Yes you are right.
Separate names with a comma.