Is there advantage an for a student to know out of the course question in Ph.D. interview? I am an undergrad student with Physics Major, currently in my last year. I'm from India, and recently I have given my first interview for an integrated Ph.D. in Physics. And there are several interviews that are ahead of me.

What I need to know is this: suppose the interviewer asks a student a question, which is off-topic that is to say, rarely thought of at the undergraduate level in any of the universities. The  student might simply say, &quot;I don't have a knowledge of the topic as we haven't learned this in our undergrad course.&quot; Or there are some students who go beyond their course and learn the topic by themselves. So suppose another student gives the answer to the same question because they learned it by themself. Can the interviewer judge the students based on this question? Is there any advantage for student B to read stuff that is out of the course?
Saying you don't know the answer to a question is poor interview technique regardless of the situation.
Of course, you can't be expected to know or study every possible topic you might be asked questions about, but that's generally not the aim of these more difficult or esoteric questions. Instead, the interviewer wants to find out how you think and whether you are able to understand the question and reason your way to an answer even if you're unfamiliar with the subject matter.
After all, that's what research is all about: finding answers to questions that no one's studied before. The ability to apply previous reasoning to a new question is particularly important in physics, as we believe the same physical laws should hold anywhere in the Universe. Therefore, a good answer to a such a question would be &quot;I'm not sure, but I would expect from my knowledge of X that Y might happen.&quot; And then go into detail as to why. It's also ok to ask for a moment to think before answering.
For what it's worth, PhD interviews are not typically like an exam, and you probably won't be aggressively grilled on your knowledge of a specific topic. More likely, the interviewers will get you to explain any previous research projects you might have done, and discuss your ideas for the future. That's not to say you shouldn't study topics outside your course, if you're interested in them and they're relevant to your intended PhD topic.