I applied at career fair. Two days after, recruiter reached out to schedule a time to do an online written exam. An HR was monitoring using the camera and there were about 30 people at the same time.

It's not a formal interview. Knew about this company at a annual conference. Talked with one of the scientists at their booth for a summer internship position. The guy asked me about the project I was working on and then suddenly asked if I knew eigenvalues. I told him the eigenvalue's math definition, but at the same time, I was unsure about his purpose of asking this question. Every college student must have learned eigenvalue problems. Rather than asking an interviewee directly what eigenvalues are, this concept is usually tested with the mixture of other knowledge by giving a concrete problem, like those in quantum mechanics, functional analysis, and linear algebra. Obviously, the guy was not satisfied with my answer and asked further what an eigenvalue is in physics. Again, it's a question without a point. Eigenvalues can have different meanings in various areas of physics. For the work I was doing, I told him it means ground state energy in Schrodinger equation. During the conversation, the guy seemed very impatient and kept playing a toy ball in the hand and from looking at me. After I gave him the answer, he barely talked with me furthermore. Don't know where went wrong, but if this is the manner of the interviewer, I couldn't imagine how to get along with him if I were hired.