Whom should I approach in the Physics Department to act as a referee for a job or internship application?
If you need a personal reference then please ask your college mentor or Senior Tutor. For an academic referee it is normal to use your physics academic adviser. If your adviser is unavailable, or if you need a second academic referee, then you could ask your current or previous physics tutor. If you are a Natural Scientist and have no physics tutor this year due to your module choices, then you may approach the Physics Department's Natural Sciences Liaison Officer (see http://www.dur.ac.uk/physics/students/natsci/) to act as academic referee. The Director of the Natural Sciences programme is also able to provide a general (not subject-specific) academic reference.
If you are taking a module with significant laboratory component (such as the Level 3 Laboratory Project) then you may consider asking your Laboratory Supervisor to act as second academic referee.