Good luck with your interview. You may have chosen a bad time to ask questions, as many schools will be on their Easter break (we had ours from 25th March to 11th April). One of the reasons to ask you to present something, may be to encourage you to research current specifications (syllabii). I don't know any which include Doppler effect at KS4: it used to be on my previous A level course but it's not on my current one! Pitching a lesson at too high a level is a common fault with "new" teachers. That's not to say don't be demanding or settle for low aspirations, just check that your (and your students) efforts are focussed on producing appropriate outcomes. Quickly check the specs: if you're unlucky and you've already prepared your talk you could possibly preface your talk by "I'm aiming to extend the gifted, more able child and encourage him or her to persue A level Physics". Good luck again.