In film school, I wanted to project a film of mine in portrait (vertical) orientation rather than landscape (horizontal) orientation. However, my film professor told me that the projector couldn't be tipped on its side; oil in parts of the projector would drip over the film and onto the light, possibly making it explode.
How did I rotate the projected image 90 degrees without tipping the film projector?
HINT:
I used 2 flat mirrors.