I agree with whoever said the whole mirrors and light switches thing is based mostly on expectations, but I also think that IRL, light switches work of their own accord. You simply flip them. In dreams, however, your mind has to do all the work, flip the switch and turn on the light. The mind has to recreate how the world works in a dream, which is why many things aren’t as they should be… your brain did it wrong. It messes up because we don’t think about such things–light switches, how many fingers we have, how mirrors work, etc–24 hours a day. Once we become lucid, however, our conscious brain can remember how things work–or, if you want, you can recreate things in a totally different manner.
Ah, how I love to ramble.