Mirrors are just things that bounce back light. In ancient times they were made of polished brass or obsidian, but they can be of anything that takes a smooth enough finish to reflect the scene. for a long time they have usually been made of glass with a coat of silver or aluminium, with layers of coatings.
" . . .most often coated with silver or aluminium, implemented by a series of coatings:
Tin chloride
Silver
Chemical activator
Copper
Paint"
Feel free to ignore this bit of unrequested information, but I thought STB might like the input.
My bathroom mirror makes me look much better than any others in the house, so I try to use that one as much as possible.