Mirror

It is one block tall and one block wide, and it sits on the wall flush to the block its placed on. You can craft one with a glass pane in a stonecutter and it has no border, although you can add one if you want by surrounding it with different ingots in the crafting table, and maybe also diamonds and emeralds. It reflects light so that way you can have a cool light system in your mine by have one torch and the mirror reflects the light to other mirrors, thus spreading the light. You could also connect a daylight sensor to the beam of the light so that way you could make different things turn on that way.