No matter the size of the room, light plays an important role in how the space feels. The more you make use of natural light in those spaces, the fewer lighting fixtures you will need and the less energy you will use. Light also enhances the color and beauty of the other elements within the room. Mirrors reflect light instead of absorbing it, so they trick the eye into thinking the room is brighter and larger, depending on where they are placed.
So instead of adding more lights to a dim or dull room, place mirrors near a light source to give the effect of more lighting. For standing and table lamps, place the mirror behind them and at an angle to spread the light across the room. To reflect natural light from a window, hang a mirror on the wall across from the window. Doing this also gives the illusion of another window.