A mirror or a window?

Why does the mirror reflect light and the window lets it in? Contrary to what may seem, it isn't so easy.
Look at the transparencies: when there are only few of them, they are transparent, and when you add some more of them they act like a mirror.

All surfaces let some of the light go through as well as reflect some of it. A 'transparent' sheet of glass reflects about 4% of the light.

Such a 'half-mirror' can also be found in a car. It can be switched over when the headlights of the car behind dazzle you.