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.

Every surface reflects or transmits the light into some degree. For example, glass (i.e. silicone dioxide) lets visible light go through and reflects the infrared light. A greenhouse and the widows at your house work in this way. Silicone, the grey semiconductor, is opaque within visible wavelength range and transparent to infrared light. Why? Electrons in atoms show characteristic oscillation frequencies, determined by their bonding energy. For the glass, this frequency lies in ultravioletWhen such light falls on glass, a resonant absorption appears. Thus ultraviolet does not penetrate glass.

The reflection in a traditional mirror, i.e. a coating of silver under glass is due to the fact that electromagnetic waves do not penetrate metals. The reflection properties depend on the concentration of free electrons in metal and the length of wave, silver reflects well.

A selective reflection in so-called coated lenses or sunglases comes about due to proper selection of dielectric constant and the thickness of the coating - similarly as in a soap bubble.

A sheet of glass reflects ca. 4% of the light intensity falling on it. When we pile up many sheets of glass, we can consider them a mirror, since the beams reflected by it will sum up.