As you see!

Not the window we are looking through is the object of our considerations. Look at the house in the background. Behind the green tree!

You've got it?

Lets zoom it up.

The windows look black! But we all are shure that the walls inside are not painted black. The situation is similar to that of the black cube with white paper inside. (see Blacker than black): The window swallows light! Evidently!! Obviously we install windows in the walls of a house, to have the swallowed light inside.

We have learned to make unvisible light visible. Nowadays we use CCD-cameras that are sensitive for infrared light. When those images are shown on a monitor the different kinds of the infrared light bring out different colours. On these false-colour-pictures the dark blue or green colours indicate only little intensity of emitted light while the bright red or yellow colours mean that rather a high intensity is coming out of the windows as you see on the picture on the left.

Human eyes cannot see the strong absorption rate at the same time as the high emission rate. On the other hand it is not adviseble to heat living rooms to show red glow, so we used the trick with the false-colour-picture. The kind of light that a black body emits and absorbs with high rates depends on the temperature of the material of this black body. Atabout 800 to 1000°C it changes from infrared to visible light. That's why at this temperature almost all kinds of material begin to show the phenomena of red glow. At even higher temperatures the light will turn into ultraviolet light.