4. The Spontaneous Emission

Thermal radiation is ubiquitous! It cannot be prevented by any means! Even the best isolated calorimeter is not a radiation-free space! Finally, it consists of energy bearing particles that store thermal energy at their quantized energy levels. Both, the particles of the substances investigated in the calorimeter and the particles of the calorimeter itself ceaselessly and spontaneously emit and absorb thermal energy.

In this fourth chapter, we will deal with the spontaneous emission because the spontaneity of this phenomenon is a very helpful key to the understanding of spontaneous thermodynamic processes.

The shelf model allows the respective emission spectrum to be calculated using very simple mathematical means for different substances and material portions. These calculations show that quantized energies and Planck's Radiation Law are mutually dependent. From the change in these spectra during the different thermodynamic processes, the transfer of thermal energy can be very well understood.

4.1. Blacker than Black

Three black cubes!

Black as black can be ! ! !

BUT WAIT.........Isn't there really anything blacker than black ? ? ?

Turn the basis to the front and you will see cubes having an aperture looking blacker than the black colour of the black carton

You can see the openings at the left and the right cubes quite well, becaus they are blacker than black. But what about the cube in the middle? It, too, has an aperture! Look at the next picture! A piece of wire disappears in the opening.

The cube in the middle has an aperture of the same size as the left cube, but it is not to be seen?! The opening of the right cube is smaller and it looks blacker than the carton. How is it possible that the opening of the middle cube cannot be seen except in case of putting something through the aperture into the inside of the cube? Who can solve this mistery?

If we put an object greater than the wire into the aperture, than you can see that the opening is of the same size as the one of the left cube. Have a look at it!

The size of the opening decreases the more, the larger the diameter of the object that we place in the cube and the opening becomes visible. Obviously smaller apertures look blacker than bigger ones. But how can we understand, that at first the aperture of the middle cube was not visible at all.

Now let's solve the mistery!

The left cube only consists of black carton, inside as well as outside. The insides of the other cubes are covered with white paper as you can see on the picture above.

A hollow cube absorbs much light through an aperture, maybe yet more than its black outer surface does. Even a cube with white insides may absorb just as much light as its black surface. This depends on the ratio between the opening area and the hollow space behind it. The aperture of the cube in the middle and the surface showed the same absorption rate. If the aperture becomes smaller (by any means) it will look blacker, i.e. the absorption rate inceases.

The opening of an inside black coloured hollow absorbes more light than the "normal-black" surface of any object!

But wait - we are not yet at the end of our mistery! The "hole" as a black coloured body not only absorbes more light than the surface of a massive black body, it even emits more light than the surface does! Unbelievable!

The light emitted by the aperture of a hollow is usually not visible for our human eyes. However, if the hollow space is heated strongly until it shows red glow, the phenomena will change. But -- how to make a carton glow red??

Well, you perhaps might enjoy to click on "The Black Emitter" in the navigation frame above?