QUANTUM VIEW OF BLACKBODY RADIATION

A blackbody absorbs all radiation incident upon it, and emission from such an object depends solely on the temperature of the body. At hotter temperatures it tends to

Figure 1.1. Energy level populations at various temperatures.

have a higher blue content, and at lower temperatures the emission is seen as red. Examining the energy levels in this situation (as in Figure 1.1), we see that at low temperatures only low energy levels will be excited. The atoms in this case will have enough energy to cause transitions creating a reasonable amount of infrared light and some red light, but very few atoms will have enough energy to allow the production of blue light. As the temperature of the object is increased, higher atomic levels will be excited and blue emissions will be seen. This shift is due to the fact that blue light is literally more energetic than red light, as evident from the Planck relationship E = hv. It may also occur to the reader that blackbody radiation is broadband. Unlike line spectra produced by most gas discharges, thermal light tends to occur in the form of a continuum of wavelengths spanning a large range. The answer is in the spacing of energy levels in the emitting substance. Gases usually have well-defined discrete wavelengths, due to the fact that each atom in the gas is ultimately identical and completely independent (i.e., no interactions between the atoms in the gas lead to changes in the discrete atomic energy levels). In solids, atoms are closely packed and interact with each other, which serves either to widen energy levels to the point where they overlap or create new electron energy levels. This leads us to speak instead of the energy levels in solids as energy bands. In some cases these bands are simply a discrete level that has been broadened (i.e., spans a range of possible energies) through various mechanisms. If the discrete energy levels in Figure 1.1 were replaced by wide bands of possible energies, it is easy to see that the average light emitted will be broadband.