Hence the atom is said to be raised to higher energy levels when an electron is nudged to an outer orbit. The energy input can be of many different kinds. Examples are heat, pressure, electrical current, chemical energy, and various forms of electromagnetic radiation. If too much energy is put into the elevator it goes flying out the roof. If too much energy is put into the atom, one or more of its electrons will go flying out of the atom. This is called ionization, and the atom, now minus one of its negative electrons and therefore positively charged, is called a positive ion.
But if the right amount of energy is put into the atom, one of its electrons will merely be raised to a higher energy level. Shown in [Figure 9], for instance, are the ground state (Circle No. 1) and two possible higher energy levels. As you can see there are three possible transitions.
Figure 9 Schematic representation of the electron orbits and energy levels of an atom. Each circle represents a separate possible orbit and each arrow a possible energy level difference.
The higher energy levels are abnormal, or excited, states, however, and the electron will shortly fall back to its normal (ground state) orbit (assuming some other electron has not fallen into it first). In order for the electron to do this (go back to its normal orbit), it must give off the energy it has acquired. This it does in the form of electromagnetic radiation.
The energy difference between the two levels will determine what kind of radiation is emitted, for there is a direct correlation between energy and frequency.[8] If the energy difference between the two levels is such that the frequency of emitted radiation is roughly between 10¹⁴ and 10¹⁵ cycles per second, we see the radiation as light. When more energy is added, the radiation emerges as ultraviolet or X rays. In other words the higher the energy difference, the higher the frequency, and vice versa. Thus it is that cosmic rays, with the highest frequencies known to man, can pass right through us as if we weren’t there.
This simple picture of energy levels and associated frequencies doesn’t quite hold for ordinary white light, however. Such light is generally produced by a process called incandescence, which results from the heating of a material until it glows. True, the atoms of the incandescent material are being raised to higher energy levels by chemical energy (as in fire), electricity (light bulb), or nuclear energy (the sun). In a hot solid, however, the explanation becomes more complicated. Many different electronic configurations are possible and the differences in energy among the various levels (which can be many more than the three shown in [Figure 9]) vary only slightly from one another. The result is a wide band of radiation.
Thus, while the incandescent electric bulb is a great advance over fire, it is still a very inefficient source of light. Because it depends upon incandescence, a considerable portion of the electrical input goes into the production of unwanted heat, for the bulb’s filament radiates in the infrared as well as the visible region.
For providing illumination, the fluorescent tube is far more efficient than the incandescent lamp: a 40-watt fluorescent tube gives as much light as a 150-watt incandescent light. This is because its radiation is more controlled, operating more in accord with our description of electronic energy levels. Hence more of its output is in the desired visual region of the spectrum.
In certain types of lighting, particular energy level changes may predominate, leading to the characteristic colors of neon tubes and vapor lamps. Although the resulting radiation bandwidth is narrow enough in these devices to appear as a definite color instead of the broad spectrum we know as white, it is still quite broad. In other words, the radiation is still frequency incoherent—and it is still spatially incoherent.