At this point in our story we have a situation where low temporally coherent radio waves and microwaves can be generated, but nothing of higher frequency. Communications engineers have gazed wistfully, but almost hopelessly, at light waves, whose frequencies are millions of times higher than radio waves. Thus, just by way of example, some 15 million separate TV channels could operate in the frequency range between red and orange in the visible band.
What, then, is the problem?
Why is light so much more difficult to handle?
LIGHT AND THE ATOM
Since light waves have such high frequencies, a different mode of generation comes into play. We can no longer count on the controlled movement of free electrons outside atoms and molecules. Rather, light and all the radiations in the higher frequencies are generated by the movement of electrons inside atoms and molecules.
Let us review momentarily the modern, albeit highly simplified, conception of an atom. Remember that no one has yet seen one. We describe the atom on the basis of how it acts, as well as how it reacts to things scientists do to it.
For the present purpose, the best model we have of the atom is that of a miniature solar system, with a nucleus or heavy part at the center and a cloud of electrons dashing around the nucleus in fixed orbits.
The term “fixed orbits” is used advisedly.
Our planet moves in a certain orbit around the sun. If we attached a large enough rocket to the earth we theoretically could move it closer to or farther away from the sun. In the atom, we have learned, this cannot be done. An electron can only exist in one of a certain number of fixed orbits; different kinds of atoms have different numbers of orbits.
We might think in terms of an elevator that can only stop at the various floors of an apartment building. Each upper floor is like an orbit of the electron. But you get nothing for nothing in the world of physics, and just as it takes energy to raise an elevator to a higher floor, it takes energy to move an electron to an outer orbit.