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.
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.
[Illustration: 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.
To understand this, let us return for a moment to the group of radio
antennas we showed in Figure 8. All of them, you will recall, could be
made to radiate in phase. In the production of light, however, each
antenna is replaced by a single atom!
This creates two problems. First, because the energy stored in the atom
is quite small, it comes out not as a continuous wave but as a tiny
packet of radiation—a _photon_.[9] It has an effect more like the hack
of an ax than the buzz of a power saw.
Second, atoms are notoriously “individualistic”. When a batch of atoms
in a material has been raised to higher energy levels there is no way to
know in what order, or in what direction, they will release their
energy.
This kind of process is called _spontaneous emission_, since each atom
“makes up its own mind”. All we know is that within a certain period of
time—a short period, to be sure—a certain percentage of these higher
energy atoms will release their photons.
[Illustration: Figure 10 _Ordinary light is a jumble of frequencies,
directions, and phases._]
What we have, then, is incoherent radiation—a jumble of frequencies (or
colors), directions, and phases. Such light, symbolized in Figure 10,
works well enough in lighting up this page, but is almost worthless as a
carrier of information (and in other ways, as we shall see shortly).
About the best that can be done with it is to turn it on and off in a
sort of visual Morse code, which is exactly what is done on the blinker
communication systems sometimes used for ship-to-ship communication.
In other words, ordinary light cannot be modulated as radio waves can.
It is of interest to note, however, that ordinary white light _can_ be
made coherent, to some extent, but at a very high cost in the intensity
of the light. For example, we might first pass the light through a
series of filters, each of which would subtract some portion of the
spectrum, until only the desired wavelength came through. As can be seen
in Figure 11, only a small fraction of the original light would be left.
[Illustration: Figure 11 _Obtaining coherent radiation the hard way.
Filters and pinhole block all but a small amount of the original
radiation._]
Incoherent
Filters
Coherent in time
Pinhole
Coherent in time and space
We would then have monochromatic (one color) light, which is temporally
coherent radiation, but it would still be spatially incoherent. In our
diagram, we show three monochromatic waves. If we then passed this light
through a tiny pinhole as shown, most of these few remaining waves would
be blocked; the ones that got through would be pretty much in step. (In
a similar manner, a true point source of light would produce spatially
coherent radiation; but, as in the process described here, there
wouldn’t be very much of it.)
We have, finally, obtained coherent light.
The important thing about the laser is that, by its very nature, it
produces coherent light automatically.
Now....