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.
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.
