OPTICAL RESONANCE
We have a great wealth of evidence that atoms behave like sharply tuned oscillators in the processes of emitting and absorbing light. Whenever the emission of light occurs under such conditions that the radiating atoms are effectively isolated from each other, as in a gas at low pressure, the spectrum consists of discrete, very narrow lines; i.e., the radiated energy is concentrated at particular wavelengths. An incandescent solid-e.g., the filament of a light bulb-emits a continuous spectrum, but the situation here is quite different, because each atom in a solid is strongly linked to its neighbors, causing a drastic change in the dynamical state of the electrons chiefly responsible for visible or near-visible radiation. We have just spoken of atoms as oscillators that emit their characteristic frequencies. But how does this fit in with the photon description of radiation, and with the picture of the radiative process as one in which the atom undergoes a quantum jump? The answer is by no means obvious. Before the advent of quantum theory, one could visualize an electron describing a circular orbit within an atom, and emitting light of a frequency equal to its own orbital frequency. But now we can only say that the frequency of the light is defined (through E = hy) by the