Quantum physics teaches us that we may talk about radiant energy as traveling either as waves in space or as discrete packages called either photons or quanta. When radiation impinges on matter, it has a probability of being absorbed which depends on (1) the energy in the photon (or its equivalents in wave terms: the wavelength or the frequency of the light), and (2) the nature of the matter.

From the photon view, the photon may be absorbed if and only if there are two energy states (roughly two electron orbits) whose energy difference corresponds exactly to the photon energy, as sketched in the figure. And, of course, there must be an electron in the lower of the two states, for there must be some particle to absorb the energy. If the structure of the matter is such that there is no energy difference equal to that of the impinging photon, the photon may impart its energy to the electron, but the electron will rapidly return to its original state and reradiate the energy. Thus, in effect, this kind of matter doesn't absorb the energy at all; because of the time taken for the absorption and re-emission, however, the reradiated photon is delayed relative to its initial path, and we say that its phase has been shifted.

Substances (such as glass) in which the energy gap between the electrons and the next higher energy state is so great that virtually none of the incident energy of visible light is absorbed, make suitable insulators.

Electrical conductors, on the other hand, are composed of substances in which, by definition, the electrons are free to move, which means that there are so many permissible energy states that almost every incident photon would be absorbed. In advanced physics texts it is shown why the photons re-emitted by electrons in metals are virtually all in the backward direction. The result is that even a thin sheet of metal (e.g.,household aluminum foil) is quite opaque, because the incident light is reflected backward.

When light is absorbed, the energy does not remain very long in the absorbing substances. The fact that the colors of substances do not change during illumination may be taken to mean that the absorbing substances quickly revert to their original state, so that they are once again ready to absorb the same wavelengths. There1 are five main fates for the bulk of the light energy absorbed: (a) chemical reactions, (b) fluorescence, (c) phosphorescence, (d) energy transfer, and (e) internal conversion. (a) Photochemistry—the study of the chemical reactions resulting from the absorption of light—is among the more fascinating provinces of current biochemical research. One of its most significant aspects is photosynthesis. Since the latter is dealt with in standard introductory biology courses, it will not be covered here. The more esoteric chemical aspects are beyond the scope of these writings. Thus the only segment to be covered in this monograph is that of action spectra, to be dealt with in a later section. (b) Fluorescence is the simplest kind of result of light absorption: the re-emission of most of the light. If the light is emitted very soon after absorption (in no more than one microsecond), the re-emission is termed fluorescence. Because the system cannot emit more energy than it absorbed, the fluorescence is quite generally of a longer wavelength (more toward the red end of the spectrum) than the incident absorbed light. Because of rapid rearrangement of internal electron orbits [these are the changes shown going from (b) to (c) in the figure], fluorescence is normally due to a transition from a particular excited electron orbit to a particular lower orbit very near the normal electron orbit; thus the light is usually monochromatic. If a substance absorbs in several regions of the spectrum (e.g., chlorophyll absorbs strongly both in the blue and in the red) the internal rearrangements referred to yield the result that the fluorescence is as though only the longer wavelength is absorbed (chlorophyll fluoresces only in the red). (c) Phosphorescence is the emission of light considerably later than fluorescence emission—phosphorescences lasting a few seconds are not at all uncommon. The light emitted is primarily the same as in fluorescence. The reason for the delayed light emission in phosphorescence is that the electron is trapped in an orbit (perhaps E2 ) from which, according to the rules of quantum physics, it cannot readily jump down to the normal orbit. After a time, which may be as long as a few seconds, the electron manages to get to the particular excited orbit from which it can jump with the emission of the normal fluorescent light. Because the molecules exhibiting appreciable phosphorescence are, de facto, in high energy states for considerable periods, they tend to be very reactive chemically. (d) Energy transfer refers to the transfer of the absorbed light energy from the receiving molecule to another molecule. It will occur if the molecules are sufficiently close to each other and if the energy of the excited electron in the absorbing molecule chances to be matched to a possible excited energy state of the other molecule. (e) Internal conversion covers a number of experimental situations. When electrons are excited by absorption of photons, they usually do not reach the next highest orbit but an orbit somewhat higher, so that they execute a vibration around the new orbit (the vibration, naturally, must be one allowed by the rules of quantum physics). This extra vibrational energy is dissipated by being communicated to the surrounding medium or to vibrations of the lattice if the atom is part of a larger structure. The electron thus reaches the lowest state of vibration of the new orbit, and this is the particular excited electron orbit referred to above in the discussion of fluorescence. The extra vibrational energy may be the source of a number of other effects ranging from rearrangements of the atoms composing a molecule to the dissociation of molecules.

The general picture, then, is that photons absorbed by electrons are usually re-emitted backward by free electrons, as in metals, and may be re-emitted primarily in the incident direction if the photon energy does not match any of the energy states available to the electrons.

These possibilities are sufficient to enable us to learn many things through the use of light. The phenomena which we shall discuss include (1) light absorption, (2) birefringence, (3) dichroism, and (4) action spectra.