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.
The light which is not absorbed is re-emitted in all directions, producing what is called scattering of the light; we mentioned the two extreme cases of total reflection and total transmission. Most cases are intermediate. If electron oscillations are equally possible in all directions, the incident unpolarized light beam will emerge essentially as it entered the suspension—except for the phase shift already mentioned. If, however, the particles or molecules in suspension are not isotropic, i.e., the electrons can oscillate more readily in one direction than in another, an incident unpolarized light beam will be split, because waves oscillating in one direction will have their phases shifted more than those oscillating in another direction. The net result is that so-called anisotropic molecules produce two plane polarized emergent beams. Since these emerge in somewhat different directions, the phenomenon is called double refraction or birefringence.
One of the problems which perturb biophysicists is the reason for the size and shape of biological structures. In this category comes the question of why we have a concave retina. In the early 1800's, one Johannes Müller was trying to figure out the answer to this question, and in speculating on the problem he remarked on some interesting aspects.
One important question about vision has to do with the number of photons required for seeing. The question has to be refined, because an immediate problem arises in deciding where these photons must be absorbed. Do they all have to be absorbed in the same rod (cones are neglected, since we know they are used for high light-intensity vision)? Can a single rod effectively absorb more than one photon in any arbitrarily short time interval? Do rods or groups of rods cooperate in vision? Do they cooperate in experiments to determine the minimum number of photons that may be detected?
Matter waves are of two types, which differ only in the direction of the vibration relative to the direction of propagation. In transverse waves the vibration is perpendicular to the direction of propagation (a plucked violin string, for example). In longitudinal waves the vibration is parallel to the direction of propagation (the pressure waves from a blast, or in front of a piston, for example). Most of the matter waves which are of interest here are, like water waves, a combination of both.