Light is no longer thought of as an ``undulation in a luminiferous aether''. First Maxwell defined the equations of light acting as a wave and then Einstein succeeded in showing that light also behaves as a particle, a photon. Each photon carries a particular packet, or quantum, of energy with it which is related to the wavelength of the light through Maxwell's equation. Although it seems paradoxical, light must both be considered to be a wave and a particle. Modern formulations consider a photon to be similar to a standing probability density wave function known as a soliton.
These quanta of energy are often considered to be one dimensional numbers, and so are thought of as being able to be represented as points along an axis. In fact, it can be shown that a single photon cannot be represented as a one--dimensional number. The second dimension is commonly thought of as a phase--angle if one is considering the light to be a wave, or as a polar orientation if one is considering the light to be a particle. This second dimension can be clearly demonstrated with a polarizing filter. The multidimensional nature of the photon has been largely overlooked in research on color vision and only the one--dimensional quantum energy value of photon has been considered to be perceptible.
A light source will emit photons with various specific energy quanta. The values of most of these quanta are determined by the potential energy changes in electron configurations in the atomic structure of the light emitting substance. The light emitted by a heterogeneous body such as the sun is composed of photons of a wide distribution of energy quanta. As Newton noted, if one passes sunlight through a prism, photons of differing energy quanta are refracted at differing angles.
But the resulting spectrum is not uniform. Not all points along the energy axis are represented in sunlight sampled at the surface of the earth. There are dark lines in the spectrum. Some of these lines are due to the fact that not all values of energy are emitted by the sun, and some of these lines are due to the fact that some specific quanta of energy are highly likely to be absorbed by molecules in the atmosphere, primarily water (see Figure 6).
Figure 6. Light is radiated by the Sun creating an emission spectrum. The light passes through some medium such as the atmosphere, or reflects off of some surface creating an absorption spectrum. The additive and subtractive components of this process comprise the photons of light available for perception by the eye.
Figure 7. Light is radiated by the Sun creating an emission spectrum. Histogram showing the number of photons present for each of eight quantum energy values contained within a hypothetical sample of visible light. A distribution of photon energy quanta which falls on a single point on the retina will produce the sensation of a single color.
The bright lines in the spectrum are called emission lines and their range is called the emission spectrum. These lines are caused by high probabilities of photons with particular values of energy quanta being present in the light. The dark lines in the spectrum are called absorption lines and their range is called the absorption spectrum. The absorption spectrum is caused by particular values of energy quanta of photons having a high probability of being absorbed rather than reflecting off of the atoms comprising a particular surface.
Thus the light reaching the eye is composed of a distribution of photons of various energy quantum values as shown in the idealized histogram in Figure 7. This distribution is interpreted during the process of visual perception and is assigned a single subjective value --- a color. Colors do not exist independently nature, they are a by--product of perception.