handprint : causes of color

the causes of color

With few exceptions — the spectrum colors of the rainbow, prism or compact disc, the iridescence of feathers and oil slicks, the red or blue hues of the sky — all color arises from the exchange of energy between individual light photons and the electrons of individual atoms.

I doubt that knowledge of these processes can help a painter make an image on canvas or paper, but it can help the painter understand the world of light and the origins of sometimes mysterious or counterintuitive color effects. At a minimum, it may inoculate you against Bauhaus color mysticism!

Photons, Atoms and Electrons. The basic mechanism that creates color in objects and surfaces is a transformation in the spectral emission profile of reflected light. This happens either because some wavelengths of the reflected light are taken away from or added to the reflected light.

All atoms have a stable basic structure: a nucleus of protons of a positive electric charge, with some number of neutrons having no electric charge, surrounded by a number of electrons of a negative electric charge. In the simplest case the number of protons and electrons is exactly the same (equal to the atomic number of the element), so the atom as a whole has no electric charge.

The quantum structure of the material world dictates that the smallest entities of energy or matter cannot exist in random, infinitely divisible quantities, but are limited to discrete quanta or steps of energy. As a result, electrons swarm around the atomic nucleus at very specific energy states, called electron shells. The number of these shells is determined by the number of positively charged protons in the atomic nucleus and by the number of electrons occupying each shell at the same time. In addition, the maximum number of electrons that can occupy each electron shell (energy level) is fixed, though — an important detail — each shell is not necessarily full of electrons.

Quantum theory also dictates that a photon of light must represent a discrete quantum of energy, which increases exponentially with decreasing light wavelength and roughly equally with increasing light frequency.

What happens when light strikes an atom? Two things: it can be deflected by the electromagnetic field around the atom (a process called Mies scattering), or it can strike one of the electrons orbiting the nucleus. In the second case, two things can happen: the electron rebuffs or reflects the light particle, or it absorbs the particle.

If an electron absorbs a photon, the energy of the photon is added to the electron. Frequently this gives the electron sufficient energy to jump to a higher energy electron shell, farther from the nucleus. This is not the most stable perch for the electron, so it generally gives up the excess energy by emitting a new photon or photons, which moves it back to its original position.

the interaction of atoms, electrons and photons
A: an electron raised in energy by absorbing light; B: an electron lowered in energy by emitting light; C: different wavelengths emitted by electrons moving to different lower energy electron shells

The quantum energy implications of the shells are different from one atom to the next, as the electrons influence one another, and electrons in other shells affect the influence exerted by the nucleus.

Frequently the emitted photon or photons are at a lower wavelength than the absorbed photon. The most common case is when the energy of visible light is emitted as heat, at infrared wavelengths.

Electrons can cohabit with atoms in other configurations than locations within stable electron shells. In some cases an electron is shared between two atoms as part of a chemical bond, or skips across vacancies in the electron shells of several atoms in a molecule or crystal; or the behavior of electrons in shells is altered by the local arrangement of atoms in a crystal; or the electron is torn free of the atomic nucleus and exists as a free particle. In these various situations the electrons still interact with light, though the behavior changes in each case.

Finally, color is created by the scattering or refractive properties of matter. This produces many of the optical properties of matter, including refraction and reflection.

The table below summarizes the major variations in the causes of color.

the causes of color
process	description and major examples
electron transitions in free atoms and ions	atoms (often as a gas or vapor) are stripped of electrons by exposure to a large amount of invisible energy (such as electricity, x rays or heat); as the electrons reattach to the nucleus and descend through the electron shells, they lose energy by emitting photons (in gases or vapors, these are at fixed wavelengths)
	1. electron excitations: the energy of electrons is raised by chemical or physical energy, causing them to emit light: sodium vapor lamps, neon signs, flames, sparks, lightning, incandescence of heated metals 2. electron vibrations: a molecule is slightly deformed in a solid or liquid state, causing it to absorb tiny amounts of "red" light and emit tiny amounts of heat: blue green color of pure water or ice
crystal field colors	the interconnected structure of atoms combined into molecules, or condensed as liquids or solids, creates specific opportunities for light absorption and emission at low electron excitation levels
	3. transition metal compounds: certain atoms (chromium, iron, copper, cobalt) have incomplete inner (low energy) electron shells, and at low energies the electrons are responsive to light or low energy electrons: most inorganic pigments and fluorescing materials (light), some lasers and phosphors (electrons) 4. transition metal impurities: color produced by a clear crystal containing small amounts of a light sensitive transition metal, whose color behavior is affected by the specific atoms in the clear crystal around it: chromium (ruby, emerald), iron (jade, aquamarine, red garnet), copper (turquoise, malachite). 5. color centers: damage from x rays or radioactivity cause tiny holes in some crystals; these holes can be occupied by isolated electrons, which interact with light: amethyst, smoky quartz
transitions between molecular orbitals	in many molecules and some crystals, electrons are displaced by the valence bonds among the atoms, allowing the electrons to orbit several atoms; in this less bounded state the electrons are more likely to interact with or emit light
	6. charged transfer: a "shared" electron between atoms in a crystal jumps from one to the other if it absorbs light (blue sapphire, iron magnetite) 7. conjugated bonds: large molecules, especially of carbon rings joined with nitrogen and oxygen, share electrons in single and double bonds, and these shared electrons are more sensitive to light, heat or chemical reactions: organic dyes and pigments (most plant and animal colors, including chlorophyl and blood), lapis lazuli (ultramarine), fabric brighteners, fireflies
transitions in materials with energy bands	many heavy metal atoms and some types of crystals have energy levels so closely spaced across all atoms that tiny amounts of energy cause electrons to move freely from one atom to another, even across unlimited distances
	8. metallic conductors: most heavy metals absorb all light wavelengths and quickly emit at the same wavelength, producing a lustrous color (copper, nickel, mercury, silver, gold) 9. pure semiconductors: atoms connected by exactly 4 electron bonds form a gap in the available energy levels; the size of the gap determines a minimum energy required for electrons to jump to excitation levels across the gap, only light above this minimum energy is absorbed (carbon black, vermilion, cadmium yellow, diamond) 10. doped semiconductors: an impurity added to a colorless semiconductor introduces energy levels within the gap, lowering the minimum energy absorbed (blue or yellow diamond, light emitting diodes)
geometrical and physical optics	the interactions of light with matter or light with light change the direction in which light is traveling
	11. refraction: light is slowed as it passes from one transparent medium to another, and this slowing deflects its direction more or less, depending on wavelength (rainbows, prisms, chromatic aberration) 12. scattering: tiny particles bend light at their edges, and very tiny particles bend light passing close by, and the deflection is very large for small wavelengths (blue sky, red sunset, moonstone, star sapphire) 13. interference: reflections from the top and bottom boundaries of a very thin plate or liquid reflect light in different phases, producing color (oil slick colors, pearl, mica, some insect colors) 14. diffraction: reflections from a surface of closely spaced grooves or ridges reflect light in different phases, producing color (compact disc reflections, opal, some insect or bird colors)
Source: Kurt Nassau, The Causes of Color