Why are things colored?

These fifteen proposed causes of color are derived from a variety of physical and chemical mechanisms and are summarized in three groups in this exhibit: made light, lost light and moved light. The fifteen proposed causes of color are also sometimes simplified into five groups: simple excitations and vibrations, transitions between molecular orbitals, transitions involving energy bands, transitions involving ligand-field-effects, and geometric and physical optics.

Made light:

• Simple excitations and vibrations explain the colors of incandescence (e.g. light bulbs) and gas excitations (lightning, aurora).

Lost light:

• Simple excitations and vibrations explain vibrations and rotations (blue ice and water).
• Molecular orbitals explain the colors of organic compounds (chlorophyll and indigo in plants, carotenoids found in algae or small shrimp that create the vivid pink of flamingos) and charge-transfer compounds (blue sapphire and lapis lazuli).
• Energy bands are involved in the colors of metals and alloys (gold, brass), pure semiconductors (cadmium yellow, vermilion) and doped semiconductors (yellow and blue diamond). Energy bands are also responsible for color centers (amethyst, smoky quartz).
• Ligand-field-effect colors (crystal field theory) are seen in transition metal compounds (turquoise, chrome-oxide green) and transition metal impurities (emerald and ruby).

Moved light:

• Geometric and physical optics cause dispersion, polarization (rainbow, sun dogs, halos, fire in gemstones), scattering (blue or black sky, blue eyes, red sunsets), interference (feathers of peacocks, soap bubbles, iridescent beetles), and diffraction (opal).

Complex processes in our eye and brain allow us to see the world in color. Not everyone perceives color in the same way. Small physiological differences are responsible for various types of colorblindness, and a better understanding of this phenomenon allows us to design images that can be recognized by the colorblind as well as those who perceive color. Color vision systems in the animal kingdom take on a wide variety of forms, with some that are very simple and others that are far more sophisticated than ours.

The study of color theory is an ongoing investigation. Through the ages, this field has engaged scientists, artists, and psychologists in a heated debate, as it involves deep questions ranging from how we perceive and use color, to the nature of light itself.