Why is the sky blue? Why is fire yellow? What about flamingos or emeralds? Scholarshave learned that all the colors in the universe originate from a merefifteen fundamental physical causes. These causes appear over and over,lending color to the world around us. Some common causes seem logical-- for example, both light bulbs and candles are colored byincandescence -- and others are surprising -- did you know that thecolors of peacock feathers and bubbles are both caused by interference?

These fifteen causes of color derived from avariety of physical and chemical mechanisms are summarized in fivegroups in this exhibit. Vibrations and simple excitations explain thecolors of incandescence (e.g., flames), gas excitations (neon tube,aurora), and vibrations and rotations (blue ice and water).Ligand-field-effect colors are seen in transition-metal compounds(turquoise, chrome-oxide green) and impurities (ruby, emerald).Molecular orbitals explain the colors of organic compounds (indigo,chlorophyll) and charge-transfer compounds (blue sapphire, lapislazuli). Energy bands are involved in the colors of metals and alloys(gold, brass), of semiconductors (cadmium yellow, vermilion), dopedsemiconductors (blue and yellow diamond), and color centers (amethyst,topaz). Geometrical and physical optics are involved in the colorsderived from dispersive refraction (rainbow, green flash), scattering(blue sky, blue eyes, red sunset), interference (soap bubbles,iridescent beetles), and diffraction (the corona aureole, opal).
webexhibits.org/causesofcolor/