After showing that light traveled as waves, scientists went on to show that the colors of light seen in the light spectrum are a function of the wave length, the distance between peaks of successive waves. Type into your browser:
light spectrum wavelengths images
Note that the wave lengths are measured in nanometers (nm). A nanometer is one billionth of a meter, or one millionth of a millimeter. Have students try to visualize a millimeter (about the thickness of a dime) divided into a million parts.