Chapter 9 301 characteristic specular reflection peak associated with luster.4,5 The characteristics of the specular reflection band are thought to be dependent on the cuticle geometry, including thickness, and the periodicity of the cuticle cells.4 Interference: A thin film of oil on water, a soap bubble, and certain bird feathers are just a few examples of materials that produce interference patterns, resulting in the presentation of multiple colors at the surface. Interference occurs when a light wave is split into two parts then later recombined. Light may reflect from the front surface of a film, or penetrate a thin film until it reaches the back surface layer, where it is reflected upward until it exits the thin film. At the point where the reflected light exits the film, the light reflected from the front and back surfaces recombines. The recombined waves can be in phase or out of phase—i.e. the crests and valleys will either match up or not. If the recombined waves are in phase, we observe constructive interference. On the other hand, recombined waves that are out of phase produce destructive interference. With regard to hair, interference was postulated as an explanation for perceived micro-color patterns observed in hair, which is clearly visible with an optical microscope. Cuticle cells are transparent—they do not contain melanin—and could act as a thin film. Gamez-Garcia and Lu reported that cuticle cells subjected to mechanical and thermal stresses exhibited interference patterns, while undamaged cells did not.6 The authors attribute such patterns to a combination of thin film interference and light diffraction. Absorption: Molecules, or chromophores, absorb electromagnetic radiation, which provide objects with their characteristic color (unless they absorb in the UV region and do not flouresce). Normally, chromophores have a highly conjugated molecular structure, which allows electrons to pass freely from one part of the molecule to the other. A material’s chromophoric composition will determine its color. In the case of hair, the most prevalent chromophore is melanin, which exists in two forms: 1) eumelanin provides black/brown tones, while 2) pheomelanin results in