Chapter 9 299 All materials have an index of refraction greater than 1. Like vacuum, air has a value very close to 1. Most of the polymeric and oil substances used in cosmetic hair care products have index of refraction values that range from 1.4–1.6—oils are closer to 1.4 and polymers usually range from 1.5–1.6. Diamond is an example of a material with a very high refractive index (ca. 2.417). Hair has an average value of 1.54 however, it is important to note that this is an average value and that each morphological component of hair interacts differently with light. A material’s refractive index will determine how much light will bend when it reaches the interface. If the light wave decreases speed as it enters a medium, light will bend toward the normal. In contrast, if the light wave increases speed as it enters a medium, light will bend away from the normal. The refractive index dependence of light’s direction of travel when confronting an interface is dictated by Snell’s law: Eq. 2 For example, if we were to assume n 1 = 1.00029 for air and n 2 = 1.54 for hair with light incident at an angle of 45 degrees, we could predict the direction of light travel within the hair medium—in this case ~ 27 degrees. An illustration of light refraction when light waves enter and exit a fiber is provided in Figure 2. The direction of the light rays is arbitrarily shown for illustration in order to demonstrate that light rays will change the direction of propagation when they enter the fiber. In the cosmetic industry, it was always rumored that higher refractive index polymers or oils provide greater luster. To some extent this is true. For example, for many years phenyltrimethicone was considered the standard luster treatment among the oils. And, in fact, it does have a higher refractive index (1.46) than most oils. On the other hand, most polymers used in hair care applications have refractive indices between 1.50 and 1.56. It is not surprising that polymers in this class provide greater specular reflection