Chapter 9 297 Interaction of Light with Hair Sunlight and artificial light reach the surface of hair and interact in a variety of different manners. Light may be reflected or scattered from hair, either from the hair’s outermost surface or its interior structures. Refraction, on the other hand, occurs when light changes its speed of travel when going from one medium to another, causing it to bend, or change its angle of incidence. Interference is another phenomenon experienced by light, which is usually caused by thin films and results in the observation of iridescent colors. Light may also be diffracted, causing separation into its individual components—a common technique employed in the design of optical spectrometers utilizing diffraction gratings. Absorption is the ultimate fate of light and is due to absorption of light by molecular entities (chromophores) causing their electrons to enter excited states, which upon relaxation to the ground state lose energy in the form of infrared radiation or fluorescence. In this section, we will summarize how these different light phenomena occur in and affect the optical properties of hair fibers. Reflection: Reflection can be characterized as either specular or diffuse reflection. In the case of specular reflection, incoming light is reflected from the surface in a manner similar to that of a mirror where both the incoming and outgoing light are oriented at equal angles relative to the surface normal hence, the angle of incidence equals the angle of reflection. In hair we commonly observe specular reflection as a ring or halo of glittery light that forms a band around the crown of the head in people with healthy, straight hair. Specular reflection may be observed in other locations also, for example the concave or convex regions of curly hair. Diffuse reflection occurs when light is reflected from the surface at multiple angles, rather than just a single angle as in the case of specular reflection. Diffuse reflection occurs due to increased surface roughness, when the asperity on the surface is equal to or larger than the incident wavelength of light. In hair, asperities on the surface favor diffuse reflection. Therefore, smooth hair will provide greater specular reflection (less diffuse reflection) than rough hair.3 As we will see in