13 Optical Properties of Materials 13.1.Interaction of Light with Matter The most apparent properties of metals,their luster and their color,have been known to mankind since materials were known. Because of these properties,metals were already used in antiq- uity for mirrors and jewelry.The color was utilized 4000 years ago by the ancient Chinese as a guide to determine the compo- sition of the melt of copper alloys:the hue of a preliminary cast indicated whether the melt,from which bells or mirrors were to be made,already had the right tin content. The German poet Goethe was probably the first one who ex- plicitly spelled out 200 years ago in his Treatise on Color that color is not an absolute property of matter(such as the resistiv- ity),but requires a living being for its perception and descrip- tion.Goethe realized that the perceived color of a region in the visual field depends not only on the properties of light coming from that region,but also on the light coming from the rest of the visual field.Applying Goethe's findings,it was possible to ex- plain qualitatively the color of,say,gold in simple terms.Goethe wrote:"If the color blue is removed from the spectrum,then blue, violet,and green are missing and red and yellow remain."Thin gold films are bluish-green when viewed in transmission.These colors are missing in reflection.Consequently,gold appears reddish-yellow.On the other hand,Newton stated quite correctly in his "Opticks"that light rays are not colored.The nature of color remained,however,unclear. This chapter treats the optical properties from a completely different point of view.Measurable quantities such as the index of refraction or the reflectivity and their spectral variations are13 The most apparent properties of metals, their luster and their color, have been known to mankind since materials were known. Because of these properties, metals were already used in antiq￾uity for mirrors and jewelry. The color was utilized 4000 years ago by the ancient Chinese as a guide to determine the compo￾sition of the melt of copper alloys: the hue of a preliminary cast indicated whether the melt, from which bells or mirrors were to be made, already had the right tin content. The German poet Goethe was probably the first one who ex￾plicitly spelled out 200 years ago in his Treatise on Color that color is not an absolute property of matter (such as the resistiv￾ity), but requires a living being for its perception and descrip￾tion. Goethe realized that the perceived color of a region in the visual field depends not only on the properties of light coming from that region, but also on the light coming from the rest of the visual field. Applying Goethe’s findings, it was possible to ex￾plain qualitatively the color of, say, gold in simple terms. Goethe wrote: “If the color blue is removed from the spectrum, then blue, violet, and green are missing and red and yellow remain.” Thin gold films are bluish-green when viewed in transmission. These colors are missing in reflection. Consequently, gold appears reddish-yellow. On the other hand, Newton stated quite correctly in his “Opticks” that light rays are not colored. The nature of color remained, however, unclear. This chapter treats the optical properties from a completely different point of view. Measurable quantities such as the index of refraction or the reflectivity and their spectral variations are Optical Properties of Materials 13.1 • Interaction of Light with Matter