13.2.The Optical Constants 。 251 The ratio between the reflected intensity IR and the incom- ing intensity lo of the light is the reflectivity: R=会 (13.17) Quite similarly,one defines the ratio between the transmitted in- tensity,Ir,and the impinging light intensity as the transmissiv- ity: (13.18) The reflectivity is connected with n and k(assuming normal in- cidence)through: R=n-12+k2 (n+1)2+k2 (13.19) (Beer equation).The reflectivity is a unitless material constant and is often given in percent of the incoming light (see Table 13.1).R is,like the index of refraction,a function of the wave- length of the light.For insulators (k=0)one finds that R de- pends solely on the index of refraction: R=-1)2 (13.20) (n+1)2 Metals are characterized by a large reflectivity.This stems from the fact that light penetrates metals only a short distance,as shown in Figure 13.2 and Table 13.2.Thus,only a small part of the impinging energy is converted into heat.The major part of the energy is reflected (in some cases as much as 99%,see Table 13.1).In contrast to this,visible light penetrates into glass (and many other dielectrics)much farther than into metals,that is, approximately seven orders of magnitude more;see Table 13.2. As a consequence,very little light is reflected by glass.Never- theless,a piece of glass about 1 or 2 m thick eventually dissipates a substantial part of the impinging light into heat.(In practical applications,one does not observe this large reduction in light intensity because windows are,as a rule,only a few millimeters thick.)It should be noted that window panes,lenses,etc.,reflect the light on the front as well as on the back side. An energy conservation law requires that the intensity of the light impinging on a material,Io,must be equal to the reflected inten- sity,IR,plus the transmitted intensity,I7,plus that intensity which has been extinct,IE,for example,transferred into heat,that is, Io IR IT IE. (13.21)The ratio between the reflected intensity IR and the incom￾ing intensity I0 of the light is the reflectivity: R
I I R 0 . (13.17) Quite similarly, one defines the ratio between the transmitted in￾tensity, IT, and the impinging light intensity as the transmissiv￾ity: T
I I T 0 . (13.18) The reflectivity is connected with n and k (assuming normal in￾cidence) through: R
(13.19) (Beer equation). The reflectivity is a unitless material constant and is often given in percent of the incoming light (see Table 13.1). R is, like the index of refraction, a function of the wave￾length of the light. For insulators (k
0) one finds that R de￾pends solely on the index of refraction: R
. (13.20) Metals are characterized by a large reflectivity. This stems from the fact that light penetrates metals only a short distance, as shown in Figure 13.2 and Table 13.2. Thus, only a small part of the impinging energy is converted into heat. The major part of the energy is reflected (in some cases as much as 99%, see Table 13.1). In contrast to this, visible light penetrates into glass (and many other dielectrics) much farther than into metals, that is, approximately seven orders of magnitude more; see Table 13.2. As a consequence, very little light is reflected by glass. Never￾theless, a piece of glass about 1 or 2 m thick eventually dissipates a substantial part of the impinging light into heat. (In practical applications, one does not observe this large reduction in light intensity because windows are, as a rule, only a few millimeters thick.) It should be noted that window panes, lenses, etc., reflect the light on the front as well as on the back side. An energy conservation law requires that the intensity of the light impinging on a material, I0, must be equal to the reflected inten￾sity, IR, plus the transmitted intensity, IT, plus that intensity which has been extinct, IE, for example, transferred into heat, that is, I0
IR IT IE. (13.21) (n  1) 2 (n 1)2 (n  1)2 k 2 (n 1)2 k2 13.2 • The Optical Constants 251