134 BIOLOGICAL EFFECTS OF RADIATION 



Table 5. — Terms Relating to the Rectilinear Transmission of Homogeneous 



Radiant Energy through a Homogeneous, Isotropic, Nonmetallic Medium 



IN THE Form of a Plate with Plane, Polished, Parallel Surfaces 



Perpendicular to the Direction of Propagation* (56, cf. page 177) 



Let . 



h = distance between the bounding surfaces. 

 El = radiant energy incident on the first surface. 

 E' = radiant energy reflected by the first surface. 

 El = radiant energy transmitted by the first surface. 

 Ei = radiant energy incident on the second surface. 

 E" = radiant energy reflected by the second surface. 

 Ell = radiant energy transmitted by the second surface. 

 Then 



p = E'/Ei = E"/Ei = reflectance. 

 v = El/ El = E11/E2 = admittance. 

 T = Ell/El = transmission, t 



T = E2/E1 = transmittance. 



t = -^/T = transmissivity. J 



= 1 — T = obstruction. 



A = 1 — T = absorptance. 



D = — logio T = density. 



i = — loge t = transmissive exponent. 



k = logio< = transmissive index. 



* In deriving these relations, multiple reflections between the bounding surfaces have been neglected. 

 This is permissible for a single plate of the ordinary refractive index for nonmetallic substances. It 

 is not permissible in case p has a high value. 



t In addition to this restricted meaning of transmission, the term ia also used, as are absorption and 

 reflection, to denote a general subject or phenomenon. 

 J This relation is known as Lambert's law. 



' It is obviously a matter of great importance to determine the char- 

 acteristics of materials used experimentally with regard to their reflect- 

 ance and transmissivity. Certain ideal conceptions are a convenience. 

 A perfect reflector would have a reflectance 1. A perfectly transparent 

 body would have a transmissivity 1. While such ideal reflectors and 

 transmitters are closely approached for certain wave-lengths, one is 

 never completely free from surface reflection. For a polished surface, 

 the reflection R of highly transparent materials depends upon the index 

 of refraction n of the material {i.e., the ratio of the velocity of radiation 

 in free space to that in the body). In ease of normal incidence 



R = 



Vn + l/ 



For glass, the reflection varies between 4 and 8 per cent in the visible. 

 A black body may be described as one of zero reflectance and zero trans- 

 mission. Practically, it may be approached by a distribution of lamp- 

 black over a surface. The small opening into an inclosure proves 

 equivalent to a black surface. 



