140 RADIATION BIOLOGY 



Thus, for radiant energy passing from air to water at normal incidence, 

 ni would be 1.0 and rio would be approximately 1.3. The reflectance 

 loss would be about 1.7 per cent. For window glass in air, iia is approxi- 

 mately 1.5, and the reflectance is 4 per cent. For a pane of glass, the 

 total reflectance loss at normal incidence for the air-glass and glass-air 

 interfaces is about 8 per cent. At larger angles of incidence the losses 

 are larger and may be calculated from data given in various physical 

 tables and handbooks. 



When the reflecting surface is highly irregular, diffuse reflection results. 

 Diffuse reflection also occurs at surfaces made up of fine particles of trans- 

 parent substances of large refractive indexes. Snow and deposits of fine 

 crystals such as zinc oxide, titanium dioxide, and the other white paint 

 "pigments" produce diffuse reflections, because the energy is refracted 

 sharply at the crystal-air interfaces and internal reflection occurs, send- 

 ing the beam back again in much the same manner as in a total-reflecting 

 prism. This is the basis upon which white paint pigments are selected. 



OPTICS OF IMAGE FORMATION 



For many optical situations one can choose either a curved mirror or 

 a lens. The same basic image formulas apply to both, and each has its 

 own peculiar properties. Since the concave mirror and positive lens 

 (plano-convex, double convex, or positive meniscus) are of most general 

 use in simple condensing, coUimating, and focusing systems, only these 

 types will be discussed here. For further details reference is made to 

 various texts in general physics and optics (Barrows, 1951; Habell and 

 Cox, 1948). 



Image Formulas. If the focal length is / (Fig. 3-3), the distance from 

 the object to the surface of the mirror or the center of the lens is p, and 

 the distance to the image is q, then the following equation apphes to both 

 mirrors and thin lenses: 



1// = Up + Uq. (3-6) 



For real images the image is on the same side of the mirror as the object, 

 whereas with the lens the object and image are on opposite sides. The 

 distances p and q are known as the "conjugate" distances. When p is 

 infinite, / = ?, and the image is formed at the principal focus F. This 

 is the condition for parallel rays incident on a positive lens or concave 

 mirror. If p < /, i.e., if the object is inside the principal focus, no real 

 image is formed ; the rays appear to come from a virtual image (see Fig. 

 3-3), and q has a negative value. 



The magnification of the image is given by the ratio of image to object 

 distances : 



