154 PHYSIOLOGY FOR NURSES 



or six hundred degrees. Chemical combinations pro- 

 duce light also by heat, since luminous flames are simply 

 gases which contain solids heated to incandescence. 

 Phosphorescence is exhibited by decaying wood, or fish, 

 by glowworms, etc. 



Some bodies, like wood, metals and many others, com- 

 pletely stop light and are called opaque. Others, like 

 glass, air, and clear water allow light to pass and are 

 called transparent or diaphanous; while others permit 

 only a portion of the light to pass, thin porcelain for 

 instance, and are said to be translucent. No body trans- 

 mits all of the light which falls upon it, but even the 

 most transparent absorb some of the rays. 



Propagation of Light. A medium is any space or sub- 

 stance which light can traverse (Ganot) and is called 

 homogeneous when its density is the same in all parts, 

 like air. Through all homogeneous media light is trans- 

 mitted in a straight line. Light emanates from a lumi- 

 nous body in all directions (Ganot), but changes its di- 

 rection when it strikes a medium of different density. If 

 the new medium is impenetrable, light will be reflected; 

 if penetrable, refracted. 



When a beam of sunlight passes through a small open- 

 ing it forms a small pencil-like bundle of rays. If these 

 strike a polished surface they are bent or turned away 

 from the surface at the same angle at which they struck it. 

 The angle at which the entering sunbeam struck the 

 polished surface is called the angle of incidence; while 

 that at which it turns away is called the angle of re- 

 flection. The rule, therefore, is that "the angle of re- 

 flection is equal to the angle of incidence." As is the 

 case of passing through a medium, not all the light is 

 reflected, but some is always absorbed by the reflecting 

 surface while some is irregularly reflected as diffused 



