NATURE OF LIGHT 



109 



Different effects produced by them vary with their wave 

 length and also with the substance upon which the rays 

 fall. For convenience, however, those which produce 

 heat in a body are called heat radiations ; those which 

 cause chemical changes, actinic rays; and those which 

 affect the eye to produce sight are called light waves. 



126. Luminous and Illuminated Bodies. All objects 

 are seen by means of the light waves that pass from them 

 to the eye. These light waves may have their origin 

 (starting point) in the body itself, or they may fall 

 upon it from some outside source and then be directed 

 to the eye from that object. Bodies which give out light 

 waves from themselves are called luminous ; those which 

 give off only waves which have fallen upon them from 

 some other source are said to be illuminated. The sun, 

 lamp flames, glowing coals, the electric arc, and very 

 hot iron are examples of luminous bodies they are 

 sources of light waves. Such bodies may be seen when 

 no other source of light is present ; whereas illuminated 

 objects chairs, tables, books, flowers, clothing, the 

 earth, plants, animals, the moon, 

 and many others disappear from 

 sight as soon as all sources of light 

 waves are taken away. 



127. Rays. Light waves start 

 from a luminous point, as o (Fig. 80), 

 md extend in all directions in 

 straight lines. The straight line 

 marking the direction of any one wave is called a ray, oa 

 (Fig. 80). Note that no wave ever goes in a curved path, 



FIG. 



