LIGHT. 



57 



that none of the specific colors predominates. The reason that even 

 under the influence of this white light most objects show a distinct 

 color is not, as might be supposed, an inherent color possession of 

 these objects, but an effect of the light. Without light all objects 

 are black i. <?., without light there is no color. There is a great 

 diversity in the behavior of different substances toward white light. 

 It may either be absorbed or reflected, or partly absorbed and partly 

 reflected. If all be absorbed, black is the result ; if all rays except 

 the red ones be absorbed, the body appears red, etc. 



Not only has the number of wave oscillations per second been 

 determined, but also their length. The longest waves are those pro- 

 ducing heat ; the shortest ones are the actinic waves. Between them 

 are the light waves ranging from 650 millionths of a millimeter in 

 length for red light, to 442 millionths of a millimeter in length for 

 violet light. 



Light rays. Light travels through homogeneous media, as air, 

 water, and glass, in straight lines, and a very narrow cylinder of light 

 is called a ray, beam, or pencil of light. Those bodies which readily 

 transmit luminous rays are said to be transparent, those not trans- 

 mitting light are called opaque, while translucent bodies are those 

 permitting light to pass through them to a limited extent. 



A body may be self-luminous, like the sun or a flame ; or it may 

 be luminous by reflected light, like the moon or any object that is 

 illuminated by daylight or by any luminous body. 



Light of itself is invisible, as can be shown by admitting a sun- 

 beam through a small hole into a dark room. If the air be free 

 from dust, the beam is invisible ; but if the eye, or any object upon 



which it mav strike, is placed in its path, 



~ V. 

 we are made aware of its presence, not 



by seeing the light, but by seeing the ob- 

 ject which emits it or intercepts it. 



Reflection. When a ray of light strikes 

 a mirror or a polished surface obliquely 

 we notice that a ray of light is thrown off 



or reflected from the mirror. On measuring 

 ,1 , . ,_.. . , , . 



the angle ^ (Fig. 17) made by the entering 

 or incident ray CB and a perpendicular NB, and the angle r made 

 by the reflected ray AB and the perpendicular, they will be found 

 equal. In other words, in plane mirrors the angle of incidence is 

 always equal to the angle of reflection. 



FIG. 17. 



Reflection. 



