Ch. IX] VISIBLE AND INVISIBLE RADIATION 271 



The infra-red waves are only shown up to a length of 2^1, although 

 many of much greater length exist. 



In the ether of space the different visible waves move with equal 

 velocity, but in the various transparent bodies on the earth, the ve- 

 locity depends upon the wave length — the shorter the waves the 

 slower the motion (§ 451). 



§ 443. Light moves in straight lines. — In a uniform medium 

 light moves in straight lines. Any body in which hght can move 



x2. 



\0.7/t 



x0.4/»_ 



Visible 

 Radiation 



Ultra-Violet Infra-Red 



Fig. 151. Visible and Invisiblk Radiation. 



The segment in white shows the visible radiation, which lies between Xo.4/i 

 and Xo.7;it. Radiation shorter than Xo.4/^ is known as ultra-violet, and longer 

 than Xo.7;U as infra-red. This is a normal spectrum magnified 20,000 vertically 

 and 50,000 times horizontally (see also fig. 144). 



freely is said to be transparent. If light meets a body in which it 

 cannot move it is either reflected (§ 444) or absorbed; if absorbed 

 it is changed to some other form of energy, usually heat. 



§ 444. Reflection. — If Hght meets a surface which is opaque or 

 only partly transparent, it is changed in its course or reflected; or 

 it may be absorbed. 



If the surface is smooth and the light is reflected, the incident and 

 the reflected ray wiU be in the same plane and wiU make equal angles 

 on opposite sides of a normal erected at the point of reflection (fig. 152). 

 The eye can see the light only when in the path of the ray, or when 

 light is deflected from the ray by dust, etc. (§ 117). 



