NATURE OF LIGHT 115 



curve. All rays parallel to these three would be simi- 

 larly reflected to the point/; this is called the principal 

 focus of the mirror. 



Experiment 90. Using a concave (in-curving) mirror, reflect 

 the sun's rays to a focus. Hold a narrow strip of paper in front 

 of the center of the mirror, and try to find this focus by moving 

 the paper back and forth. When found, the focus will appear as 

 a small spot of light, but very bright. Why is this spot so much 

 brighter than is common in sunlight ? 



If now any luminous source be placed at the focus of 

 a concave surface, those of its waves which strike the 



FIG. 85 FIG. 86 



surface will be reflected away in parallel lines, as in 

 Fig. 86. Smooth curved surfaces used in this way are 

 called reflectors; they are much used in locomotive 

 headlights, search lights', and signal lights. They catch 

 many rays that would be lost sidewise and back of a 

 light, bending them all in the direction where they are 

 needed. 



135. Intensity of Illumination. From a luminous 

 source light waves move in straight lines in every 

 direction. But it is evident that the rays, moving in 

 straight lines away from a point, must be spread farther 

 apart as the distance from the point increases. That is, 



