74 



PRACTICAL LESSONS IN SCIENCE. 



angle IAP equals the angle PAR, that is the angle of reflection 

 PAR equals the angle of incidence IAP. IA is called the incident 

 beam, AR the reflected beam, and A the point of incidence. 



Rays of light may be parallel, or converging or diverging. 

 They cannot have any other relation. As the angle of reflection 

 equals the angle of incidence in the case of plane mirrors, they do 

 not change the relations of rays of light falling upon them. The 

 reflected rays are parallel, converging or diverging after as 

 before reflection. In the case of concave mirrors the effect on the 

 pays of light is different and more interesting. Let MM in Fig. 



F.c9 



9 represent a concave mirror, C the center of curvature, G the 

 vertex of the mirror, and CG, its principal axis. The lines CA, 

 CB, and CG are radii, and perpendicular to the mirror. Light- 

 waves emanating from the point C would be reflected to their 

 source at C, but rays of light from E, as EA and EB, fall upon 

 the mirror at the points A and B, and as the angle of reflection 

 is equal to the angle of incidence, they would be reflected to 

 H; while if H were a source of light the rays HA and HB would 

 be reflected to E. The diverging rays in each case are by reflec- 

 tion made to converge to a point called a focus. As the light 

 waves which emanate from either of these points are brought 



