BEFLECTION OF LIGHT. 



309 



Fig. 243. Tbus, if A B, Fig. 243, were a concave spheri- 



cal mirror, of whicli C were the center, rars issu- 

 ing from C would, in obedience to the law that 

 the angle of incidence and reflection are equal, 

 meet again at C. 



Diverging rays falling on a spheri- 

 cal concave mirror, if they issue from 

 the principal focus, half way between the center of the sur- 

 face and the center of the curve of the mirror, will be re- 

 flected in parallel lines. 



Thus, in Fig. 244, if F represent a can- ' pj^, 244 



die placed before a concave mirror, ABC, 



half way between the center of its surface, 



B, and the center of its curve, C, its rajs, 



falling upon the mirror, will be reflected ^" 



in the paraUel lines d ef g h. 



This principle is taken advantage of in 



the arrangement of the illuminating and 



reflecting apparatus of light-houses. The lamps are placed before a concave 



mirror, in its principal focus, and the rays of light proceeding from them are 



reflected parallel from the surface of the mirror. 



■^hen the rays issue from a point, P, Fig. 

 245, beyond the center, C, of the curve of the 

 mirror, they will, after reflection, converge to 

 a focus, / between the principal focus, F, and 

 the center of the curve, C. 



On the contrary, if the rays issue from a 

 point between the principal focus, F, and the 

 surface of the mirror, they will diverge afl:er 

 reflection. 



667. Images are formed by concave mirrors 

 in the same manner as by plane ones, but they 

 are of difterent size from the object, their gen- 

 being to produce an image larger than the 



Fig. 245. 



How are images 

 formed by con- 

 cave mirrors? 



eral effect 

 object. 



"When an object is placed between a concave 

 image formed mirror and its principal focus, the image will 

 mirror b^"iSlg! appear larger than the object, in an erect posi- 

 tion and behind the mirror. 

 This will be apparent from Fig. 246. Let a be an object situated 

 within the focus of tiie mirror. The rays from its extremities will fall 

 divergent on the mirror, and be reflected less divergent to the eye at b, 



