SPHERICAL REFLECTORS. 



Since, in all the cases of reflection from plane mirrors, the rays 

 diverge as if they had issued from points behind the mirror, the 

 images are always virtual or imaginary. 



9. IMAGES PRODUCED BY SPHERICAL REFLECTORS. 



Curved reflecting surfaces may have various forms, but 

 those which are most important are spherical; that is, such 

 as consist of a part of the surface of a globe of greater or 

 less diameter. A concave spherical reflector is a part of the 

 surface of a globe seen from the inside, and a convex, seen from 

 the outside. 



10. Let A c (fig. 6) be the section of a concave reflector, whose 

 centre is o. The line o B through the middle of the reflector 



Fig. 6. 



and the centre, o, is called its axis. Let F be the middle point of 

 the radius o B. 



If an object be placed before the reflector at any place, such as 

 L M, beyond its centre o, an image of this object m l y will be 

 found at a certain point between F and o. The pencils of rays 

 which radiate from each point of the object, after encountering 

 the surface of the reflector, will be reflected, converging to the 

 corresponding points of the image. Thus the rays which proceed 

 from L will be reflected, converging to I, and those which proceed 

 from M will be reflected, converging to m. 



The image m I will therefore be inverted with relation to the 

 object, the top of the one corresponding to the bottom of the other, 

 the right to the left, and vice versa. 



It is evident also, that the linear dimensions of the image will 

 bear to those of the object the exact proportion of their respective 

 distances m o and :vr o from the centre of the reflector. 



11. The production of such an image can be easily verified 

 experimentally. Let the object L M be a candle, and let a small 

 piece of card be held between o and F at right angles to o B. An 

 image of the candle will be seen upon the side of the card pre- 

 sented to the reflector. The image will at first be nebulous and 

 indistinct, but by moving the card alternately to and from the 



87 



