438 SPHERICAL MIRRORS. 



slight curvature, whether the reflection takes place 

 from the internal or the external face that is, whether 

 the reflecting surface be concave or convex. 



Spherical mirrors are thus either concave or convex 

 mirrors. The centre of the hollow sphere of which 

 the mirror forms part, c in fig. 248, is called the 



FIG. 248. 



centre of curvature. Lines drawn from the centre of 

 curvature to any point of the mirror, as a c, b c, d c, e c, 

 are radii of curvature ; further, the infinite straight line 

 g A, which is conceived to pass through the middle of 

 the mirror, </, and the centre of curvature, c, is the 

 principal axis of the mirror. 



The mode of reflection of light from curved mirrors 

 is deduced from the laws of reflection from plane 

 mirrors, by considering the surface of the former as 

 made up of infinitely small plane surfaces, which are 

 called its elements. A radius of curvature drawn from 

 the centre to any point of the mirror is perpendicular to 

 the surface at that point ; hence rays of light which 

 emanate from a luminous point situated in the centre 

 of curvature are reflected back again in their own 

 direction to the centre. 



