chap. xxiv. CONCAVE MIRRORS. 303 



mirror is a part is the principal axis of the mirror. 

 The centre of the sphere is called the centre of curva- 

 ture. The distance between the centre of curvature and 

 the surface of the mirror is the radius of curvature. 

 A secondary axis is any line passing through the 

 centre of curvature to the mirror, but not through the 

 centre of the figure. The aperture of the mirror is 

 the angle formed by 

 lines drawn from the 

 circumference of the M ~ 



mirror to the centre of G ~ 



r r ^-'- "F 



curvature. 



These points are M ^_ 



shown in Fig. 129, 



where AB is the mirror, _. . . p 



Fig. 129. Principal Focus of a Con- 

 O its centre, C the cave Mirror. 



centre of curvature, 



LCO the principal axis, CO the radius of curvature, 

 and the angle ACB the aperture. CA and CB are 

 secondary axes. 



Concave mirrors. 1. Let rays of light par- 

 allel to the principal axis fall upon a concave mirror 

 (for practical purposes rays from the sun are considered 

 parallel), they will be reflected according to the laws 

 of reflection, and will meet in a point F on the prin- 

 cipal axis of the mirror (Fig. 129). By drawing the 

 normals CH, CD, etc., it can be shown, that because 

 the angle of incidence GDC is equal to the angle of 

 reflection FDC, CF and FD are equal. FD is equal to 

 FO, and so CF and FO are equal to one another. That 

 is, the reflected rays meet in a point which bisects 

 the radius of curvature. F is called the principal 

 focus of the mirror, and the distance FO is the prin- 

 cipal focal distance. Thus, rays parallel to the prin- 

 cipal axis, falling on a concave mirror, are reflected to 

 meet in the principal focal point, ^uhich is at a distance 

 from the mirror equal to half the radius of curvature. 



