CONCAVE MIRRORS. 



447 



centre of curvature and focus. A ray Afd passing 

 through the focus is reflected parallel to the axis, in 

 the direction da\ a ray A e, parallel to the axis, is 

 reflected in a direction efa, which passes through the 

 focus. The point of intersection a is therefore the 

 image of A. Similarly the rays emitted by B intersect 

 at b, as seen by following the directions Bfg b and 

 B hfb ; hence b is the image of B. 



If an object is in the focus, no image is formed ; the 

 reflected rays are parallel to the axis ; they do not in- 

 tersect at, nor do they appear to proceed after reflection 

 from, definite points. 



Finally, let an object be between the mirror and the 

 focus, as in fig. 253. The ray A d from A, parallel to 



LI B 1! A it > 



SIVKUSITV ' 

 lALiKuiiXL 



FIG. 253. 



the axis, is reflected towards the focus /; the ray A e, 

 which is in a line with the centre of curvature, is re- 

 flected back upon itself, but the reflected rays df and 

 e A c do not intersect, they diverge and appear to 

 proceed from a, a point behind the mirror ; hence a is a 

 virtual image of the point A. The rays Bgf and 



