VISION 



1009 



incidence the same angle as the incident ray. Let D (Fig. 405) be the 

 middle point of the mirror, and C its centre of curvature i.e., the 

 centre of the sphere of which it is a segment. Then CD is the principal 

 axis, and any other line through C which cuts the mirror is a secondary 

 axis. When the mirror is a small portion of a sphere, rays parallel to 

 the principal axis are focussed at the principal focus F midway between 

 C and D; rays parallel to any secondary axis are focussed in a point 



^i^^"""^"^^^^^^" --^^^^^^^^ 



Fig. 405. Reflection irom a , Concave Fi^. 406. Formation of Real Inverted 

 Spherical Mirror. Image by a Concave Spherical Mirror, 



lying on that axis ; and rays diverging from a point on any axis are 

 focussed in a point on the same axis. 



These facts afford a simple construction for finding the position of 

 the image of an object formed by a concave mirror. Let AB be the 

 object (Fig. 406). Then the image of A is the point in which all rays 

 proceeding from A and falling on the mirror, including rays parallel to 

 the principal axis, are focussed. But the ray AE, parallel to the prin- 

 cipal axis, passes after reflection through the principal focus F, there- 

 fore the image of A must lie on the straight line EF. .If any secondary 

 axis ACD be drawn, the 

 image of A must lie on 

 ACD. It must therefore 

 be the point of intersec- 

 tion, a, of EF and ACD. 

 Similarly, the image of B 

 must be the point of inter- 

 section, b, of GF and BCH. 

 The image ab of an object 

 AB farther from the mirror 

 than the principal focus 

 is real and inverted. The 

 Purkinje-Sanson image re- 

 flected from the concave 

 anterior surface of the 

 vitreous humour (Fig. 421) 

 is an example. 



After reflection from a convex mirror, rays of light always diverge, and 

 only erect, virtual images are formed i.e., images which do not really 

 exist in space, but which, from the direction of the rays of light, we, judge 

 to exist. The position of the image of an object AB (Fig. 407) may be 

 found by a construction similar to that for reflection from a concave 

 mirror. The image of a flame reflected from the anterior surf ace of the 

 cornea or lens is erect and virtual. It diminishes in size with increase 

 in the curvature or convexity of the reflecting surface (Fig. 421). 



Refraction. A ray of light passing from one medium into another 

 has its velocity, and consequently its direction, altered. It is said to 



64 



Fig. 407. Formation of Image by a Convex 

 Mirror. 



