chap, xxiv.] FORMATION OF IMAGES BY MIRRORS. 307 



rays ; the image of a is therefore formed at A, and b at 

 B. AB would thus become the image. If, then, the 

 object be between the centre of curvature and the 

 principal focus, the 

 image is real, and in- 

 verted, but larger than 

 the object; and the 

 nearer the object ap- 

 proaches to the princi- 

 pal focus, the larger will Fig. 132. Formation of a real Image 

 be the image. Should, b > T a Concave Mirror. 



however, the object be 



at the principal focus, the incident rays are reflected 

 in a direction parallel to one another (page 303). No 

 conjugate focus is formed, and hence no image. 



Finally, suppose the object to be nearer to the 

 mirror than the principal focus, then, as already noted 



(page 305), the reflected 

 rays are divergent. They 

 do not meet in front of the 

 mirror, and no real image 

 is formed. If the reflected 



Fig. 183. Virtual Image of Concave raVS be prolonged back- 



wards, however, they will 



meet behind the mirror and so form a virtual image. 

 Thus, let MN (Fig. 133) be a concave mirror, xQx' its 

 principal axis, c its centre of curvature, and F its 

 principal focus. Within F place an arrow AB. Let AG 

 AE be incident rays from A. They are reflected in the 

 directions p and R. Rays BH BL from B are reflected 

 in the directions s and T. Prolonged backwards, the 

 former met at a and the latter at b. Thus ab becomes 

 the image of AB. It is behind the mirror, virtual; 

 is ERECT, and larger than the object. The nearer the 

 object is to the principal focus, without coinciding with 

 it, the larger is the virtual image, the nearer the object 

 is to the surface of the mirror, the smaller is the image. 



