GENERATION, CONTROL, AND MEASUREMENT 



141 



M ^ q/p. 



When the object and image distances are equal, M 

 same size as the object, and p = q =^ 2/. 



(3-7) 

 1, the image is the 



POSITIVE 

 LENS 



Fig. 3-3. Image formation in positive lenses and mirrors. The focus is at F, and the 

 focal length is/; the object is at 0, and the object distance is p; the image is at /, and 

 the image distance is q. The radius of curvature is /■. 



For a mirror the principal focus is located on the axis halfway between 

 the center of curvature C and the mirror surface (see Fig. 3-3); thus 

 r = 2/ for a mirror. The focal length of a thin lens in air is determined 

 by the radii of curvature of the lens surfaces and the refractive index of 



the lens: 



1//= {n - l)(l/ri + l/r^), (3-8) 



