FOCI. 333 



remark applies wherever the points of incidence A and B are 

 situated, so that all reflected rays meet at the point F; this point is 

 accordingly called the geometrical focus or principal focus of the 

 mirror, and is situated midway between C and G, the point on the 

 surface of the mirror where the line CFG drawn through C and F 

 cuts that surface.* The length OF = FG is called the focal length 

 of the mirror. 



Expt. 362. Virtual Focus of a Convex Mirror. If the rays 

 of light in the last experiment fall not on a concave mirror, but on a 

 convex one, a different result will be brought about. Fig. 178 

 illustrates the track of the two rays SA, SB, reflected from the 

 points A and B, in accordance with the law of reflection. Here 



Fig. 178. Virtual Focus of Convex Mirror. 



the reflected rays, AM and BN, make equal angles respectively, 

 with the normals CAD and CBE, and diverge from one another as 

 though they had emanated from a point, F, behind the mirror. 

 This point is the virtual geometrical focus, and is situated just 

 as far behind the mirror as the geometrical focus of a concave 



* The term ' ' focus " is derived from the circumstance that a concave mirror 

 concentrates radiant heat at F as well as visible light (Expt. 378), so that in- 

 flammable bodies placed at F can be fired by a " burning mirror " (focus, 

 Latin for fireplace). Strictly speaking the different reflected rays do not all 

 pass through the same point, F, when the curvature of the mirror is that of a 

 sphere, but only pass very close to it; this want of exact coincidence is termed 

 spherical aberration, and introduces certain difficulties in the way of obtain- 

 ing clear definition with optical instruments in which concave mirrors are 

 employed. Certain other forms of curvature are free from this defect, 

 especially "parabolic" mirrors (mirrors where the curved surface is that pro- 

 duced by the revolution about its axis of a parabolic curve) ; hence these are 

 preferable to spherical mirrors for accurate optical instruments, particularly 

 reflecting telescopes (Expt. 375). 



