448 CONCAVE MIRRORS. 



B li B c give similarly the point b as image of B, and 

 hence a b is the image of A B. (The image of an object 

 between the mirror and its focus is erect, larger than the 

 object, and virtual.} 



The distance from the mirror of the image may be 

 found by the following rule, if the distance of the 

 object from the mirror and the focal length are'known : 

 Divide the product of the distance of the object into the 

 focal length by the difference of the two quantities ; the 

 quotient is the distance of the image. Thus for a 

 mirror of 20 cm focal length and a distance of 30 cm be- 

 tween object and mirror, the distance of the image 



OA x 90 

 is X = GO (that is, in front of the mirror) ; if 



oO 20 



the object is 10 cm from the mirror, we obtain by the 



10 x 20 

 same rule -^ = 20 cm (that is, behind the 



mirror, as indicated by the opposite sign which precedes 

 the result). 



The relative magnitude of object and image is always 

 in the ratio of their respective distances from the centre 

 of curvature. In the last example the centre of curva- 

 ture is 40 cm , the object 10 cm in front of the mirror; their 

 distance from one another is therefore 30 cm . Again, the 

 image which is 20 cm behind the mirror is 60 cm from the 

 centre of curvature ; that is, its distance from it is twice 

 as great as that of the object; hence the image has twice 

 the size of the object. In the previous case the 

 distance of the object from the centre of curvature is 

 40 - 30 = 10 cm , that of the image 60 - 40 = 20; 

 the image is therefore in that case also twice as large as 

 the object. 



