390 REFLECTION OF LIGHT. 



608. Construction for Virtual Images formed by 

 Concave Mirrors. Let AB represent an object in front of a 

 concave mirror at a distance from it less than the focal distance. 

 Draw the secondary axes for the points A and B, and produce them 

 back of the mirror as dotted lines. From A and B, draw the inci- 

 dent rays Ao and Bi, parallel to the principal axis. After reflection 

 they will pass through the principal focus (601 ) Produce these 

 rays back of the mirror as dotted lines until they intersect the 

 prolongations of the secondary axes at a and b, which will be the 

 virtual conjugate foci for A and B. The conjugate foci for other 

 points in AB will be between a and b. Therefore, if the object be 

 between the principal focus and the mirror, the image will be 

 virtual, erect and enlarged. 



FIG. 290. 



6O9. Images of the Observer formed by a 

 Concave Mirror. A person at a considerable distance 

 before a concave mirror, sees his image, real, inverted and 

 smaller than the object. As he approaches the centre of 

 curvature, the image increases in size. As the observer 

 moves from the centre to the principal focus, the image is 

 formed back of him and is, therefore, invisible to him. As 

 he moves from the principal focus toward the mirror, the 

 image becomes virtual, erect and magnified, but gradually 

 growing smaller. The eye will not always recognize real 

 images as being in front of the mirror. It may some- 



