14 



GEOMETRICAL CONSTRUCTION OF IMAGES 



[Ch. I 



6j» 



§ 12. Construction of virtual images. — (i) For these the object 

 must be somewhere between the principal focus and the lens. 



(2) From some point in the object draw a line to the refracting 

 plane of the lens, parallel to the principal axis, and from this point 



through the principal 

 focus, and continue it 

 indefinitely. 



(3) From the same 

 point of the object as 

 in (2) draw a secondary- 

 axis through the optical 

 center of the lens and 

 extend it indefinitely. 



The two fines will not 

 cross above the lens, but 

 if they are extended be- 

 low the lens (fig. 12) 

 they will cross, and the 

 crossing point locates 

 the image. But as there 

 are no light rays ex- 

 tending in this direction 

 the image is imaginary 

 or virtual. That is, it 

 looks as if the rays 

 reaching the eye orig- 

 inated from the point 

 where the rays would 

 cross if extended back- 

 ward. 



§ 13. Relative position of object and image. — The general law 

 is that the nearer the object to the principal focus the farther away 

 is the image ; and conversely, the nearer the image is to the principal 

 focus the farther from it must be the object. And from the law of 

 similar triangles, the size of the image is to the size of the object as 

 the distance of the image from the center of the lens is to the distance 



Fig. 13-14. Real Image with the Object far 

 from and near to the principal focus. 



Axis, Axis The principal optic axis extended 

 above and below the lenses. 



/> /> /> / The principal foci of the lenses. 



L c, L c The same lens with the object farther 

 from and nearer to its principal focus. 



A B, B' A' The object and its inverted image 

 when the object is far from the principal focus. 



A B, B' A' The object and larger inverted real 

 image when the object is near the principal focus. 



