12 



GEOMETRICAL CONSTRUCTION OF IMAGES 



[CH.I 



FIG. 9. CONCAVE LENS 

 SHOWING VIRTUAL Focus 

 (F). 



(2) The obliquity with which the light strikes the second medium. 



The greater this obliquity the greater the bending of the light, in 



accordance with the law of sines. (For 

 further discussion see Ch. IX.) 



10. Geometrical construction of im- 

 ages. In this book the lenses shown are 

 thick, but the course of the rays, for sim- 

 plicity, is shown to be as if the lenses were 

 infinitely thin, that is, they show all the 

 bending at one plane (the refracting plane, 

 fig. 11-12). In reality there is one refrac- 

 tion at the incident or entering surface and 

 one at the emerging surface. With thick 

 lenses like those figured, there will be no 



angular deviation for rays traversing the optical center of the lens, 



but there will be a cer- 

 tain amount of dis- \\\|/// 



placement, although 



the emerging ray will 



remain parallel to the 



entering or incident ray 



(fig. si). 



For the construction 

 of images it is necessary 

 to know the position of 

 the principal focus and 

 the optical center of the 

 lens. 



FIG. 10. 



LENS WITH A PRINCIPAL Focus ON 

 EACH SIDE. 



lOa. Geometrical con- 

 struction of images. It 

 should be remembered in 

 making the drawings for 

 the geometrical construc- 

 tion of images that there 

 are two fundamental laws 

 which must always be 

 obeyed. 



(i) Light rays extend in straight lines in a transparent medium of uniform 

 density, and whenever the direction is to be changed the light must meet a 



Axis The principal optic axis. 



F The principal focus, the point on the axis 

 at which rays parallel with the principal axis 

 cross. 



The arrows indicate the direction of the light. 



