Ch. I] 



REFRACTION AND LENSES 



II 



faces. Natural transparent 'minerals may also be given a lenticular 

 form, e.g. fluorite, quartz, etc. 



The lens is usually a segment of a sphere or of .two spheres (fig. 8) . 

 In dealing with lenses mention must frequently be made of the optical 

 center of the lens, the principal axis, secondary axis, and the principal 

 focus. These are illustrated in fig. 8, 11-12, and are briefly: 



(i) Optical center. — The point in or 

 near a lens through which, if rays pass, 

 they wiU suffer no angular deviation, and 

 the emerging ray will be parallel to the 

 incident ray (fig. 8 c.l) . 



(2) Principal axis. — The axis passing 

 through the centers of curvature of the 

 two spheres whose surfaces bound the 

 lens (fig. 8). 



(3) Secondary axis. — Any axis oblique 

 to the principal axis, but passing through 

 the optical center of the lens (fig. 11-12). 

 A ray along a secondary axis undergoes 

 no angular deviation, although it may 

 suffer displacement as a ray in travers- 

 ing a piece of plane glass (fig. 51). 



(4) Principal focus. — The point where 

 rays of light, parallel to the principal axis, 

 cross after traversing the lens (fig. 10). 

 Every lens has two principal foci, one on 

 each side (fig. 10) . 



With concave lenses the foci are virtual (fig. 9) . 



§ 9. Refraction. — By this is meant the change in direction of 

 light in passing from one transparent medium into another. The 

 possibility of the production of images by lenses depends upon re- 

 fraction. 



The amount of refraction depends upon two things: 



(i) The difference in density of the two media. The greater 

 the difference, the greater the amount of bending of the light in 

 passing from one medium to another. 



Fig. 8. Lens with Out- 

 lines OE THE Two Spheres 

 OF WHICH IT IS Segments. 



Axis The principal optic 

 axis, the line joining the two 

 centers of curvature (c c'). 



c c' Centers of curvature, 

 — centers of the two spheres 

 from which the lens is de- 

 rived. 



r r' Parallel radii. 



1 1' Tangents at the term- 

 inal points of the radii. 



d Center of the lens, — 

 point where the line joining 

 the radii at the tangential 

 points crosses the principal 

 axis. 



