REFRACTION THROUGH LENSES. 



469 



refracting surfaces must, in that case, be more inclined 

 to one another at the top and less inclined at the base 

 than they actually are. Now this is precisely the effect 

 of a spherical lens. Its form permits a gradual change 

 in the inclination of the two bounding surfaces, and if its 

 surfaces are accurately formed, all rays which impinge 

 upon it from a luminous point which is not too close to 

 the lens, and is either in the axis or near it, will converge 

 after refraction to one point. 



Incident rays parallel to the axis will converge after re- 

 fraction to a point/, fig. 268; this point is the principal 



FIG. 268. 



focus, and its distance from the centre of the lens is the 

 4 focal length ' of the lens. The focal length of lenses 

 does not depend solely on the radius of curvature as is 

 the case with mirrors ; it also depends on the kind of 

 glass of which the lens is made. In a common double 

 convex lens, in which the radii of its two surfaces are 

 equal, the focal length is somewhat less than the radius 

 of curvature, while in a plano-convex lens it is some- 

 what less than twice the radius. 



Rays of light which proceed from the focus are 

 refracted so as to pass, after emerging from the lens, in 

 directions parallel to the principal axis; such rays there- 

 fore do not again converge in a point. 



