Focal Length of a Lens The distance beyond a convex lens at which parallel rays converge to a common focal point is
called the focal length of the lens. The diagram at the top of Figure 49-6 demonstrates this focusing of
parallel light rays.
Figure 49-5 A, Focusing of light from a point source to a line focus by a cylindrical lens.
B, Two cylindrical convex lenses at rightangles to each other, demonstrating that one lens
converges lightrays in one plane and the other lens converges light rays in theplane at a right
angle. The two lenses combined give the samepoint focus as that obtained with a single
spherical convex lens. Focal pointsLight from distant source Point sourceFigure 49-6 The
two upper lenses of this figure have the samefocal length, but the light rays entering the top
lens are parallel, rays on the focal distance is shown.
