The Thin-Lens Formulas
Our plan is to consider each lens surface as a separate refracting surface, and to use the image formed tJy the first surface as the object for the second. We start with the thick glass “lens” of length L in Fig. 35-21a whose left and
right refracting surfaces are gr ound to radii r’ and ,.H. A point object 0′ is placednear the left surface as shown. A ray leaving 0 ‘ along the central axis is not deflected on entering or leaving the lens.A second ray leaving 0′ at an angle c.r with the cenrral axis intersects the left surface at point a’, is refracted, and intersects the second (right) surface at point 0″. The ray is again refracted and crosses the axis at I”, which, being the intersection of two rays from 0 ‘, is the image of point 0 ‘,formed after refraction at two surfaces. Figure 35-21h shows that the first (left) surface also forms a virtual image of 0′ at I’. To locate I’. we use .