45. (a) We use Eq. 35-10, with the conventions for signs discussed in ż35-5 and ż35-6.
(b) For the bi-convex (or double convex) case, we have
-1 -1
1 1 1 1
f = (n - 1) - = (1.5 - 1) - =40 cm.
r1 r2 40 cm -40 cm
Since f >0 the lens forms a real image of the Sun.
(c) For the planar convex lens, we find
-1
1 1
f = (1.5 - 1) - =80 cm,
" -40 cm
and the image formed is real (since f >0).
(d) Now
-1
1 1
f = (1.5 - 1) - = 240 cm ,
40 cm 60 cm
and the image formed is real (since f >0).
(e) For the bi-concave lens, the focal length is
-1
1 1
f = (1.5 - 1) - = -40 cm ,
-40 cm 40 cm
and the image formed is virtual (since f <0).
(f) In this case,
-1
1 1
f = (1.5 - 1) - = -80 cm ,
" 40 cm
and the image formed is virtual (since f <0).
(g) Now
-1
1 1
f = (1.5 - 1) - = -240 cm ,
60 cm 40 cm
and the image formed is virtual (since f <0).