Question

A convex lens of focal length 40cm is in contact with a concave lens of focal lwngth 25cm.The power of combination is

Answer 1

Diopter=1/f . In convex lens focal is positive

So , diopter=1/0.4m (diopter is measure in meter so we first change focal length form cm to m)

Diopter in convex lens =2.5

Now,in concave focal length should be taken negative

Changing focal length to m from cm

25cm=0.25

Putting in formula, diopter=1/f

1/-0.25

=- 4 d

Now adding both power

-4+2.5

=-1.5