Question

A converging lens of focal length 20 cm is placed in contact with a diverging lens of focal length 30 cm. The focal length of this combination is

Answer 1

Answer:

The focal lenth (F) =+10.0cm

Explanation:

The formular for combined focal length (F) is given as;

$1/f = 1/f_{1} + 1/f_{2}$

In this  question,

F1 = 20cm

F2  = -30cm

Plugging the values into the formuar above,

$1/f = 1/20 + 1/(-30)$

$1/f = 0.05 - 0.033[tex]1/f = -0.017f = [tex]1/ -0.017$

f = 58.82cm

i.e. the combination behaves as a converging lens (because of the postive sign) of focal length 58.82cm .