Question

A convex lens of focal length 20 cm is placed in contact with a concave lens of focal length 12.5 cm in such a way that they have the same principal axis. Find the power of the combination​

Answer 1

Answer :

A convex lens of focal length 20cm is placed in contact with a concave lens of focal lens 12.5cm.

We have to find power of the combination$.$

Focal length of convex lens is taken positive and that for concave lens is taken negative.

Relation b/w focal length and power of lens is given by

• P = 1 / f

◈ Power of convex lens :

» P = 1 / f

» P = 1 / 20

» P = 0.05D

◈ Power of concave lens :

» P' = 1 / f'

» P' = 1 / (-12.5)

» P' = -0.08D

◈ Power of the combination :

➝ Peq = P + P'

➝ Peq = 0.05 + (-0.08)

➝ Peq = -0.03D

(Combination of lens will behave as concave lens.)

☃ God_Bless :)

Answer 2

Remember that :

Focal length of convex lens is taken positive and that for concave lens is taken negative.

Formula Used

1/ F = Power

Power of convex len

1/20 = p1

p1 = +0.05 D

Power of concave len

1/-12.5 = p2

p2 = -0.08 D

power of the combination

p1+ p2

0.05D + (-0.08D )

-0.13 D