Question

A convex lens of focal length 10 cm is placed in closed contact with a concave lens of focal length 10 cm. The power of combination is
20 D
10 D
40 D
Zero

Answer 1

Answer:

F1 = +10cm for convex lens

F2=-10 cm for concave lens

the following lens of the combination is given below

1/f = 1/F1 + 1/F2

1/f = 1/ +10cm + 1/ -10cm

= 1/20cm

f= +20 cm

the correct option is number (a)

Answer 2

Answer:

The power of combination is Zero.

Step-by-step explanation:

Step -1: For convex lens, focal length = +10 cm

            For concave lens, focal length = -10 cm

Suppose the focal length for combined lens = f

$\frac{1}{f}$ = $\frac{1}{10}$ + $\frac{1}{-10}$ =0

∴ f is infinite.

Step-2: Power of a lens = $\frac{1}{f}$ = $\frac{1}{infinite}$ = 0

So, Power of the combined lens is zero.