Question

The focal lengths of two lenses in contact to each other are 20 cm and 50 cm respectively. Their combined power is:
(a) + 7.0 D (b) + 70 D

(c) + 3.0 D (d) + 30 D​

Answer 1

Answer:

(a) 7 D

Explanation:

100/20+100/50

5+2

7 D

Answer 2

Answer:

a) 7D

Explanation:

Power of lens is defined as reciprocal of focal length.

P = 1/f

For convex lens (positive focal length) and for concave lens (negative focal length).

Given that the focal lengths of two lenses in contact to each other are 20 cm and 50 cm respectively.

Let's say P1 = 20 cm, P2 = 50 cm and Combined power = P.

Combined Power = Power of lens of 1st lens + Power of lens of 2nd lens

P = P1 + P2

P = 1/f1 + 1/f2

f1 = 20cm = 0.2 m and f2 = 50 cm = 0.5 m

P = (1/0.2 + 1/0.5) D

P = (5 + 2) D

P = 7 D

Dioptre (D) is the unit of power of lens.

Hence, the combined Power of lens is 7D.