Question

2.The magnitude of focal legth of a concave lens is 20cm.What is the power of the lens?
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Answer 1

Power of lens is given by formula:

• D = 1/f (in metre)

Focal length=20cm=0.2m

Therefore,

D = 1/0.2 = +5

Fact:

Power of convex lens is positive

and

Power of concave lens is negative.

Answer 2

Given:

• The magnitude of focal length of a concave lens is 20cm.

To Find:

• The power of the lens.

Solution:

We have,

f = 20cm

D = ?

We know that,

D = 1m/f

D = 100cm/f

D = 100/25

D = 5

As, concave lens focal length is negative, so answer is -5.