Question

a concave lens is kept in contact with a convex lens of focal length 20cm the combination works is a convex lens of focal 50centimetre find the power of concave lens

Answer 1

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Answer 2

Answer: The power of the concave lens is -3D.

Explanation:

Given that,

Focal length of convex lens f = 20 cm

Focal length of combination $f_{c}=50 cm$

Now, the power is

$P = \dfrac{1}{f}$

Now, power of the convex lens is

$P = \dfrac{1}{0.2}= 5 D$

The power of the combination of the lenses

$P_{c}= \dfrac{1}{0.5}= 2D$

We know that,

The power of the concave lens is

$P_{c}= P+P'$

Here, P' is the power of concave lens

$P'= 2D-5D$

$P' = -3 D$

Hence, The power of the concave lens is -3D.