Question

A convex lens of power 20 D and a concave
lens of power 15 D are placed in contact with
each other. The focal length of the equivalent
lens will be​

Answer 1

SOLUTION

GIVEN

A convex lens of power 20 D and a concave lens of power 15 D are placed in contact with each other.

TO DETERMINE

The focal length of the equivalent lens

EVALUATION

Here it is given that a convex lens of power 20 D and a concave lens of power 15 D are placed in contact with each other

So the power of the equivalent lens

= + 20 D - 15D

= + 5D

Hence the required focal length of the equivalent lens

$\displaystyle \sf{ = \frac{1}{5} \times 100 \: \: \: cm }$

$\displaystyle \sf{ =20 \: \: \: cm }$

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