The power of a combination of two lenses A and B is 5D.The focal length of A is 15cm. What is the focal length of B?

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+15 cm

-20 cm

-60 cm

+3cm

answer is C.

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Detailed Solution

Concept: The formula used to determine the power of lenses equivalent to 1f is as follows: ${"mathml":"<math class="image_resized" style="width:102px;height:40px"><span style="font-family:$ , where ${"mathml":"<math class="image_resized" style="width:19px;height:16px"><span style="font-family:$ and ${"mathml":"<math class="image_resized" style="width:19px;height:16px"><span style="font-family:$ are the focal lengths of the two lenses.
Here in the question the focal length of A is given as 15cm let it as ${"mathml":"<math class="image_resized" style="width:19px;height:16px"><span style="font-family:$ Since 1D=1/m, we may state that 5D=5/m.
In order to answer the question, we now change the units from meters to centimeters.
${"mathml":"<math class="image_resized" style="width:144px;height:36px"><span style="font-family:$ And now put these value in the formula

Hence , the correct option is 3.

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