What is meant by power of a lens? Define its ...

CBSE Class 10 Science 2015 Term II Outside Delhi Set 1

Question : 22 of 36

Marks: +1, -0

What is meant by power of a lens? Define its S.I. unit.

You have two lenses

A

and

B

of focal lengths

+10\,\text{cm}

and

-10\,\text{cm}

respectively. State the nature and power of each lens. Which of the two lenses will form a virtual and magnified image of an object placed

8\,\text{cm}

from the lens? Draw a ray diagram to justify your answer.

Solution:

Power of a lens is defined as the ability of lens to converge or to diverge a beam of light. It is also defined as reciprocal of focal length in metres.

For lens A :

f_{A}=+10\,\text{cm}=0.1\,\text{m}

Since the focal length is positive, it is a convex lens.

Now,

P_{A}=\;\frac{1}{f_{A}}=\;\frac{1}{0.1}=+10\,\text{D}

For lens B :

f_{B}=-10\,\text{cm}=-0.1\,\text{m}

Since the focal length is negative, it is a concave lens.

P_{B}=\;\frac{1}{f_{B}}=\;\frac{1}{-0.1}=-10\,\text{D}

In case of a convex lens, image is virtual and magnified if object is placed between focus and optical centre.

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