CBSE Class 10 Science 2015 Term II Outside Delhi Set 1

Question : 22

Total: 36

What is meant by power of a lens? Define its S.I. unit.
You have two lenses A and B of focal lengths +10‌cm and −10‌cm 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 8‌cm 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 :
fA=+10‌cm=0.1m
Since the focal length is positive, it is a convex lens.
Now, PA=‌

=‌

0.1

=+10D.
For lens B :
fB=−10‌cm=−0.1m
Since the focal length is negative, it is a concave lens.
PB=‌

=‌

−0.1

=−10D
In case of a convex lens, image is virtual and magnified if object is placed between focus and optical centre.

Go to Question:

Prev Question

Next Question