question_answer

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

You have two lenses A and B of focal lengths \[+\mathbf{10}\text{ }\mathbf{cm}\]and \[\mathbf{10}\text{ }\mathbf{cm}\]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 cm from the lens? Draw a ray diagram to justify your answer.

Answer:

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

For lens A:

\[{{f}_{A}}=+10\,cm=0.1\,m\]

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

Now,     \[{{P}_{A}}=\frac{1}{{{f}_{A}}}+\frac{1}{0.1}=+10D\]

For lens B:

\[{{f}_{B}}=-10\,cm=-\,0.1\,m~~\]

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

\[{{P}_{B}}=\frac{1}{{{f}_{B}}}=\frac{1}{-\,0.1}=-10D\]

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