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# What is meant by power of a lens. Define its S.I unit. You have two lenses A and B of focal length $+10\text{ cm}$ 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 cm from the lens? Draw a ray diagram to Justify your answer.

Last updated date: 06th Sep 2024
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Hint: Power of a lens is the visibility to converge or diverge the light ray falling on it. Power of the lens is the reciprocal of its focal length.

Power of Lens: The Degree of convergence or divergence of light rays achieved by a lens is expressed in terms of power. The power of a lens is defined as the reciprocal of its focal length. It is represented by P.
The power of a lens is related to focal length as below,
$\text{Power of a lens}=\dfrac{1}{\text{Focal length in meter}}$
The SI unit of power is dioptre.
The power of a convex lens is positive, the power of a concave lens is negative as a concave lens has a negative focal length.
In the given question lens A is having Focal length $+10\text{ cm}$and lens B is having Focal length$-10\text{ cm}$. Hence, Lens A is convex and Lens B is concave. Convex lenses form virtual and magnified images of an object placed between F and O.

A virtual, erect and enlarged image is formed at the same side of Lens, when an object is placed between Principal focus $\left( {{F}_{i}} \right)$ and optical center $\left( O \right)$ of a convex lens.

Note:
There are various possibilities to form various (different) types of image at positioning the objects at different positions. For convex lenses there is only one possibility to form Image which is virtual, erect and enlarged.