Question

When a real object is placed $25\text{cm}$ from a lens, a real image is formed. Mark the wrong statement from the following:

• A. The lens is a converging lens.
• B. The image may be magnified or diminished.
• C. The focal length of the lens is less than $25\text{cm}$.
• D. The focal length of the lens may be greater than $25\text{cm}$.

Answer 1

The correct option is D The focal length of the lens may be greater than $25\text{cm}$.

Since a real image is formed, the light rays must converge after refraction from lens.
Hence, it is a converging lens. Thus the focal length is positive.
For formation of image of an object $25\text{cm}$ from lens.
By lens formula,
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

Substituting the value of $u=-25\text{cm}$ we have,
$\frac{1}{v}-\frac{1}{-25}=\frac{1}{f}$

$\Rightarrow \frac{1}{v}=\frac{1}{f}-\frac{1}{25}$

$v$ should be positive.

Hence, $f<25\text{cm}$

Since magnification $=\frac{v}{-25}$ which may be greater than or less than $1$ depending upon the focal length.

Thus, option $\left(d\right)$, is right choice.