Question

The graph of magnitude of object distance and magnitude of image distance for the various position of an object in front of a convex lens is shown in the figure.
The object distance & image distance is in $\text{cm}$, based on graph the focal length of lens is :

• A. $20\text{cm}$
• B. $40\text{cm}$
• C. $10\text{cm}$
• D. $30\text{cm}$

Answer 1

The correct option is C $10\text{cm}$

From the given graph, at the object distance of $20\text{cm}$ the image distance and object distance are equal.
$\Rightarrow |u|=|v|=20\text{cm}$
Thus, it is possible only when object is kept at a distance of ${}^{\prime }2f^{\prime }$ from the lens.
$\Rightarrow |u|=2f$
or, $20\text{cm}=2f$
$\therefore f=10\text{cm}$