$A$ convex lens of focal length $f$ produces an image $\frac{1}{n}$ times the size of the object. The distance of the object from the lens is:

The formation of a real image using a biconvex lens is shown below:
If the whole setup is immersed in water without disturbing the object and the screen positions,what will one observe on the screen?

$A$ biconvex lens is cut into two halves along $(i) XOX'$ and $(ii) YOY'$ as shown in the figure. Let $f, f', f''$ be the focal lengths of the complete lens, of each half in case $(i),$ and of each half in case $(ii),$ respectively. Choose the correct statement from the following:

An achromatic telescope objective is to be made by combining the lenses of flint and crown glasses. This proper choice is

In the displacement method,the distance between the object and the screen is $70 \; cm$ and the focal length of the lens is $16 \; cm$. Find the separation between the magnified and diminished image positions of the lens. (in $; cm$)

$A$ screen is placed $100 \,cm$ from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by $20 \,cm$. The focal length of the lens is (in $\,cm$)