$A$ lens behaves as a converging lens in air and a diverging lens in water. The refractive index of the material of the lens is:

$A$ convex lens forms a real image of a point object placed on its principal axis. If the upper half of the lens is painted black,the image will

The radius of the convex surface of a plano-convex lens is $20 \ cm$ and the refractive index of the material of the lens is $1.5$. The focal length of the lens is......$cm$.

$A$ thin converging lens of focal length $f=25 \,cm$ forms the image of an object on a screen placed at a distance of $75 \,cm$ from the lens. The screen is moved closer to the lens by a distance of $25 \,cm$. The distance through which the object has to be shifted, so that its image on the screen is sharp again, is (in $\,cm$)

$A$ convex lens of focal length $1/3 \ m$ forms a real,inverted image twice the size of the object. The distance of the object from the lens is: (in $m$)

$A$ convex lens of focal length $30 \,cm$ produces $5$ times magnified real image of an object. What is the object distance (in $\,cm$)?