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

$A$ biconvex lens of refractive index $1.5$ has a focal length of $20 \,cm$ in air. Its focal length when immersed in a liquid of refractive index $1.6$ will be:

$A$ convex lens is made of three different materials as shown in the figure. How many images will be formed for a point object placed on its principal axis?

$A$ convex lens of focal length $f$ produces a real image whose size is $n$ times the size of an object. The distance of the object from the lens is

An object is placed first at infinity and then at $20 \ cm$ from the object-side focal plane of a convex lens. The two images thus formed are $5 \ cm$ apart. The focal length of the lens is $...... \ cm$.

$A$ turnip sits before a thin converging lens,outside the focal point of the lens. The lens is filled with a transparent gel so that it is flexible; by squeezing its ends toward its center,you can change the curvature of its front and rear sides. Suppose that the image must be formed on a card which is at a certain distance behind the lens,while you move the turnip away from the lens,then you should