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

If the radii of curvature of both convex surfaces of a lens are $20 \, cm$,find the focal length of the lens in the given arrangement. (in $, cm$)

An object approaches a convergent lens from the left of the lens with a uniform speed $5 \ m/s$ and stops at the focus. The image:

An equiconvex lens has a power of $2 \ D$. If it is made of glass of refractive index $1.25,$ then the radius of curvature of each of its surfaces will be $.... \ cm$.

Is power negative or positive for a concave lens?

$A$ convex lens made of glass (refractive index $= 1.5$) has a focal length of $24 \ cm$ in air. When it is totally immersed in water (refractive index $= 1.33$),its focal length changes to: (in $cm$)