The focal length of a biconvex lens made of glass with equal radii of curvature is $f$. If the lens is dipped in water,what will be its new focal length? (Take the refractive index of glass as $3/2$ and water as $4/3$)

Double-convex lenses are to be manufactured from a glass of refractive index $1.55$,with both faces of the same radius of curvature. What is the radius of curvature (in $cm$) required if the focal length is to be $20\;cm$?

$A$ bulb is located on a wall. Its image of equal size is to be obtained on a parallel wall with the help of a convex lens. The lens is placed at a distance $d$ ahead of the second wall. The required focal length will be:

$A$ square card of side length $1\, mm$ is being seen through a magnifying lens of focal length $10\, cm$. The card is placed at a distance of $9\, cm$ from the lens. The axis is perpendicular to the plane of the card. The apparent area of the card through the lens is......$cm^2$

The radii of curvature of the faces of a double convex lens are $10\,cm$ and $15\,cm$. Its focal length is $12\,cm$. What is the refractive index of glass?

The power (in diopters) of an equiconvex lens with radii of curvature of $10 \ cm$ and refractive index of $1.6$ is