The refractive index of the material of a double convex lens is $1.5$ and its focal length is $5 \,cm$. If the radii of curvature are equal, the value of the radius of curvature (in $cm$) is

$A$ convex lens is used to form an image of an object on a screen. If the upper half of the lens is blackened,so that it becomes opaque,then

The distance between an object and the screen is $80 \, cm$. $A$ lens produces an image on the screen when the lens is placed at either of the positions $20 \, cm$ apart. The focal length of the lens is ....... $cm$.

If the image of an object is at the focal point $f$ to the right side of a convex lens, the position of the object on the left of the lens is at

There is an equiconvex glass lens with radius of each face as $R$,$_a{\mu _g} = 3/2$,and $_a{\mu _w} = 4/3$. If there is water in the object space and air in the image space,then the focal length is:

If the focal lengths of red and violet light rays for the same lens are $f_R$ and $f_v$ respectively,then which of the following is true?