$A$ double convex thin lens made of glass (refractive index $\mu = 1.5$) has both radii of curvature of magnitude $20 \ cm$. Incident light rays parallel to the axis of the lens will converge at a distance $L$ such that $L = ...... \ cm$.

$A$ convex lens is made from glass material having a refractive index of $1.4$ with the same radius of curvature on both sides. The ratio of its focal length to its radius of curvature is . . . . . . .

If the focal length of a double convex lens for red light is $f_R$,what is its focal length for violet light?

$A$ converging lens of focal length $20\, cm$ and diameter $5\, cm$ is cut along the line $AB$. The part of the lens shown shaded in the diagram is now used to form an image of a point $P$ placed $30\, cm$ away from it on the line $XY$,which is perpendicular to the plane of the lens. The image of $P$ will be formed:

$A$ biconvex lens has a focal length $2/3$ times the radius of curvature of either surface. The refractive index of the material is:

In an experiment with a convex lens,the plot of the image distance $(v')$ against the object distance $(u')$ measured from the focus gives a curve $v' u' = 225$. If all the distances are measured in $cm$,the magnitude of the focal length of the lens is $cm$.