An object is placed at a distance of $10 \, cm$ from a lens. The image is formed at a distance of $20 \, cm$ behind the lens. The power of the lens is ......

The distance between the object and the screen is $100 \, cm$. $A$ lens produces an image on the screen when it is placed at either of two positions $40 \, cm$ apart. The power of the lens is (approximately): (in $, D$)

$A$ convex lens made of glass has a focal length of $0.15 \,m$ in air. If the refractive index of glass is $\frac{3}{2}$ and that of water is $\frac{4}{3}$, what is the focal length of the lens when immersed in water (in $\,m$)?

$A$ symmetric thin biconvex lens is cut into four equal parts by two planes $AB$ and $CD$ as shown in the figure. If the power of the original lens is $4 \text{D}$, then the power of one part of the divided lens is: (in $\text{D}$)

$A$ divergent lens will produce

If in a plano-convex lens,the radius of curvature of the convex surface is $10 \,cm$ and the focal length of the lens is $30 \,cm$,the refractive index of the material of the lens will be ..........