$A$ divergent lens will produce

When the object is at distances $u_1$ and $u_2$ from a lens,the images formed are real and virtual respectively,and both have the same size. Then the focal length of the lens is:

The formation of a real image using a biconvex lens is shown below:
If the whole setup is immersed in water without disturbing the object and the screen positions,what will one observe on the screen?

$A$ glass concave lens is placed in a liquid in which it behaves like a convergent lens. If the refractive indices of glass and liquid with respect to air are $^a{\mu _g}$ and $^a{\mu _l}$ respectively,then

The radius of curvature of a convex lens is $40 \,cm$ for each surface. Its refractive index is $1.5$. Its focal length is: (in $\,cm$)

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 ......