Is it possible for a lens to act as a convergent lens in one medium and a divergent lens in another?

One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.

Define $1$ dioptre of power of a lens.

$(a)$ $A$ thin converging lens forms a:
$(i)$ Real magnified image.
$(ii)$ Virtual magnified image of an object placed in front of it.
Write the positions of the objects in each case.
$(b)$ Draw labelled ray diagrams to show the image formation in each case.
$(c)$ How will the following be affected on cutting this lens into two halves along the principal axis?
$(i)$ Focal length,
$(ii)$ Intensity of the image formed by half lens.

Find the focal length of a convex lens,if its power is $+2.0 \ D$. (in $m$)

$A$ doctor has prescribed a corrective lens of power $+1.5\, D.$ Find the focal length of the lens. Is the prescribed lens diverging or converging?