For what position of the object does a convex lens form an erect and virtual image?

Name the type of lens that can be used as a magnifying glass. Give reason$(s)$ and draw a ray diagram to support your answer.

What is the expression for magnification of a lens?

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

$A$ point object is placed at a distance of $12 \, cm$ from a convex lens on its principal axis. Its image is formed on the other side of the lens at a distance of $18 \, cm$ from the lens. Find the focal length of the lens. Is the image magnified? Justify your answer.

Name the type of lens that can be used to obtain:
$(i)$ $A$ magnified and virtual image.
$(ii)$ $A$ diminished and virtual image of the object.