How many centres of curvature does a lens have?

An object of height $5 \, cm$ is placed perpendicular to the principal axis of a concave lens of focal length $10 \, cm$. If the distance of the object from the optical centre of the lens is $20 \, cm$,determine the position,nature,and size of the image formed using the lens formula.

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.

From which point of a lens are all the distances measured?

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

What is the $SI$ unit of the power of a lens?