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

What is the nature of the final image formed by an astronomical telescope compared to the object?

An object is placed $50\, cm$ from a lens that produces a virtual image at a distance of $10\, cm$ in front of the lens. Calculate the focal length of the lens. (in $, cm$)

If the image formed by a convex lens is of the same size as that of the object,what is the position and nature of the image with respect to the lens?

$A$ $5 \, cm$ tall object is placed perpendicular to the principal axis of a convex lens of focal length $20 \, cm$. The distance of the object from the lens is $30 \, cm$. Find the $(i)$ position,$(ii)$ nature,and $(iii)$ size of the image formed.

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