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

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

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?

Give one use of a concave lens.

$(a)$ Define power of a lens and write its $SI$ unit.
$(b)$ $A$ convex lens forms a real and inverted image of a needle at a distance of $50\, cm$ from it. Where is the needle placed in front of the lens, if the image size is equal to the object size? Also, find the power of the lens.

How will you identify a convex lens by touching it?