What is the focal length of the objective of an astronomical telescope as compared to that of the eyepiece?

When an object is placed at $2F$ in front of a convex lens,where will its image be formed?

Which of the following statements is correct?

The diagram given below shows an object $O$ and its image $I.$ Copy the diagram and draw suitable rays to locate the lens and its focus. Name the type of lens in this case.

What is the focal length of a convex lens having power $+5.0 \,D$?

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.