What type of image is always formed by a concave lens?

The figure below shows an object $AB$ placed on the principal axis of a convex lens placed in air. $F_{1}$ and $F_{2}$ are the two foci of the lens. Copy the diagram.
$(i)$ Draw a ray of light starting from $B$ and passing through $O$. Show the same ray after refraction by the lens.
$(ii)$ Draw another ray from $B$ which passes through $F_{2}$ after refraction by the lens.
$(iii)$ Locate the final image formed.
$(iv)$ Is the image real or virtual?

Define $1$ dioptre of power of a lens.

$A$ 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.

What kind of image is formed when an object is placed between the principal focus $(F)$ and the optical center $(O)$ of a convex lens?

What is the power of a concave lens of focal length $25\, cm$?