$A$ convex mirror has a focal length $f$. $A$ real object is placed at a distance $f$ in front of it. The mirror produces an image at

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?

Which of the following is the lens formula?

State the sign convention used to measure the length of the image and the object.

Copy the figure and show the image formation with the help of rays.