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?

How will you distinguish between a convex and a concave lens without touching them?

How many images are formed by two parallel mirrors?

One half of a convex lens of focal length $10\, cm$ is covered with a black paper. Can such a lens produce an image of a complete object placed at a distance of $30\, cm$ from the lens? Draw a ray diagram to justify your answer.
$A$ $4\, cm$ 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 $15\, cm$. Find the nature,position and size of the image.

Which of the lenses with focal lengths $10 \, cm$,$20 \, cm$,$25 \, cm$,and $50 \, cm$ has the maximum power (in $, cm$)?

Two lenses have powers of $(a) +2 \text{ D}$ and $(b) -4 \text{ D}$. Find the nature and focal length of each lens.