$A$ thin lens has a focal length of $-12\, cm$. Is it a convex lens or a concave lens?

$(a)$ Calculate the distance at which an object should be placed in front of a convex lens of focal length $10\, cm$ to obtain a virtual image of double its size.
$(b)$ In the above given case,find the magnification,if the image formed is real. Express it in terms of the relation between $v$ and $u$.

You are given water,mustard oil,glycerine,and kerosene. In which of these media would a ray of light incident obliquely at the same angle bend the most?

Define the term reflection.

If an object is placed symmetrically between two plane mirrors, inclined at an angle of $72^{\circ},$ then the total number of images formed is

Which of the following can produce a magnification of $+5$ as well as $-5$?