If the focal length of a spherical mirror is $20 \ cm$,what is its radius of curvature (in $cm$)?

Define power of a lens. What is its unit? One student uses a lens of focal length $50\, cm$ and another of $-50\, cm$. What is the nature of the lens and its power used by each of them?

What will be the focal length of a lens whose power is given as $+2.0 \ D$?

When a plane mirror is rotated through a certain angle,the reflected ray turns through twice as much and the size of the image

An object is brought towards a concave mirror. How does the position and size of the image change?

$(a)$ 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 $u$.