The power of a lens is $2.5$ dioptre. What is the focal length and the type of lens?

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

$A$ convex lens of focal length $20 \, cm$ can produce a magnified virtual as well as real image. Is this a correct statement? If yes,where shall the object be placed in each case for obtaining these images?

The figure below shows a point source of light $S$,a convex lens $L$,and a plane mirror $M$. The three are placed such that rays of light from $S$ return to it after reflection from $M$. $(i)$ What is the distance $OS$ called? $(ii)$ To which point (left of $S$,on $S$,or right of $S$) will the rays return if $M$ is moved to the left and brought in contact with $L$?

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

$A$ concave lens of focal length $15 \,cm$ forms an image $10 \,cm$ from the lens. How far is the object placed from the lens? Draw the ray diagram.