The focal length of a lens is $10 \ cm$. What is the power of the lens in dioptre (in $D$)?

$A$ thin glass (refractive index $1.5$) lens has an optical power of $-5\,D$ in air. Its optical power in a liquid medium with a refractive index of $1.6$ will be: (in $,D$)

$A$ glass concave lens is placed in a liquid in which it behaves like a convergent lens. If the refractive indices of glass and liquid with respect to air are $_a \mu_g$ and $_a \mu_l$ respectively,then

If ${I_1}$ and ${I_2}$ are the sizes of the images for the two positions of the lens in the displacement method,then the size of the object is given by:

The focal length of a convex lens of refractive index $1.5$ is $2 \ cm$. What will be the focal length of the lens when it is immersed in a liquid of refractive index $1.25$?

Which of the following measurements require 'index correction'?