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

Define absolute refractive index. Absolute refractive indices of medium $A$ and medium $B$ are $n_{a}$ and $n_{b}$ respectively. What is the refractive index of medium $B$ with respect to medium $A$? How does the velocity of light vary with change in the optical density of the media?

Where should an object be placed so that a real and inverted image of the same size is obtained by a convex lens?

Write down the equation connecting the object distance,image distance,and focal length of lenses and mirrors.

What is the nature of the image formed by an object placed at a distance of $10 \text{ cm}$ in front of a concave mirror with a focal length of $20 \text{ cm}$?

What is the power of a convex lens with a focal length of $50 \ cm$?