$A$ person cannot see objects clearly beyond a distance of $40 \, cm$. What power of lens should be prescribed for them?

If the least distance of distinct vision for a boy is $35 \ cm$,then the lens to be used by the boy for correcting the defect of his eye is

$A$ person can see objects clearly only when they lie between $50\, cm$ and $400\, cm$ from his eyes. In order to increase the maximum distance of distinct vision to infinity, the type and power of the correcting lens the person has to use will be

$A$ person can see clearly only up to a distance of $25 \, cm$. He wants to read a book placed at a distance of $50 \, cm$. What kind of lens does he require for his spectacles and what must be its power?

$A$ person cannot see distinctly at a distance less than $1 \, m$. Calculate the power of the lens that he should use to read a book at a distance of $25 \, cm$.

When the power of the eye lens increases,a defect of vision is produced. This defect is known as: