The image formed on the retina is:

$A$ presbyopic patient has a near point of $30 \, cm$ and a far point of $40 \, cm$. The dioptric power of the corrective lens required for seeing distant objects is: (in $, D$)

If the distance of the far point for a myopia patient is doubled,the focal length of the lens required to cure it will become

$A$ man cannot see clearly the objects beyond a distance of $20 \, cm$ from his eyes. To see distant objects clearly,he must use which kind of lenses and of what focal length?

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 when they lie between $40 \ cm$ and $400 \ cm$ from his eye. In order to increase the maximum distance of distant vision to infinity, the type of lens and power of the correction lens required respectively are