$A$ 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 $, D$)

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

For a normal eye,the cornea of the eye provides a converging power of $40\, D$ and the least converging power of the eye lens behind the cornea is $20\, D$. Using this information,the distance between the retina and the cornea-eye lens can be estimated to be ......... $cm$.

$A$ person has a near point of $50\, cm$. What lens should they use to reduce it to $25\, cm$?

$A$ man can see objects clearly between $15 \ cm$ and $30 \ cm$. He uses a lens to see far objects. Due to the lens used,the new near point will be at........$cm$.

$A$ person suffering from hypermetropia cannot focus on objects closer than $120 \, cm$. To read at a distance of $40 \, cm$,what should be the focal length of the lens required?