Give the name of a natural optical instrument.

$A$ person cannot see objects beyond $3 \, m$. What should be the focal length of the lens required to see objects at $12 \, m$?

$A$ man can see clearly up to $3 \, m$. Prescribe a lens for his spectacles so that he can see clearly up to $12 \, m$.

If you are using eyeglasses of power $2 D$, what is your near point (in $cm$)?

$A$ person is suffering from a myopic defect. He is able to see clear objects placed at $15 \,cm$. What type and of what focal length of lens should he use to see clearly the objects placed $60 \,cm$ away?

$A$ person cannot see objects clearly placed at a distance of more than $40 \, cm$. He is advised to use a lens of power: