$A$ defective eye cannot see close objects clearly because their image is formed

$A$ person wears normal spectacles in which the distance between the glasses and the eyes is approximately $2 \, cm$. The power required is $-5 \, D$. If he wears contact lenses,then the required power is ......... $D$.

$A$ 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?

The impact of an image on the retina remains for......$sec$.

$A$ 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

The near point of a hypermetropic person is $75\, cm$ from the eye. What is the power of the lens required to enable the person to read clearly a book held at $25\, cm$ from the eye?