i) The parts of the human eye controlling the amount of light entering it are:

Iris and Pupil.

Iris is the part of the eye surrounding the pupil. It adjusts the size of the pupil based on the intensity of light received by the eye. (contracts the eye if light received is large, dilates the pupil if light received by the eye is less.

Pupil is the opening in the iris through which light passes before reaching the lens and getting focussed at the retina. Hence the pupil allows light to enter the eyes.

ii) Retina is a thin and delicate tissue lining the inner eye. It varies in thickness from 0.1mm to 0.5 mm (thickest near the optic disk). The sensory photoreceptor cells i.e. rods and cones are present here. These generate excitatory responses to light and sends electrical signals to the brain due to which visual information is processed. The retina thus also acts as a screen on which image is formed.

iii) Yes, the corneal-impairment can be cured by replacing the defective cornea with the cornea of the donated eye.

We can encourage eye donation because of a number of reasons and the following initiatives can be taken:

a) Various campaigns can be conducted to support the cause.

b) Millions of people are suffering from corneal blindness hence donating an eye can be helpful for them who can see with those eyes donated since eyes are functional even after death of a person so people can be encouraged to donate their eyes after death.