The eye can detect colours because of the cone cells in the retina at the back of the eye. These cells, if functioning properly, can detect the three primary colour:- red, blue and green, and combinations of them.
For someone who has colour-blindness, these cells do not work properly, resulting in their inability to see certain things in their actual colour.
A person who has hypermetropia/hyperopia/long-sightdness has blurred vision when looking at objects close to them, but clearer vision when looking at objects in the distance.
This is because images of the objects they are looking at are formed behind the retina. By placing a convex lense in front of the eye with this defect, this condition can be corrected.
On the other hand, a person who has clear vision when looking at objects close to them, but blurred vision when looking at objects in the distance has a vision defect called myopia.
This happens because light from the objects form their respective images before it reaches the retina. By placing a concave lense in front of the eye with this defect, the images move back to the retina. Thus, a clear vision can be obtained.
Another condition in which one has blurred vision when looking at objects both near and far is called presbyopia. It is a perfectly natural condition that will eventually affect us all, and can be corrected by wearing bifocal glasses.
Positioning a cylindrical lense in front of an eye that has astigmatism will refract light in one direction more than in the other.
Thus, a person who has distorted vision as a result of this condition will then be able to see clearly.
Schematic diagram of the human eye