Question

nearsightedness and farsightedness
nearsighted people experience blurred vision while looking at distant objects, such as street signs viewed from inside a car. conversely, farsighted people experience blurred vision while looking at nearby objects, such as words on a page. the causes of these two conditions differ as well. nearsightedness, or myopia, occurs either when a persons eye is too long from back to front or when the cornea (the clear layer at the front of the eye) is too curved. on the other hand, farsightedness, or hyperopia, results when the eye is too short or when the cornea is not curved enough. fortunately, corrective lenses can restore normal vision in both cases. as you might expect, these lenses are of opposing types: nearsightedness is improved with a lens that is thinner in the middle and thicker at the edges, while farsightedness is compensated for with a lens that is thicker in the middle and thinner at the edges.
what is the main, or central, idea of the passage?
in nearsightedness, the eye may be too long from front to back, while in farsightedness, the opposite may be true.
nearsightedness and farsightedness produce contrasting vision problems, have opposite causes, and require different types of lenses.

Explanation:

The passage first describes the contrasting vision - problems of nearsightedness and farsightedness, then their opposite causes, and finally the different types of corrective lenses for each.

Answer:

Nearsightedness and farsightedness produce contrasting vision problems, have opposite causes, and require different types of lenses.