The main idea of the passage is Nearsightedness and farsightedness produce contrasting vision problems, have opposite causes, and require different types of lenses. How does the detail in bold develop the main idea?
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 person's 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.

1.It contrasts the cause of farsightedness with that of nearsightedness.
2.It addresses a common misunderstanding about the causes of farsightedness.