What refractive error occurs when parallel rays focus behind the retina?

The condition where parallel rays of light focus behind the retina is known as hyperopia, or farsightedness. In hyperopia, the eyeball may be too short or the cornea may have insufficient curvature, which prevents light rays that are coming from distant objects from converging properly on the retina. Instead, these rays converge behind the retina, leading to difficulties in seeing objects clearly at close range, while distant objects may be seen more clearly.

In contrast, myopia, or nearsightedness, occurs when light rays focus in front of the retina, making distant objects blurry. Astigmatism results from an irregular shape of the cornea or lens, leading to blurred vision at all distances due to light being focused at different points. Presbyopia is a condition related to aging, where the eye's lens loses its ability to accommodate for near vision. Each of these refractive errors has a distinct mechanism and effect on vision, highlighting why hyperopia specifically is characterized by the focal point being located behind the retina.