When parallel rays of light enter the eye they are refracted (bent) by the crystalline lens and projected into the eye. The healthy eye adjusts for distance, focusing each image perfectly on a minute hollow in the retina at the back of the eye called the fovea. In refractive errors, the image either falls short of the fovea or lands behind it. This causes blurred vision as a result of inadequate adjustment of the eye, irregular axial length (distance from the front to the back of the eyeball), or incorrect curvature of the cornea. These problems can usually be fixed with corrective lenses or surgery, which adjusts the shape of the cornea.
Science EncyclopediaScience & Philosophy: Verbena Family (Verbenaceae) - Tropical Hardwoods In The Verbena Family to WelfarismVision Disorders - Refractive Errors, Hyperopia/presbyopia, Other Refractive Errors, Strabismus, Nonparalytic Strabismus, Paralytic Strabismus - Myopia, Astigmatism, Glaucoma, Secondary glaucomas, Degeneration of the macula, Retinal dystrophies