Kids Research Express

Vision, physiological sense of sight by which the form, color, size, movements, and distance of objects are perceived. Vision in Humans The human eye functions somewhat like a camera; that is, it receives and focuses light upon a photosensitive receiver, the retina. The light rays are bent and brought to focus as they pass through the cornea and the lens. The shape of the lens can be changed by the action of the ciliary muscles so that clear images of objects at different distances and of moving objects are formed on the retina. This ability to focus objects at varying distances is known as accommodation.

Eyeglasses, also called glasses or spectacles, lenses set in frames for wearing in front of the eyes to aid vision or to correct such defects of vision as myopia, hyperopia, and astigmatism. In 1268 Roger Bacon made the earliest recorded comment on the use of lenses for optical purposes, but magnifying lenses inserted in frames were used for reading both in Europe and China at this time, and it is a matter of controversy whether the West learned from the East or vice versa. In Europe eyeglasses first appeared in Italy, their introduction being attributed to Alessandro di Spina of Florence. The first portrait to show eyeglasses is that of Hugh of Provence by Tommaso da Modena, painted in 1352. In 1480 Domenico Ghirlandaio painted St. Jerome at a desk from which dangled eyeglasses; as a result, St. Jerome became the patron saint of the spectacle-makers’ guild. The earliest glasses had convex lenses to aid farsightedness. A concave lens for myopia, or nearsightedness, is first evident in

Myopia, also called nearsightedness and shortsightedness, visual abnormality in which the resting eye focuses the image of a distant object at a point in front of the retina (the light-sensitive layer of tissue that lines the back and sides of the eye), resulting in a blurred image. Myopic eyes, which are usually longer than normal from front to rear, are somewhat more susceptible to retinal detachment than are normal or farsighted eyes. Severe myopia can be associated with other eye problems as well, most of which affect the retina or the choroid (i.e., pathologic blood vessel growth from the choroid).

Presbyopia, loss of ability to focus the eye sharply on near objects as a result of the decreasing elasticity of the lens of the eye. The eye’s ability to focus on near and far objects—the power of accommodation—depends upon two forces, the elasticity of the lens of the eye and the action of the ciliary muscle (a roughly ring-shaped muscle that encircles the lens and is attached to it by suspensory ligaments). When the ciliary muscle is relaxed, the ring enlarges away from the lens and the suspensory ligaments are tautened, flattening the lens into a shape suitable for viewing distant objects. When the muscle contracts, the ligaments are loosened, and, because of the elasticity of the lens, the surface of the lens—particularly the front surface—becomes more curved, in keeping with viewing near objects. Ordinarily the lens gradually becomes less elastic (it hardens) with age, so the power of accommodation is lost progressively. The loss is most rapid in the decade of the 40s, the age w

Hyperopia, also called farsightedness, refractive error or abnormality in which the cornea and lens of the eye focus the image of the visual field at an imaginary point behind the retina (the light-sensitive layer of tissue lining the back and sides of the eye). The retina thus receives an unfocused image of near objects, though distant objects may be in focus. Hyperopia frequently occurs when an eye is shorter than normal from front to rear; the lens is then unable to increase its convexity sufficiently to focus the images of close objects onto the retina. Corrective lenses for hyperopia are designed to supply the additional convexity needed for focusing. Hyperopic laser in situ keratomileusis (H-LASIK) and photorefractive keratectomy for hyperopia (H-PRK) are common surgical methods that reshape the cornea to improve vision in hyperopic patients.