Image formation in the eye

Light rays from an object passes through the conjunctiva, cornea, aqueous humour, lens and vitreous humour to the retina. The light rays are refracted as they pass through the various regions and are finally brought to a focus (converged) on the fovea of the retina. A real image, which is inverted and smaller than the object, is formed. The cones and rods are stimulated by the light falling on the retina. Impulses are then sent via the optic nerve to the visual centres of the cerebral cortex of the brain the brain interprets the impulses and an upright impression of the object, of the correct size and distance from the eye, is perceived. The eye is able to adjust automatically the focal length of the lens according to the distance of an object from the eye, such that images of objects from far and near are brought to a focus on the fovea of the retina. The ability of the eye to do this is termed accommodation. Eye Defects Eyes defects are caused by eye strain, old age, and abnormal shape of the eyeball. Three types of eye defect are common in humans, namely myopia, hypermetropia and astigmatism. Myopia (Short-sight) Individuals with this type of eye defect can see near objects clearly but not distant objects. It is caused by the eyeball being too long, so that images from distant objects are brought to a focus in front of the retina. It is corrected by wearing spectacles with diverging lenses (concave lenses). This type of lens diverges parallel light rays coming from the object so that they are brought to a focus on the retina. Hypermetropia (Long-sight) Individuals with long-sight can see far objects clearly but not near objects. It is caused by the eyeball being too short, so that images from near objects are brought to a focus behind the retina. It is corrected by wearing spectacles with a converging lens (convex lens). This type of lens converges the parallel light rays from the object, so that hey are brought to a focus on the retina. Astigmatism This is the most common of all eye defects. It is caused by the surface of the cornea not being perfectly smooth or spherical. Parallel light rays are therefore not brought to a sharp focus on the retina, resulting in blurred vision. Every eye has some degree of astigmatism but it is most common in older people. It is corrected by wearing spectacles with cylindrical lenses. The smooth surface of the lens bring s the light rays to a sharp focus on the retina.