Defects of the eye

Homeostasis and response · The human nervous system · note 7 of 8

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Defects of the eyeSpec 4.5.2.3

Two common defects of the eye are myopia (short sightedness) and hyperopia (long sightedness). In both, rays of light do not focus on the retina.

	Myopia (short sightedness)	Hyperopia (long sightedness)
Effect	Distant objects look blurred	Near objects look blurred
Where light rays focus	In front of the retina	Behind the retina
Correction with spectacle lenses	Concave lens, which spreads the light rays slightly before they enter the eye	Convex lens, which bends the light rays inwards before they enter the eye

Ray diagrams: in myopia light from a distant object focuses in front of the retina and a concave lens corrects it; in hyperopia light from a near object focuses behind the retina and a convex lens corrects it. — Myopia is corrected with a concave lens; hyperopia is corrected with a convex lens. Both bring the focus onto the retina.

Generally these defects are treated with spectacle lenses which refract the light rays so that they do focus on the retina. New technologies now include:

hard and soft contact lenses
laser surgery to change the shape of the cornea
a replacement lens in the eye.