33.10 - Nearsightedness

Nearsightedness: Ability to focus on nearby

objects but not distant objects.

To people with nearsightedness, or myopia, faraway objects are blurry and look like the

image above.

To view an object that is far away, the eye’s ciliary muscle completely relaxes, causing

the lens to be as flat as possible. In a nearsighted eye, the eye is too long, causing

images of faraway objects to form in front of the retina, even when the lens is

completely relaxed. You see this in the conceptual diagram in Concept 2.

To correct this problem in focusing, a diverging lens can be placed in front of the eye.

As illustrated in Concept 3, the lens spreads out the parallel rays before they reach the

eye, so that the rays do not come to a focus quite so soon, but farther back upon the

retina where they should.

More recently, laser surgery has become a popular method for correcting

nearsightedness. A laser directed at the eye reshapes the cornea, causing its outer

surface to be flatter so that an image can form more crisply on the retina. The angle of

incidence for incoming light rays is less, causing them to converge farther back in the

eye, at the retina.

Contact lenses can also correct nearsightedness; they change the radius of curvature of

the front surface of the eye. Since the contact lenses are thicker at the edges than the

center, they work by flattening the eye’s surface.

Nearsightedness

Distant objects out of focus

The problem

Image forms in front of retina

A solution

Corrected with diverging lens

33.11 - Farsightedness

Farsightedness: Inability to clearly see objects that are relatively close.

With farsightedness, or hyperopia, the image of a nearby object forms behind the retina. The eyeball is too short, and no matter how hard the

ciliary muscle strains, it cannot contract the lens enough to focus the image on the retina. You see this shown in Concept 2.

People with hyperopia have difficulty reading things that are close and often use reading glasses as a result. A related condition called

presbyopia (“old vision”) occurs in many people starting in their forties. With presbyopia, the eyeball is not too short, but the lens itself has

become stiff with age and is unable to contract enough to focus images on the retina.

A converging lens can compensate for this problem by creating a virtual image beyond the eye’s near point, so that the lens can focus an

image on the retina. This solution is shown in Concept 3.

Farsightedness

Close objects out of focus

(^616) Copyright 2007 Kinetic Books Co. Chapter 33