Light Refraction

When a person sees something, either the object is giving off light, or light waves are reflected from it. These light waves enter the eye, and an image of what is seen focuses upon the retina. The light rays must bend to be focused, a phenomenon called refraction (re-frak'shun).

Refraction occurs when light waves pass at an oblique angle from a medium of one optical density into a medium of a different optical density. For example, as figure 12.36 shows, when light passes obliquely from a less dense medium such as air into a denser medium such as glass, or from air into the cornea of the eye, the light is bent toward a line perpendicular to the surface between these substances. When the surface between such refracting media is curved, a lens is formed. A lens with a convex surface causes light waves to converge, and a lens with a concave surface causes light waves to diverge (fig. 12.37). Clinical Application 12.5 discusses some familiar problems with refraction.

Light Refraction Problems

Figure

When light passes at an oblique angle from air into glass, the light waves bend toward a line perpendicular to the surface of the glass.

Figure

When light passes at an oblique angle from air into glass, the light waves bend toward a line perpendicular to the surface of the glass.

Refraction Disorders

The elastic quality of the lens capsule lessens with time. People over forty-five years of age are often unable to accommodate sufficiently to read the fine print in books and newspapers or on medicine bottles. Their eyes remain focused for distant vision. This condition is termed presbyopia, or farsightedness of age. Eyeglasses or contact lenses can usually make up for the eye's loss of refracting power.

Other visual problems result from eyeballs that are too short or too long for sharp focusing. If an eye is too short, light waves are not focused sharply on the retina because their point of focus lies behind it. A person with this condition may be able to bring the image of distant objects into focus by accommodation, but this requires contraction of the ciliary muscles at times when these muscles are at rest in a normal eye. Still more accommodation is necessary to view closer objects, and the person may suffer from ciliary muscle fatigue, pain, and headache when doing close work.

People with short eyeballs are usually unable to accommodate enough to focus on the very close objects. They are farsighted. Eyeglasses or contact lenses with convex surfaces can remedy this condition (hyperopia) by focusing images closer to the front of the eye.

If an eyeball is too long, light waves are focused in front of the retina, blurring the image. In other words, the refracting power of the eye, even when the lens is flattened, is too great. Although a person with this problem may be able to focus on close objects by accommodation, distance vision is in variably poor. For this reason, the person is said to be nearsighted. Eyeglasses or contact lenses with concave surfaces that focus images farther from the front of the eye treat nearsightedness (myopia) (figs. 12A and 12B).

Another refraction problem, astigmatism, reflects a defect in the curvature of the cornea or the lens. The normal cornea has a spherical curvature, like the inside of a ball; an astigmatic cornea usually has an elliptical curvature, like the bowl of a spoon. As a result, some portions of an image are in focus on the retina, but other portions are blurred, and vision is distorted.

Without corrective lenses, astigmatic eyes tend to accommodate back and forth reflexly in an attempt to sharpen focus. The consequence of this continual action is often ciliary muscle fatigue and headache. ■

The convex surface of the cornea refracts light waves from objects outside the eye, providing about 75% of the total refractive power of the eye. The light is refracted again by the convex surface of the lens and to a lesser extent by the surfaces of the fluids within the eye chambers.

If the shape of the eye is normal, light waves are focused sharply upon the retina, much as a motion picture image is focused on a screen for viewing. Unlike the motion picture image, however, the one formed on the retina is upside down and reversed from left to right (fig. 12.38). When the visual cortex of the cerebrum interprets such an image, it somehow corrects this, and objects are seen in their proper positions.

Light waves coming from objects more than 20 feet away are traveling in nearly parallel lines, and they are focused on the retina by the cornea and by the lens in its more flattened or "at rest" condition. Light waves arriving from objects less than 20 feet away, however, reach the eye along more divergent lines—in fact, the closer the object, the more divergent the lines.

Divergent light waves tend to focus behind the retina unless something increases the refracting power of the eye. Accommodation accomplishes this increase, thickening the lens. As the lens thickens, light waves converge more strongly so that diverging light waves coming from close objects focus on the retina.

Shier-Butler-Lewis: I III. Integration and I 12. Somatic and Special I I © The McGraw-Hill

Human Anatomy and Coordination Senses Companies, 2001

Physiology, Ninth Edition

Uncorrected point of focus

Light waves

Uncorrected point of focus

Light waves

Concave lens

Corrected point of focus

Concave lens

Corrected point of focus

Point of focus

Light waves

Point of focus

Light waves

(b) Normal eye

Cornea

Point of focus

Cornea

Retina

Light waves

Point of focus

(c) Eye too short (hyperopia)

Figure 12A

(a) If an eye is too long, the focus point of images lies in front of the retina. (b) In a normal eye, the focus point is on the retina. (c) If an eye is too short, the focus point lies behind the retina.

Uncorrected point of focus

Light waves

Figure

Convex lens

Corrected point of focus

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