Astigmatism

Because the curvature of the cornea and lens is not perfectly symmetrical, light passing through some parts of these structures may be refracted to a different degree than light passing through other parts. When the asymmetry of the cornea and/or lens is significant, the person is said to have astigmatism. If a person with astigmatism views a circle of lines radiating from the center, like the spokes of a wheel, the image of these lines will not appear clear in all 360 degrees. The parts of the circle that appear blurred can thus be used to map the astigmatism. This condition is corrected by cylindrical lenses that compensate for the asymmetry in the cornea or lens of the eye.

Emmetropia (normal vision) Rays focus on retina

Emmetropia (normal vision) Rays focus on retina

No correction necessary

Hyperopia (farsightedness) Rays focus behind retina

Convex lens corrects farsightedness

Hyperopia (farsightedness) Rays focus behind retina

Convex lens corrects farsightedness

Myopia (nearsightedness) Rays focus in front of retina (b)

Concave lens corrects nearsightedness

Myopia (nearsightedness) Rays focus in front of retina (b)

Concave lens corrects nearsightedness

Astigmatism

Rays do not focus <d)

Uneven lens corrects astigmatism

Astigmatism

Rays do not focus <d)

Uneven lens corrects astigmatism

Clinical Investigation Clue

Remember that Ed had difficulty seeing print, although he never needed glasses before and his distance vision was still good. ■ What condition was most likely responsible for Ed's vision impairment?

■ Figure 10.34 Problems of refraction and how they are corrected. In a normal eye (a), parallel rays of light are brought to a focus on the retina by refraction in the cornea and lens. If the eye is too long, as in myopia (b), the focus is in front of the retina. This can be corrected by a concave lens. If the eye is too short, as in hyperopia (c), the focus is behind the retina. This is corrected by a convex lens. In astigmatism (d), light refraction is uneven because of irregularities in the shape of the cornea or lens.

Test Yourself Before You Continue

1. Using a line diagram, explain why an inverse image is produced on the retina. Also explain how the image in one eye corresponds to the image in the other eye.

2. Using a line diagram, show how parallel rays of light are brought to a focus on the retina. Explain how this focus is maintained as the distance from the object to the eye is increased or decreased (that is, explain accommodation).

3. Explain why a blurred image is produced in each of these conditions: presbyopia, myopia, hyperopia, and astigmatism.

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