Activity 2C: Keeping in Focus Topic: Vision Part of: Unit: Challenges and Changes: Sensitivity to Vision & Hearing Compromises Lesson: How We See

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Keeping in Focus Hyperopia Teacher Handout

Keeping in Focus Myopia Teacher Handout

Keeping in Focus Student Handout

 

Keywords

depth perception, visual field, focus, presbyopia, myopia, brain, visual cortex, astigmatism, hyperopia

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Activity Description

This section is designed to teach children about focusing. Listed below are descriptions of myopia, hyperopia, astigmatism and presbyopia. Pass out two copies of the line drawing "Keeping in Focus." Using the descriptions from this lesson, have the students draw out where the images from an object will focus in the eye.

Binocular, Depth Vision - Our eyes are arranged so that their visual fields overlap a great deal. Each eye sees most of the same thing, with a slightly different view. The difference between the two views is called the "binocular parallax." The visual cortex of each hemisphere synthesizes the two views into a three-dimensional picture, hence we have depth perception. Our vision is then maintained through coordination between nerves and eye muscles. Sometimes the focusing properties of our eyes are imperfect, causing either nearsightedness (myopia), farsightedness (hyperopia), or uneven focusing in different planes (astigmatism). So special lenses are made and glasses worn to correct the focusing problem.

Myopia - Myopia occurs when there is an abnormally long distance between the cornea and the lens (or between the lens and the retina, or too powerful of a lens) which results in the image of a distant object (beyond 20 feet) focusing in front of the retina. The myopic individual sees blurry images of distant objects. Corrective lenses (concave lenses of the proper power) are worn to "refocus" the image on the back of the retina thereby allowing a clear image of distant objects.

Hyperopia - Hyperopia is opposite of myopia. It occurs when there is an abnormally short distance between the cornea and the lens (or a weakened lens) which results in the image of a distant object focusing behind the retina. The hyperopic individual wears corrective lenses (convex lenses of the proper power) to "move" the image forward and "refocus" it on the back of the retina.

Astigmatism - Astigmatism is a defect caused by distortions in the curvature of the cornea or the lens. The image seen falls unevenly in front of the retina. It is also corrected by special lenses.

Presbyopia - Presbyopia occurs as we age when the lens gradually loses its elasticity, interfering with vision. Things that are within a yard or so away appear blurry. A person with presbyopia wears corrective lenses (convex) to help them clearly see the objects less than a yard away.

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