Materials Included In Kit

Additional Materials Required

Prelab Preparation

Cut apart the Astigmatism Box Charts along the dotted lines.

Safety Precautions

Although the materials are not considered hazardous, please follow all laboratory safety guidelines. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory.

Disposal

Materials may be stored for future use.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. All materials are reusable. This laboratory activity can reasonably be completed in one 50-minute class period.
• The ends of the string may be covered with tape to inhibit unraveling.
• Students whose astigmatism is corrected with glasses or contact lenses will not see the grayed blurring of the test lines unless the glasses are removed. Students with contacts may want to bring a contact lens case and solution so their eyesight can be tested.
• If students have difficulty viewing the Astigmatism Hemisphere Chart from a distance of 20 feet, allow them to move closer until the chart lines are clearly visible.

Teacher Tips

• Incorporate the activity into a unit on sight using the Snellen Eye Test Chart, AB1240, Peripheral Vision Student Laboratory Kit, FB1871, and the Visual Perception Activity Stations Kit, FB1872.
• Extend the activity into a research project on eye problems or the history of eye tests.

Sample Data

Observations

Answers to Questions

1. Why is it necessary to remove corrective eyewear such as glasses before beginning the activity?

   Corrective eyeglasses correct the vision for defects such as astigmatism. The grayed blurring will not be visible with the eyewear in place.

2. Vision problems are often first discovered when a child begins school. Why might this be so?

   School is the first time many students are required to stay at a distance while viewing objects with others. A student who is unable to see an object, such as writing on a board, that others can obviously see with ease will likely mention this fact to either the teacher or parents. Also, many schools require vision screening tests to ensure early detection of vision problems.

3. In the Background section it was stated that injury to the eye may cause astigmatism. Explain why an object damaging the eye may cause astigmatism.

   The area may be permanently distorted due to corneal damage from the object. Permanent distortion may also be caused by inflammation of the eye due to the object.

Teacher Handouts

10939_Teacher1.pdf

References

Astigmatism. http://www.webmd.com/eye-health/understanding-astigmatism-basics (Accessed September 2008).

Hole, J. W. Human Anatomy and Physiology, 5th ed.; Wm. C. Brown: Dubuque, IA, 1990; pp 449–464.

Scott, A. S.; Fong, E. Body Structures & Functions, 10th ed.; Delmar Learning: Clifton Park, NY, 2004; pp 180–191.

Introduction

Headaches and blurred vision are two symptoms that may occur due to defects in the anatomy of the eye. In this activity explore the most common eye deformity—astigmatism.

Concepts

• Astigmatism
• Myopia
• Hyperopia
• Eye anatomy and physiology

Background

The three most common types of vision problems are myopia, hyperopia, and astigmatism. Myopia, or nearsightedness, occurs when the eye is able to see close objects clearly but distant objects are blurry. Myopia is common from a young age. Myopia is caused when the cornea is curved too much or the eye is longer than normal. Instead of an image focusing precisely on the retina, light is focused in front of the retina, resulting in blurred distant objects (see Figure 1).

Hyperopia, or farsighted, is caused when close objects are blurry while distant objects remain in clear focus. Hyperopia tends to occur more commonly in middle age. Hyperopia occurs when the cornea is curved too little or the eye is shorter than normal. The focal point of the image is behind the back of the eye making nearby objects blurry (see Figure 2). Astigmatism affects both near and distant vision and often occurs with myopia or hyperopia. Astigmatism occurs when the eye is not spherical or the cornea or lens has an irregular shape. The irregularity prevents light from focusing properly on the retina, creating multiple focal points (see Figure 3). Because of the multiple focal points, part of the field of vision is blurred, regardless of distance. Astigmatism is very common; most people have some degree of astigmatism. Although the brain may be able to compensate for slight astigmatism, more severe forms require glasses or contacts to merge the multiple focal points into one cohesive image on the retina. Injury to the eye and eye disorders may cause astigmatism. Many optical tests are available for diagnosing astigmatism. The first set of vision tests developed for astigmatism is still in use today around the world. This test involves viewing a hemisphere of lines placed at a distance of twenty feet as well as viewing a set of lined boxes at a distance of eighteen inches. The distances chosen for the test is significant. Eighteen inches is a typical reading distance while objects that are clearly visible at 20 feet tend to remain clear at further distances. The orientation and line spacing are important as well. In a person with normal vision, the lines of the hemispheres or boxes will appear a steady solid black. A person with myopia will see the set of boxes clearly but the distant hemisphere of lines will be very blurry. A person with hyperopia will see blurry boxes but the hemisphere of lines will be clear. In astigmatic persons one or more sets of lines will have a gray tint near the lines.

Experiment Overview

In this activity, discover the imperfections present in your vision by using the most common vision test for astigmatism.

Materials

Safety Precautions

Although the materials used in this activity are not considered hazardous, please follow all laboratory safety guidelines. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

1. Mount the Astigmatism Hemisphere Chart on the wall with the bottom of the hemisphere at approximately eye level.
2. Use the string to measure 20 feet away from the Astigmatism Hemisphere Chart. Place a 1" piece of electrical tape on the floor to mark this distance.
3. One person should be the test subject and the other the recorder. Once the activity is completed, reverse roles.
4. The test subject should remove his or her eyeglasses, if applicable, and stand with his or her toes at the tape line.
5. The test subject should cover his or her left eye with the left hand. The test subject should look at the hemisphere mounted on the wall and disclose any lines that do not have a uniform black appearance to the recorder.
6. Using a black pen, the recorder should circle the lines indicated by the test subject on the Astigmatism Worksheet. Write the words “right eye” near the circled lines.
7. The test subject should repeat step 5, covering the right eye with the right hand.
8. Using a blue pen, the recorder should circle the lines indicated by the test subject on the Astigmatism Worksheet. Write the word “left eye” near the circled lines.
9. The test subject should hold the Astigmatism Box Chart approximately 18 inches (or a comfortable reading distance) from his or her eyes.
10. The test subject should cover the left eye with his or her left hand. The test subject should look at the four boxes on the Astigmatism Box Chart and disclose any lines that do not have a uniform black appearance to the recorder.
11. Using a black pen, the recorder should circle the boxes indicated by the test subject on the Astigmatism Worksheet. Write the words “right eye” near the circled boxes.
12. Covering the right eye with the right hand, the test subject should disclose any boxes that do not have a uniform black appearance to the recorder.
13. Using a blue pen, the recorder should circle the boxes indicated by the test subject on the Astigmatism Worksheet. Write the word “left eye” near the circled boxes.
14. Switch roles and repeat steps 4–13.

Student Worksheet PDF

10939_Student1.pdf