Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Cut the acetate sheet into 2½" x 3" pieces using scissors or a paper cutter.
2. Approximately 1" from a 2½"-long side, draw an “E” using a permanent black marker (see Figure 7).
   {12664_Preparation_Figure_7}

Safety Precautions

If a candle is used as the light source, caution students about the dangers of an open flame and of the high flammability of the “retina” paper. Follow normal laboratory safety guidelines.

Disposal

All the materials may be saved for future use.

Teacher Tips

• Enough materials are provided in this kit for four groups of students working in pairs or in groups of four. This laboratory activity can reasonably be completed in one 50-minute class period. One device can also be used as an effective demonstration if time is limited.
• Books, cloth towels or other objects, such as rulers and folders, can be used to prevent the eye model and flashlight from rolling on the tabletop.
• It is important to keep the distance between the transparency “E” and the eye lens constant throughout the experiment so the only variable is the size of the eye.
• The images on the model eye retina will be easier to see in a dark room. If candles are used as a light source a dark room will be critical for good observations. The brighter the light source, the easier it will be for students to observe the images formed on the “retina.”
• Before performing this experiment, students should have a basic understanding of convex and concave lenses and how they focus light rays. Students should refer to their physical science or physics textbook for information on the thin-lens equation to determine the focal length of the lens for the optional questions. Drawing accurate ray diagrams to show how the light rays are focused when using corrective lenses is not necessary for introductory experiments, but can be done as an extension for more advanced classes.
• Extra filter paper sheets are provided as replacements.

Sample Data

Normal Eye

Length: ___20.4 cm___
 
Distance between the “E” and the “eye’s” lens when image is in focus: ___20.2 cm___

Observations of the image:

The image of the “E” on the “retina” is larger than the real “E.” The image is upside-down compared to the real “E.”

Squeezed Eye

Length: ___13.8 cm___
 
Initial observations of the image on the “retina”:

The image is very blurry. The image of the “E” cannot be seen. A dark region is visible in the center of the bright spot.

Lens used to bring the “E” into focus: convex
 
Distance between the external lens and the “eye’s” lens when the image is in focus: ___10.3 cm___

Final observations of the focused image using a corrective lens:

The image of the “E” is in focus again. The image is smaller than the original image produced by the “normal eye.” The image is upside-down and does appear a bit clearer than before.

Elongated Eye

Length: ___23.0 cm___
 
Initial observations of the image on the “retina”:

The image is very blurry just like it was for the “squeezed eye” case. The image of the “E” cannot be seen. A gray region is visible in the center of the bright spot.

Lens used to bring the “E” into focus: ___concave___
 
Distance between the external lens and the “eye’s” lens with the image is in focus: ___10.2 cm___
 
Final observations of the focused image using a corrective lens:

The image of the “E” is in focus again. The image is much smaller than the original image produced by the “normal eye.” The image is upside-down and is not as bright as the original image.

Answers to Questions

Normal Eye

1. Compare the length of the normal eye to the position of the “E” in relation to the “eye’s” lens.

   The length of the eye is approximately the same length as the distance between the “E” and the lens.

2. (Optional) Refer to your physical science or physics textbook. Calculate the focal length of the “eye’s” lens.
   {12664_Answers_Equation_1}
   where f is the focal length of the lens, i is the image distance from the lens and o is the object distance from the lens.
   {12664_Answers_Equation_2}

Squeezed Eye

3. Which eye disorder is being demonstrated when the eye is shortened?

   The eye disorder that is demonstrated with the “squeezed eye” model is hyperopia, or farsightedness. The eye’s lens focuses the incoming light rays beyond the “retina” instead of directly on the retina. This causes the image to be blurry.

4. Draw a sketch of the eye problem that occurs when the eye is “too short.” Do the light rays focus before the retina or behind the retina? Which lens type, concave or convex, helped correct this vision problem? Include the correct lens type in your drawing and explain how the lens corrected the vision.
   {12664_Answers_Figure_8}
   When the eye is too short, the lens focuses the image of the “E” behind the lens paper “retina” and therefore the image is out of focus on the paper. A convex, or converging, lens helps bring the image of the “E” back into focus by converging the light rays before they reach the eye’s lens. The eye’s lens will focus the light rays even further so that they converge directly on the retina
5. (Optional) Refer to your physical science or physics textbook. Calculate the final image location of the lens combination. The focal length of the convex corrective lens is 15 cm. The focal length of the concave corrective lens is –15 cm.
   {12664_Answers_Equation_3}
   The negative sign indicates that the image is virtual and is located on the same side of the lens as the incoming light, 29.1‑cm away from the corrective lens. This virtual image is therefore 29.1 cm plus 10.3 cm, or 39.4 cm, away from the “eye’s” lens. This virtual image acts as the object for the “eye’s” lens. The image location produced by the “eye’s” lens is therefore:
   {12664_Answers_Equation_4}

Elongated Eye

6. What eye disorder is being demonstrated when the eye is elongated? The eye disorder that is demonstrated with the stretched eye model is myopia, or nearsightedness.

   The eye’s lens focuses the incoming light rays in front of the “retina” instead of directly on the retina. This causes the image to be blurry.

7. Draw a sketch of the eye problem that occurs when the eye is “too long.” Do the light rays focus before the retina or behind the retina? Which lens type, concave or convex, helped correct this vision problem? Include the correct lens type in your drawing and explain how the lens corrected the vision.
   {12664_Answers_Figure_9}
   When the eye is too long, the lens focuses the image of the “E” in front of the “retina,” and therefore the image is out of focus on the paper. A concave, or diverging, lens helps bring the image of the “E” back into focus by diverging the light rays before they reach the eye’s lens. The eye’s lens then converges the spread-out light rays onto the retina.
8. (Optional) Refer to your physical science or physics textbook. Calculate the focal point of the lens combination.
   {12664_Answers_Equation_5}
   The negative sign indicates that the image is virtual and is located on the same side of the lens as the incoming light, 6 cm away from the corrective lens. This virtual image is therefore 6.0 cm plus 10.3 cm, or 16.3 cm, away from the “eye’s” lens. This virtual image acts as the object for the “eye’s” lens. The image location produced by the “eye’s” lens is therefore:
   {12664_Answers_Equation_6}

Introduction

The human eye is intricate and complex. To truly understand how the eye works, various elements of physics, anatomy, physiology, biochemistry and nutrition must be studied. This activity will illuminate the physics (optics) principles involved in the functioning eye.

Concepts

• Focal length
• Hyperopia
• Converging lens
• Image inversion
• Eye anatomy
• Diverging lens
• Myopia
• Refraction

Background

A sketch of the anatomical parts of the human eye is shown in Figure 1.

A discussion of the role of each part of the eye is not included in this discussion. These details are covered in most high school biology textbooks and on many websites.

A realistic understanding of how the eye works started around the 17th century. It was realized in the 17th century that the retina at the back of the eye, and not the cornea as previously thought, was responsible for the detection of light and therefore sight. Johannes Kepler (1571–1630) of Germany and René Descartes (1596–1650) of France, both prominent scientists, made many advances in understanding how the eye functions. Kepler, most famous for his laws of planetary motion, was the first to propose that the lens of the eye focuses images onto the retina at the back of the eye. A few decades later, Descartes demonstrated that Kepler was correct. Following up on experiments performed by another scientist, Descartes surgically removed an eye from an ox and scraped the back of the eye to make it transparent. He then placed the eye on a window ledge as if the ox were looking out the window. When he looked at the back of the eye, Descartes saw an inverted image of the scenery outside. Descartes correctly hypothesized that the image was inverted as a result of being focused onto the retina by the eye’s lens. Descartes was actually one of the first to realize a corneal contact lens could be used to correct vision. In this activity, a model eyeball will be constructed and a more humane and less gory version of Descartes’ ox eye experiment will be repeated.

In addition, several common eye correction procedures will be simulated with the working eye model. The model will demonstrate two common refraction disorders—nearsightedness (myopia) and farsightedness (hyperopia). An individual who is nearsighted can easily see objects that are up close, but objects that are far away are out of focus and blurry. Farsightedness is a condition in which an individual can clearly see objects that are far away, but has difficulty seeing objects that are close. Both of these conditions are the result of the eye’s inability to focus the image directly on the retina. For nearsightedness, the lens is not expanded enough to increase the focal length when viewing objects at a distance, and for farsightedness, the lens is not squeezed enough to shorten the focal length in order to view a nearby object. The failure of the lens of the eye to change shape often occurs as the body ages because the ciliary muscles regulating the shape of the lens deteriorate and become weaker. Another reason for the lens not being able to focus light directly on the retina is because the eye is too long or too short relative to the range of focal lengths the lens can attain. The net result of a misshaped eye is that the image is focused short of the retina (2a) or beyond the retina (2b) and produces a blurred image. See Figure 2 showing the two conditions. Corrective lenses, such as eyeglasses or contact lenses, placed in front of the cornea assist the eye in converging or diverging light rays, which allows the eye’s lens to focus the image directly on the retina to produce clear vision.

Materials

Safety Precautions

Although this activity is considered nonhazardous, please follow all normal laboratory safety guidelines.

Procedure

Part A. Normal Eye

1. Remove the screw cap from the accordion bottle. Place the 38-mm “eye” lens over the bottle opening. Screw the cap back on the bottle and over the lens to secure the lens in place (see Figure 3).
   {12664_Procedure_Figure_3}
2. Use transparent tape to secure the filter paper flat and taut over the hole in the bottom of the bottle. (This paper will act as the eye model’s “retina.”)
3. Place the transparency sheet with the letter “E” into the lens holder so that the sheet is perpendicular to the tabletop and the “E” is upright (see Figure 4).
   
   {12664_Procedure_Figure_4}
4. Squeeze the accordion bottle to its shortest length and hold it in this position for one minute. Hold the bottom of the bottle at the edges to avoid ripping or puncturing the paper “retina.” Release the bottle and let it expand to an equilibrium length. (The equilibrium length will be somewhere between the shortest length and the longest length of the accordion bottle.)
5. Once the bottle has reached an equilibrium length, measure the distance between the front of the accordion bottle (the lens) and the back of the bottle (the “retina”). Record the length of the “normal eye” to the nearest 0.1 cm in the Eye Model Worksheet.
6. Place the optics items in a line as illustrated in Figure 4.
7. Shine the flashlight through the transparency with the letter “E” toward the front of the accordion bottle (the eye model’s lens).
8. Adjust the height of the “E” and the height and orientation of the flashlight so that a bright spot illuminates the center of the paper on the back of the accordion bottle. A cloth towel placed under the flashlight works well to prevent it from rolling. The towel can also be folded to support the handle of the flashlight so that the light shines parallel to the tabletop and is at the proper height to produce a bright spot at the center of the “retina.”
9. Once the flashlight, “E”, eye model lens and “retina” are properly aligned and a bright spot illuminates the center of the “retina,” move the eye model toward or away from the flashlight until the image of the “E” forms clearly on the “retina.” When the bottle is the appropriate distance away from the transparency sheet, use books or a towel to prevent the bottle from rolling out of position.
10. How does the size of the image “E” compare to the real “E”? Is the image upright, upside-down or tilted to the left or right? Record all observations of the image produced by the eye model in the Eye Model Worksheet.
11. Place a piece of tape, or use a marking pen to draw a line on the tabletop at the front end of the bottle directly below the “eye’s” lens. The front of the accordion bottle should line up with this marked location for each part of the experiment.
12. Measure the distance between the transparency “E” and the front of the eye model. Record this distance to the nearest 0.1 cm in the Eye Model Worksheet.

Part B. Squeezed Eye

13. Pick up the bottle without disturbing the locations of either the flashlight or transparency “E.” The flashlight may be turned off if this can be done without moving it significantly.
14. Squeeze the accordion bottle to its shortest length and hold it in this position. Hold the bottom of the bottle at the edges to avoid ripping or puncturing the paper “retina.”
15. Lab partner not holding the bottle: Measure the distance between the front of the accordion bottle (the lens) and the back of the bottle (the “retina”). Record the length of the “squeezed eye” to the nearest 0.1 cm in the Eye Model Worksheet. (Turn the flashlight on if it is turned off.)
16. Lab partner holding the bottle: Continue to squeeze and hold the bottle in its shortest length as shown in Figure 5.
    {12664_Procedure_Figure_5}
    Place the bottle back on the tabletop so that the front of the bottle lines up with the mark on the table and it is in line with the flashlight and the transparency “E” (see Figure 6). Adjust the orientation of the bottle until a bright spot illuminates the center of the “retina,” but do not release the bottle.
    {12664_Procedure_Figure_6}
17. Observe the image produced by the “squeezed eye.” Is the image of the “E” visible? How large or small is it in comparison to the real “E” on the transparency? Record all observations in the Eye Model Worksheet.
18. As one lab partner continues to squeeze and hold the eye model, a second lab partner will experiment with the two larger diameter “corrective” lenses.
19. Place the convex or concave lens between the eye’s lens and the transparency “E” (like an eyeglass or monocle). Move the lens toward or away from the eye model lens until the image of the “E” is clear and in focus. If a clear image is not produced, use the other lens. Which lens brings the image of the “E” into focus? Record this in the Eye Model Worksheet.
20. Measure the distance between the external corrective lens and the eye model’s lens when the “E” is in focus. Record the distance to the nearest 0.1 cm in the Eye Model Worksheet.
21. How does the image of the “E” with the corrective lens compare to the original “normal eye” image? Is the image larger, smaller or the same size? Is the image clearer, or brighter? Is the image upright, upside-down or tilted? Record your observations in the Eye Model Worksheet.

Part C. Elongated Eye

22. Pick up the bottle again without disturbing the location of either the flashlight or the transparency “E.” The flashlight may be turned off if this can be done without moving it significantly.
23. Stretch the bottle as much as you can and hold it in the fully stretched length for about one minute. Care should be taken to not rip or puncture the “retina.”
24. Release the bottle and allow it to relax back to a new equilibrium length.
25. Measure the distance between the front of the accordion bottle (the lens) and the back of the bottle (the “retina”). The equilibrium size should be longer than the first experiment. If it is not, stretch the bottle for another minute and then allow it to relax to a new equilibrium position. Repeat this step until the bottle length is longer than the “normal eye” model. Record the length of the “elongated eye” to the nearest 0.1 cm in the Eye Model Worksheet.
26. Place the bottle back on the tabletop so that the front of the bottle lines up with the mark on the table and is in line with the flashlight and the transparency “E.” Adjust the orientation of the bottle until a bright spot illuminates the center of the “retina.”
27. Observe the image produced by the “elongated eye.” Is the image of the “E” visible? How large or small is it in comparison to the real “E”? Record all observations in the Eye Model Worksheet.
28. Place the convex or concave lens between the eye’s lens and the transparency “E” (like an eyeglass, or monocle). Move the lens toward or away from the eye model lens until the image of the “E” is clear and in focus. If a clear image is not produced, use the other lens. Which lens brings the image of the “E” into focus? Record this in the Eye Model Worksheet.
29. Measure the distance between the external corrective lens and the eye model’s lens when the “E” is in focus. Record the distance to the nearest 0.1 cm in the Eye Model Worksheet.
30. How does the image of the “E” with the corrective lens compare to the original “normal eye” image? Is the image larger, smaller or the same size? Is the image clearer, or brighter? Is the image upright, upside-down or tilted? Record all observations in the Eye Model Worksheet.
31. Answer the questions on the Eye Model Worksheet.
32. Consult your instructor for appropriate storage procedures.

Student Worksheet PDF

12664_Student1.pdf