LO - Lab #06 - The Amazing Human Eye

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Carmella Gallagher
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1 LO - Lab #06 - In this lab you will examine and model one of the most amazing optical systems you will ever encounter: the human eye. You might find it helpful to review the anatomy and function of the human eye before you begin. Equipment: Your eyes, Eye charts, Human eye model, Light box, Lens box containing: a 13- mm aperture, spherical lenses with the powers +2.0, and (all measured in diopters), and cylindrical lenses with the powers of and Activity #1: What Can (and Can't) You See? Complete the following exploration activities with your own eyes. Everyone should do each activity and record your results in your logbook. You can do the activities in any order, however, be sure to clearly state what you did in your descriptions in your logbook. (a) How Do Your Eyes Accommodate? Look up from this paper at an object somewhere across the room and then look back at the paper. Were you able to clearly see both the other side of the room and the paper? This activity illustrates the process of accommodation, or focusing, for your eyes. Accommodation is automatically accomplished in your eye by a set of muscles that changes the curvature of the crystalline lens. (See figure on page 3 for details.) That is, the eye actually changes the shape of the lens. 1. Think of the glass lenses you used last week in lab. Were they able to produce clear images for any and all object distances or were they constrained by certain parameters? What does the ability of your eye to accommodate tell you about the focal length of your eyes' lenses? (b) What is Your Visual Acuity? You can measure your own visual acuity. Stand at the taped marker in the hallway and look at the eye chart hanging on the wall. Only one group should do this activity at a time. If a group is already working with this equipment, just move on to the following explorations and come back to this later. A person whose vision is rated 20/20 is able to see details at a distance of 20 feet as clearly as a "normal" individual would. A rating of 20/15 is better than average for at 20 feet the person can see details that would be clear for "normal" vision at 15 feet. When visual acuity falls below 20/200, the individual is considered to be legally blind. Page 1 of 8

2 2. Which lines can you read clearly? What is your visual acuity at twenty feet? Describe in your own words what this rating means. 3. Do you think this test is as good as an eye test at the doctor's office? If not, what things do you think would have to be improved to make it a better test? Page 2 of 8

3 (c) Where is Your Near Point? Next, measure your near point. That is, measure how close you can hold this page to your eyes and still see it clearly. The typical near point for people is 25 cm (though it can be much closer when you are young). The nearpoint is the point that can be seen clearly with maximum accomodation of your eyes. Record the distance of your nearpoint. 4. (a) Recall that the ciliary muscles determine the curvature of the eye's crystalline lens. How are the muscles changing your lens to allow you to see things this close? That is, do you need the lens to have more or less curvature to see near objects? (b) Why do you think your nearpoint changes with age? Discuss your ideas. (d) Your Blind Spot The optic disc is the region of the back of your eye where the optic nerve originates. There are no light detectors on this disc. Because light striking this area goes unnoticed, it is commonly called the "blind spot." You do not usually notice a blank spot in your visual field because involuntary eye movements keep the visual image moving and allow the brain to fill in in the missing information. Locating your blind spot: Close your left eye and stare at the cross on Eye Chart #2 with your right eye, keeping it in the center of your field of vision. Begin with the page a few inches away and gradually increase the distance as you keep staring at the cross. Note how far the paper needs to be away from your eyes to have the dot "disappear." Watch what happens if you continue to move it further away. Repeat this activity by closing your right eye and staring at the dot with your left eye. 5. How far away does the paper have to be before the dot disappears and then reappears for each of your eyes? (e) Astigmatism Astigmatism is usually caused when the cornea or lens is out-of-round. This common defect causes point-like objects to focus as lines and therefore blurs the image. Test your own eyes for astigmatism using the figure in Eye Chart #2. Look with one eye at the center of the pattern. Sharply focused lines appear dark and those that are not in focus appear dimmer or gray. Page 3 of 8

4 Record your observations for your own eyes. Page 4 of 8

Activity #2: Preparing The Eye Model Now that you have explored some of the remarkable properties of your own eyes, you will model the physical properties of the human eye with the provided human eye

5 Activity #2: Preparing The Eye Model Now that you have explored some of the remarkable properties of your own eyes, you will model the physical properties of the human eye with the provided human eye model. Here is a brief description of the parts of the human eye along with their counterparts in the eye model. Image of a Human Eye Image of the eye model you will use in lab Part of a Human Eye Cornea Iris General Description The first and most powerful lens of the eye s optical system Controls the amount of light intensity that enters the eye s optical system Part of the Eye Model Meniscus lens C (Fixed in the eye model) Aperture insert (Placed at position G1) Pupil The variable opening in the iris Aperture insert (Placed at position G1) Crystalline Lens Ciliary Muscle Vitreous Humor Retina Second lens of the eye s optical system Muscles controlling the curvature of the crystalline lens Clear colorless jelly that fills the eyeball Light sensitive membrane distributed over the back of the eyeball Lens insert (Placed at position L) The eye model is filled with water. Curved screen (Placed at position R) Fovea The most sensitive region of the Dashed markings on Page 5 of 8

6 Optic Nerve retina Conducts visual stimuli to the brain the curved screen. Shown as the spot on the curved screen. Page 6 of 8

7 The power of a lens is often measured in the unit of diopters (for instance, eyeglass prescriptions are given in units of diopters). The power of a lens is computed by taking the reciprocal of its focal length when the focal length is measured in meters. Lens power ( diopters) = 1 f (m) 6. Compute the power of the two converging lens(es) that you used in the last lab. If a lens has a higher power, then does it have a longer or shorter focal length? Remove any lenses that may have been left in the model from the last class (positions L, G and S). Verify that the curved screen, which simulates the eye s retina, is placed in the normal position (R). That is, place the screen in the middle of the three possible positions. Take your eye model into the hallway and carefully fill it with water at the sink before doing any of the following activities. Fill it so the model s cornea is completely covered, but don t fill it so full that water spills over the top.! Please be careful not to spill water in the hallway. If any water is spilled, please notify your lab instructor right away so it can be cleaned up before anyone slips. 7. Why do you think you use water in your eye model? How does this water relate to the human eye? What physical properties might it simulate?! Turn off the overhead lights before you continue so you can best judge when images are in focus. Page 7 of 8

8 Activity #3: Modeling Accommodation with the Eye Model Looking at far away objects Set up the model so that it is "looking" toward an unshaded window or other bright object 4 or 5 meters away. Use an object with features that you can recognize in the image (like your lab partner standing in front of the window). Don't use a bare light bulb, which only looks like a bright spot and does not have any distinguishing features. Describe the object and describe precisely how the image looks on the retina. Comment on important features like the size of the image, if it is right side up or upside down, etc. Find a spherical lens to insert into the groove L that gives a clear, sharp image of the far away object on the retina. Record both the power of this lens in diopters and its focal length in meters. Describe how the new image looks on the retina. Note the characteristics of the image including: whether it is erect or inverted, the image size compared to object size,... Looking at near objects Without changing anything in the eye model, turn the model so it is looking at a near object (namely, the light box). Position the light box with the radially-slotted pattern 35 cm in front of the model s cornea. Sketch the image of the light box on the retina and describe how it looks. 8. How does the quality of the image compare to the image that was formed when the model was looking far away? Accommodating for near objects Replace the crystalline lens with one that makes the image of this near object clear. Record your observations and lens choice. Carefully describe this image along with any notable characteristics. 9. How does the crystalline lens needed for the model to have clear far vision compare to the lens needed to clearly view near objects? Is this in agreement with your answer to question #4a? 10. How does the image that is formed on your retina differ from what your brain tells you that you are seeing? Explain. 11. How does the process of accommodation for your real eyes compare to and differ from the process of accommodation of the eye model? Page 8 of 8

9 ! During the remainder of this lab, you will be modeling the eye's function when it is looking at near objects. Therefore, you must leave the crystalline lens for near vision in place (position L) for the remainder of the lab. Feel free to verify that you have the correct lens by comparing with another group or asking your lab instructor. Page 9 of 8

10 Activity #4: Farsighted and Nearsighted Vision Two of the most-common defects that occur with human vision are farsightedness and nearsightedness. These two conditions are briefly defined here. Farsightedness Someone with farsighted vision is only able to clearly see objects far away. Farsightedness (hypermetropia) occurs if a person s eyeball is "short." This results in parallel light being focused behind the retina. Nearsightedness Someone with nearsighted vision is only able to clearly see near objects. Nearsightedness (myopia) occurs if a person s eyeball is "long." This results in parallel light being focused in front of the retina. Becoming an optometrist for an eye model Set aside your model of a "normal" eye from Activity #3. Your lab instructor has two patients, Martha and George, who are in need of eyeglasses. Request a patient from the instructor so you can complete the following activity. If you find that a patient is already busy with another doctor, then you can continue on to Activity #5 until the patient is available. Be sure to return the patient to your instructor once you have finished. 12. Complete a study of your patient as he/she is looking at the light box from a distance of 35 cm. As part of this study, be sure to answer all of the following questions. Explain the evidence that led you to your conclusions. Record your patient's name. Is this patient nearsighted or farsighted? What impact does their visual defect have on their ability to form a clear image? Give a careful description and/or sketch. What shape of lens is needed to correct this defect? Using the lenses provided in your box, find an appropriate lens to correct this patient's vision. Keep in mind that you are only licensed to determine a prescription for this patient (position S1), you are not licensed to do surgery! (That is, do not remove the lens L!) Make a note of your prescription and the resulting image formation. Page 10 of 8

11 13. Repeat this activity for the second patient, answering the same questions listed in #12. Page 11 of 8

12 Activity #5: Modeling Astigmatism with the Eye Model In the human eye, astigmatism is generally caused by a slight cylindrical curvature of the cornea. Thus, a change in the model s cornea would perhaps be the logical way of producing this effect. However, this is impractical since the cornea of the model is a fixed lens. However, the same effect can be accomplished by inserting an additional lens that is "out of round." Return to using your "normal" eye model from Activity #3. Put the object box at 35 cm. Insert a cylindrical concave lens (-5.5 diopters) immediately behind the cornea, producing astigmatism. Remember the crystalline lens you found for near vision in the first activity should still be in place. Turn the cylindrical lens a little to make only one line of the image sharp. Make a sketch of the blurred image and record the lenses that you are using. Becoming an optometrist for an eye model Your eye model no longer represents normal vision, but vision with astigmatism. Assume you are an optometrist and need to prescribe a corrective lens (i.e., glasses) to correct this patient s vision. Place in front of the cornea the correcting convex cylindrical lens (1.75 diopters) and turn it until the image is again sharp. Change the angle of the rear lens and repeat. 14. Explain how you think one lens is able to correct this vision defect. Note, the axis of a cylindrical lens is defined as the line along the thinnest part of the lens. Activity #6: Modeling Compound Defects with the Eye Model Astigmatism is often accompanied by farsightedness or nearsightedness You will now model these compound defects as well as attempt to correct for them. Be sure to finish Activity #4 before starting Activity #6! In order to study this phenomenon, place a concave cylindrical lens (-5.50 diopters) at G1 immediately behind the cornea with its cylindrical axis vertical. In addition, place the retina in the position to give myopia. Make a note of how you model this eye defect. Becoming an optometrist for an eye model Assume you are an optometrist and need to prescribe corrective lenses (i.e., glasses) to correct this eye's vision. Page 12 of 8

13 Correct the eye s vision by choosing the proper combination of eyeglass lenses (S1 and S2). Record the kinds of lenses used and the results of the lens combination. In actual practice the two correcting lenses are combined into a single eyeglass lens. Page 13 of 8

14 Activity #7: Modeling the Treatment of a Cataract with the Eye Model In the eye disease known as a cataract, the crystalline lens becomes opaque. When this condition exists, the crystalline lens is often removed. Return the eye model so that it represents a "normal" eye. Remove the lens L from your model (modeling cataract surgery) 15. (a) Is vision still possible for someone who has a lens removed? Explain. (b) What do you need for the eye to see clearly? With this eyeglass lens, at what distance(s) is the image still distinct? Would another lens allow vision at another distance? Test your hypothesis and describe the results. Activity #8: Applying What You Learned to Your Own Eyes 16. (a) Using what you know about image formation with lenses and with this eye model, do you think the optic nerve in your eyes is located at the center of retina, between the center and your nose, or between the center and your ear? Explain the reasoning for your choice, using sketches when helpful. Hint! Think about the results of your blind spot test! (b) Based on your reasoning, is your eye model a human right eye or left eye? Explain and draw a sketch of your evidence. Activity #9: Wrap Up 17. Describe in your own words at least two ways that the eye model is a good model for the behavior of the human eye and at least two ways that the eye model is not a good model for the behavior of the human eye. Please do the following before you leave At the conclusion of this experiment, be sure there are no lenses left in the model. Empty and rinse the eye model at the sink in the hallway and dry it with paper towel. Be sure to also dry each lens and put them back into their small metal box. You should only have one of each type of lens and aperture at your lab station. Be sure all "patients" have been returned to your instructor. Thank you! Page 14 of 8

15 * This write-up was largely taken from I. Bassett, et al. General Physics Laboratory Experiments, Brigham Young University, Provo, Utah. * Additional information and graphics were taken from H. Q. Fuller, et al. Physics Including Human Applications, Harper & Row, New York, Page 15 of 8

Aspects of Vision. Senses

Aspects of Vision. Senses Lab is modified from Meehan (1998) and a Science Kit lab 66688 50. Vision is the act of seeing; vision involves the transmission of the physical properties of an object from an object, through the eye,