by Carl J. Wenning, ISU Physics Department
(adapted from CENCO's Experiment L-51 Procedure, 1928)
1. ACCOMMODATION
Observation:
Fill the eye model tank to within 2 centimeters of the top rim with water. Set up the model so that it points toward a window at a distant scene. With the retina in the middle (normal) eye position, insert the +7 diopter lens in the groove behind the cornea. Note the focused image on the retina. Note the orientation of the image. Place the lamp box 33 cm from the cornea and turn it on. Note that if you replace the +7 diopter with the +20 diopter lens, the image is in focus. This illustrates the process of accommodation or focusing which is accomplished automatically in the eye by the set of muscles which change the curvature of the crystalline lens located just behind the cornea. If you find yourself wearing bifocal eyeglasses, this is so because your eyes' lenses are no longer supple enough to permit accommodation.
Generalization:
Is the image formed on the retina erect or inverted?
What is the size of the image relative to the object?
Application:
How will the lens in the eye change when focusing first on a distant object and then on a nearby object? (more or less convex)
If the lens becomes more convergent when accommodating, does that mean that the image is initially formed in front of or behind the retina?
Does the lens become more or less convex when making this accommodation?
2. BLIND SPOT
Observation:
The eye's blind spot is marked as a dark spot on the model's retina. In the eye, this small region is not light sensitive (the optic nerve exits the eye at this location). To demonstrate this phenomenon, examine the "O" and an "X" below. Orient the page so that the line connecting the two letters is horizontal and the "X" lies to the left. Close your right eye and hold the page out at arm's length. Using your left eye, stare directly at the "O", keeping the "X" to the left of your face. Slowly bring the images of the letters closer until you notice something happen to the "X" without directly looking at it.
X O
Generalization:
What appears to happen to the "X" while observing the "O" as instructed above?
What happens if you use your right eye to look at the "X"?
What happens if you use your right eye to observe the "O," or your left eye to observe the "X"?
Application:
Is the blind spot in your eye closest to the nose or farthest from it? How do you know? Draw a diagram to explain your reasoning.
Can you think of any situation in which the blind spot might constitute a problem for a viewer? If so, please describe the situation.
3. FAR- AND NEAR-SIGHTEDNESS
Observation:
Without moving the lamp box from its 33 cm position, make the model far-sighted by moving the retina to the position nearest the lens. Keep the +20 diopter lens in place behind the cornea. Determine whether the +2.00 diopter (convex) or -1.75 diopter (concave) spherical lens must be used in order to correct the defect by placing each lens in front of the eye and noting the sharpness of the image. Repeat this step with the retina in the rear position which corresponds to the near-sighted eye.
Generalization:
When images hopelessly form in front of the retina, are you near-sighted or far-sighted?
Application:
If you are near-sighted, what sort of correction lens do you need? Positive or negative?
4. EFFECT OF PUPIL SIZE
Observation:
Select any combination of lenses in which the image produced is out of focus. Insert the circular diaphragm either in front of or behind the cornea and note how the image is sharpened. This illustrates why some persons can see clearly outdoors on a sunny day but have poorer clarity of vision by night. Under brightly light conditions the iris closes to reduce the amount of light entering the eye; under dim conditions the iris opens to admit more light.
Generalization:
When the iris of the eye dilates (widens) is one's vision likely to be better or worse?
Why is this so?
Application:
Persons with minor eye defects may see objects more clearly when looking through a small hole produced by coiling the hand. Under what conditions might one who uses eyeglasses put this information to good use?
5. ASTIGMATISM
Observation:
With the lamp box at 33 cm, the +20 diopter lens in place behind the pupil, and the retina in the middle position, insert the -5.50 diopter cylindrical concave lens immediately behind the cornea thereby producing astigmatism. (In the human eye astigmatism is generally the result of slight cylindrical curvature of the cornea, so in the model a change of cornea would perhaps be the logical way of producing astigmatism. This is impractical and the same effect is accomplished by introducing the additional lens.) By turning the cylindrical lens slightly, one line alone of the object pattern can be made sharp, while the others are blurred. Now place in front of the cornea the correcting convex cylindrical lens marked +1.75 diopter and turn it until the image is again sharp. Notice the relative directions of the cylindrical axes (angles between the tabs on the lenses).
Generalization:
What affect does astigmatism have on the sharpness on the veins of a "radial" image?
Is the darker axis more or less well focused than the lighter axis?
Application:
How might one determine if a set of eyeglasses is designed to correct for astigmatism?
How might one determine the axis of that astigmatic correction?
6. COMPOUND DEFECTS
Observation:
Astigmatism is often accompanied by far- or near-sightedness. Combine test 3 and 5, using both a cylindrical and spherical focusing lens at the same time in front of the cornea. In actual practice the two correcting lenses are combined into a single compound eyeglass lens.
Generalization:
Application:
7. CATARACTS
Observation:
In some diseases of the eye (i.e. cataracts) the crystalline lens must be removed due to an increasing opacity. Good vision is still possible, however, with the use of a suitable correcting lens. Set up the model using the +20 diopter lens and the light source 33 cm from the front of the lens. You should have a clearly focused image. Remove the +20 diopter crystalline lens and replace it with the +7 diopter corrector lens in front of the cornea. A clear image will be formed if the lamp box is brought very near.
Generalization:
Does a +20 diopter lens in water act the same way a +7 diopter lens does in air? If not, describe the difference.
Does a +20 or +7 diopter lens work the same way in water as in air? If not, describe the difference.
Application:
If a lens is replaced with the use of glasses following a cataract operation (e.g. the natural lens was not replaced with an artificial version), does the number of diopters of the lens in air have to be greater or lower than that of the crystalline lens replaced? How do you know?
Would such a lens be positive (convex) or negative (concave)?
8. ACTION OF A SIMPLE MAGNIFIER
Observation:
With the retina in the middle position, use the +20 diopter lens above the lower partition with the lamp box at 33 cm. Now use the +7 diopter lens as a simple magnifier in front of the cornea. For a clear image the lamp box will now have to be moved nearer the eye model until the distance is only about 1/3 its former value; the image will be correspondingly 3 times larger.
Generalization:
Examine the +7 and +20 diopter lenses carefully. Are they convex or concave?
Which of the above two lenses can said to be the "stronger"? That is, which refracts light more?
Application:
What happens if a person who is strongly near-sighted removes his or her eyeglasses and brings an object very close to his or her face in order to see it more clearly?
9. EYEGLASS LENSES
Observation:
With one eye and wearing corrective glasses if this is the norm, look through the correcting spherical lenses +2.00 and -1.75 diopters and move them from side to side. Note in which case objects appear to move with the lens and which against.
Generalization:
How is the effect altered when the cylindrical lens is used?
Using your own or borrowed glasses, test as above and notice the character of the eye defect that they are intended to correct (far-/near-sightedness, relative strength of lenses, astigmatism -- rotate lens in its own plane and note any distortions.)
Application:
The image of an object at a great distance forms behind the retina. Is the person near-sighted or far-sighted?
What sort of lens (+ or -) needs to be used to correct this eye defect?
A person is said to wear "coke bottle" glasses. Are these lenses positive or negative?
What sort of defect are theses eyeglasses intended to correct? (near-sightedness or far-sightedness)
10. FOCAL LENGTHS
Observation:
Determine the focal lengths of the +7 and +20 diopter lenses by projecting a distant object onto a sheet of paper. The distance between the image and lens is roughly equal to the focal length of the lens. Measure this distance and convert it to meters. Note how the diopter designation appear to be related inversely to the focal length.
Generalization:
Which lens, the +7 diopter or the +20 diopter, has the shorter focal length?
Which of the two lenses would be able to provide a greater correction for failing eyesight? Why?
Application:
Opticians prefer to work with diopters rather than focal lengths as diopters can be conveniently added to find a resultant. Take the focal length of the +7 diopter lens to be 0.143 meters, and that of the +20 diopter lens to be 0.050 meters. Now, from the
thin lens formula:
Show, using the thin lens formula, that a +20 and +7 diopter combination of lenses has the effect of a single +27 lens.
When you have finished with this exercise, please empty out the eye model, unplug the lamp box, and lay out the lenses on a paper towel to dry.