The Human Eye & Optical Instruments

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Experiment 1: Image formation

1. Put the retina screen in the NORMAL slot and the +120 mm lens in the SEPTUM slot.

2. Fill the model with water. This simulates the effect of the aqueous and vitreous humors that actually fill the eyeball.

3. Aim the eye at a bright, distant object such as a window or lamp across the room. An image is formed on the retina screen. Observe and record the orientation of the image relative to the object (up/down, left-right). Why are you not aware of any reversals of the retinal image compared to the real-life object?
Experiment 2: Focusing at different distances: Accommodation

In the process of accommodation, muscles in the eye change the shape of the crystalline lens to change its focal length. Accommodation in the eye model is simulated by changing the lens or lenses that represent the crystalline lens.

1. Place the eye model about 35 cm from the light source and object. Place the +62 mm lens in the SEPTUM slot.

2. Is the image in focus now? Move the eye model as close as possible to the light source while keeping the image in focus. Describe the image on the retina screen as you move the eye model.

3. Measure the object distance, o, from the screen of the light source to the top rim of the eye model, as pictured below. (The front of the rim is a convenient place to measure to and marks the center of the eye model’s two-lens system.) Record this distance, which is the near point of the eye model when equipped with the +62 mm lens. Measure and record the near point for your eyes and those of your lab partner by finding the shortest distances at which you can focus. (Keep your glasses on if you wear them. Note that although we’ve given a typical value, your value may be much smaller or larger thant his value!)

4. Increase the ability of the eye model to focus on a close object by adding the +400 mm lens to slot B. This combination models a different focal length for the crystalline lens, due either to accommodation (or wearing reading glasses!) How close can the eye focus now?

5. Remove both lenses and place the +62 mm lens in the SEPTUM slot. Adjust the eye-source distance to the “near point” distance for this lens (which you found in step 3) so that the image is in focus. While looking at the image, place the round pupil in slot A. What changes occur in the brightness and clarity of the image? Move the light source closer to and farther from the eye model. Is the image still in focus? Remove the pupil and observe the change in clarity of the image. Both with and without the pupil, how much can you change the eye-source distance and still have a sharp image?
Note that this effect is not a change in resolution similar to what we discuss later. It’s simply a way to increase the depth of focus (the range of distances over which images stay approximately in focus). For a pinhole camera, images are in sharp focus for any distance, but only a tiny amount of light is transmitted. We have a pinhole camera available in the lab; you should try this out and understand how it forms images. Ask your instructor for help if necessary. For a lens of nonzero diameter, the focal plane has a limited depth. Increasing the aperture allows more light in, but decreases the depth of focus. Similar tradeoffs are important in photography.
Try on the pinhole glasses (taking off any glasses you ordinarily use) and find your near-point with the pinhole glasses on—it should surprise you! Record its value and explain qualitatively why it has this value. What could you do to make it even smaller?

6. Position the eye model (with pupil removed) so that it is looking towards a distant object. Is the image on the retina in focus? Replace the lens in the SEPTUM slot with one that makes a clear image of the distant object; this is the far vision lens. Record the focal length marked on the handle of the lens.

7. In a real human eye, accommodation is accomplished by muscles that change the curvature of the crystalline lens. When an eye changes accommodation from a distant object to a near object, does the curvature of the crystalline lens increase or decrease? Relate to your results above. Why does this explain why the eye’s range of accommodation might decrease with age?
Experiment 3: Refractive errors: near-sightedness and far-sightedness

A person affected by myopia (near-sightedness) has a longer-than-normal eye ball, making the retina too far away from the lenses. This causes the image of a distant object to be formed in front of the retina. By contrast, a person affected by far-sightedness (hyperopia) has a shorter-than-normal eye ball, making the retina too close to the lens system. This causes images of near objects to be formed behind the retina. (Aging can cause a decline in accommodation that results in similar optical effects, a condition called presbyopia.) These refractive errors of course can be compensated for with eyeglasses or laser surgery to change the curvature of the cornea. We will explore only the first condition here.

1. Set the eye model to normal, far vision (put the far lens you found in Experiment 1, step 7 in the SEPTUM slot, remove other lenses, and put the retina screen in the NORMAL position). Turn the eye model to look at the distant object. Make sure the image is in focus. Now put the retina screen in the NEAR position. Describe the image. This is what a near-sighted person sees when trying to look at a far-away object. The lens in the SEPTUM slot represents the crystalline lens in its most relaxed, flattest state, with its longest-possible focal length. Can an eye compensate for myopia by accommodation? Explain.

2. Decrease the pupil size by placing the round pupil in slot A. What happens to the clarity of the image? Remove the pupil. This underlies the operation of the pinhole glasses concept. If you or your partner is near-sighted, try on the pinhole glasses in place of your own glasses and look at distant objects.

3. You will now correct the myopia by putting an “eyeglass” lens on the model. Find a lens that brings the image into focus when you place it in front of the eye in slot 1. Record the focal length of this lens. Does rotating the eyeglasses lens in the slot affect the image? Why or why not?

4. Set the eye model to near vision (leave the corrective “eyeglass” lens in place, remove the far vision lens, put the +62 mm near vision lens in the SEPTUM slot, and leave the retina screen in the NEAR position). With the eye model looking at the nearby light source, adjust the eye-source distance so that the image is in focus. How does this compare to the near-point distance you found in Experiment 1? Why?

5. Describe an experiment similar to those above that would probe far-sightedness. Draw a diagram indicating what your experiment looks like. Perform your experiment and record which “eyeglass” lens corrects the “far-sighted” eye model.
Experiment 4: Astigmatism

In a normal eye, the lens surfaces are spherical and rotationally symmetrical; but an eye with astigmatism has lens surfaces that are not rotationally symmetrical. This makes the eye able to focus sharply only on lines of certain orientations, and all other lines look blurred. Astigmatism can be corrected with a cylindrical eyeglass lens that is oriented to cancel out the defect in the eye. Each cylindrical lens included with the eye model has its cylindrical axis marked by two notches in the edge.

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