Name: Thao Vo
Title: The Optics of the Eye
Section 202 Monday 4:30pm – 7:15pm

Course Name: Physics 246
Lab Partner’s Name: Jennifer Darlington, Hyon Hodge
Date: 11/12/2012
Instructor: Dr Vermuru
Word Count: 1650

Aim:
The aim of this experiment is to examine a scaled up model of the human eye to understand the image formed in the eye, the optics of accommodation, nearsightedness, and farsightedness.
Introduction:
The basic structure of the human eye consists of cornea, natural crystalline lens and retina, as shown in Figure 1, in which the lens of the eye does little of the bending of the light rays, most of the refraction is done at the front surface of the cornea and retina plays the role of the film in a camera.

Figure 1: Diagram of a human eye
Accommodation is the focusing adjustment in which the muscles in the eye change the shape of the crystalline lens to change its focal length.
Figure 2: Accommodation by a normal eye

a) Lens relaxed, focused at infinity

b) Lens thickened, focused on a nearby object

Nearsightedness refers to an eye that can focus only on nearby objects. Because the far point is not infinity but some shorter distance, distant objects are not seen clearly. This defect is usually caused by an eyeball that is too long and images of distance objects are focused in front of the retina. In order to correct this defect, a diverging lens should be used as it causes parallel rays to diverge which allowing the rays to be focused at the retina.
Figure 3:

a) Nearsighted eye

b) Nearsighted eye is corrected by use of diverging lens
Farsightedness refers to an eye that cannot focus on nearby objects. Although distant objects are usually seen clearly, the near point is somewhat greater than the “normal” 25 cm, which makes reading difficult. This defect is caused by an eyeball that is too short and the image is formed behind the retina. This defect can be corrected by using a converging lens.
Figure 4:

a) Farsighted eye

b) Farsighted eye is corrected by use of converging lens
In this lab, instead of using the focal length, the power of the lens which is the reciprocal of the focal length will be used to specify the strength of eyeglass lenses. P = 100f (in cm) Diapter 1f = 1do + 1di

PASCO Model of the Human Eye:
Specifically, PASCO model of human eye is a scaled up model of the human eye in which it uses a sealed plastic tank roughly shaped like the horizontal cross-section of an eye ball as shown in Figure 5. A plano-convex, glass lens, permanently mounted on the front side of the eye model acts as the cornea. The tank can be filled with water to model the aqueous humor and the vitreous humor. A replaceable lens behind the cornea models the crystalline lens of the eye. A screen, which is movable between the three slots, at the back of the eye model represents the retina.

Figure 5: PASCO Human Eye Model

Results and Discussion:
Part 1 – Image Formation in the Eye
In part 1 of the experiment, the image formation in the eye will be examined by placing the human eye model on the rail with the light source as shown in Figure 6. The retina screen was put in the middle slot marked Normal and the lens labeled +400 mm was put in Septum

Figure 6: Setup for the Optics of the Eye Experiment
Firstly, examining the hand in front of the eye model about 50 cm from the cornea of the model by using the flash light to brightly illuminate the hand, it was shown that there was an inverted image of the hand on the retina screen. When the hand was moved up, the hand’s image on the retina screen was moved down and vice versa. Similarly, when examining a letter A on a piece of paper which is held upside down in front of the eye, the letter A is shown inverted, right-side-up, on the retina which made it is easier to be read as shown below

Thus, the image formed on the retina is upside down. However, we do not see things upside down because of our brain. Human brain is able to invert the image of the retina when it processes the visual information from the eyes.
Part 2 - Accommodation
In the part 2 of experiment, the accommodation will be initially modeled by varying the focal length of the crystalline lens using the adjustable focus lens, after that accommodation is achieved by replacing the crystalline lens with fixed lenses of various focal lengths when the model is filled with water.
When placing the illuminated light source object on the rail and the eye model with the lens in the Septum about 25 cm from the object, the image cannot be seen on the retina. In order to form a clearest image possible, it is necessary to move the syringe plunger to adjust the focal length of the lens to 7.6 cm. In this case the lens is concave (diverging) and the Power of the lens is 13 Diapter. When repositioning the eye model about 50 cm from the light source object and readjusting the adjustable lens focal length by moving the plunger to form the clearest possible image, it is obvious that, comparing to the lens in previous step, the new focal length increases to 9.0 cm and the new Power of the lens decreases to 11 Diapter. When using the +400 mm lens placed in the Septum with the air in the eye model, the distance between the light source object and the eye model should be 41 cm in order to form a clear image. However, when filling the eye model with water, the image could not be seen clearly as with the air in the eye model. It can be said that the vitreous humor and aqueous humor makes the image hard to be focused, especially with the +400 mm lens. In the near point part, when placing the eye model about 35 cm from the light source object and replacing the +400 mm lens in the Septum with the +62 mm lens, the image now can be focused with the Near Point at 27 cm and the effective focal length is around 7.8 cm. If the eye model was moved as close as possible to the light source, the observed image on the retina was bigger and had tendency to be blurry. When adding a +400 mm lens in slot B, this combination of +62 mm and +400 mm lens increased the ability of the model eye focus on near object by increasing the Near Point distance to 22.5 cm. Finally, when replacing the +62 mm lens in Septum with a +120 mm lens, the eye model now could focus at the farer Near Point which was 78.3 cm and the effective focal length was around 9.64 cm. It can be said that the human eye can increase the ability to focus on a near object by increasing the focal length of the crystalline lens. In the far Vision part, when placing the +62 mm lens in the Septum and setting the distance between the model eye and the object was 687 cm, it showed that the image of the object could not be focused on the retina. However, when replacing the lens with a +120mm lens, the clear image now could be focused on the retina. This +120mm lens is called the far vision lens and the effective focal length of the eye model was 10.80 cm. In the cataracts part, the crystalline lens is removed from the eye model. The image now could not be observed on the retina. Even when the +400 mm lens was placed in slot 1 as an eye glasses lens, the vision of eye model still was not clear. However, when replacing the +400 mm lens with +120 mm lens in slot 1 and adjusting the distance object to 42.50 cm, a clear image could be seen now. It is obvious that when an eye changes accommodation from a distance object to a near object, the curvature of the crystalline lens decrease. Moreover, the eye’s range of accommodation will change with age. The eye’s lens becomes increasingly less able to change shape which also means the eyes lose their ability to focus to close distances. Comparing to the old method of surgical removal of the crystalline lens, Crystalens is the modern treatment for cataracts which is designed to bend and flex along with the muscles of the eye to allow the eye to focus naturally for near, intermediate and distance vision.
Part 3 – Farsightedness:
In this part, when placing the normal vision lens +62 mm in the Septum and setting the retina to Far Slot, with the near focus distance at 27 cm between the eye model and the light source object, the image on the retina is too blur. However, when placing the +120 mm lens in Slot 1, the image now was very clear on the retina. The effective focal length of the lens was 7.80 cm and its Power was 12.82 Diapter. If rotating the eye glasses lens, the image was still not be affected. It is obvious that a high power lens will have a shorter focal length because the subject can be brought closer to the center of projection. Between the +62 mm and +400 mm lenses, the +62 mm has more curvature. In real eye glasses, each lens has one convex and one concave surface, however, to correct for farsightedness, the convex surface needs to be more curved.
Part 4 – Nearsightedness In this part, , when placing the normal vision lens +62 mm in the Septum and setting the retina to Near Slot, with the near focus distance at 27 cm between the eye model and the light source object, the image on the retina is clear and inverted. In order to correct the nearsightedness, it is necessary to place the +400 mm lens in Slot 1. The focal length of the lens was 9.60 cm and the Power of the eye glasses lens was 10.37. Even when rotating the eye glasses lens, the image would not be affected. When removing the eye glasses lens, the distance between the object light source and the model eye should be adjusted to be 51 cm in order to focus the image on the retina. Comparing to the near focus distance of 27 cm identified above, this object distance has been increased. It is obvious that myopia cannot be compensated by the eye’s own accommodation. In the eye model, the slot marked as NEAR is farthest from the lens and the slot marked as FAR is nearest to the lens. Thus the word FAR refers to farsightedness.
Conclusion:
Information acquired during this experiment supported all the theory demonstrating in the introduction. The image formed in the eye model is always upside down, however, due to the brain’s processing, the image seen by human has been changed to right-side-up. For accommodation, the human eye can increase the ability to focus on a near object by increasing the focal length of the crystalline lens. When an eye changes accommodation from a distance object to a near object, the curvature of the crystalline lens decrease and the eye’s range of accommodation will decrease with age. It is also noted that high power lens will have a shorter focal length. In order to correct for farsightedness the convex surface of the lens need to be more curved and for the nearsightedness, the concave surface need to be more curved.