Model Science – The Human Eye

1. Model Science – The Human Eye Junior MESA Day

2. Agenda • Overview of the Human Eye • Physiology • Functions of External Structures • Functions of Internal Structures • Disorders and Diseases • Building the Model • Building the Display • Questions

3. Overview of the Human Eye • Significant sense organ • Spheroid structure with an average diameter of 24mm, about 2/3 the size of a ping-pong ball • External structures: eyelids, eyelashes, tears and fat glands, extraocular muscles, conjunctiva • Internal structures: cornea, sclera, iris, ciliary body, choroid, retina, lens, anterior and compartment, optic nerve

5. Physiology • The eye has many functions. It can distinguish light and dark, shape, color, brightness and distance. • Light is electromagnetic radiation at wavelengths which the human eye can see. • A typical human eye can see wavelengths from about 380 to 750 nm and about 790 to 400 terahertz.

6. Crystalline lens • Biconvex structure • Convergence of light to a nodal point • Inversion of image

7. Activity 1: Layers of the Eye

8. Activity 2: Focusing Images • To understand how images can be focused at one point • Materials: index card and push pin • Take index card and make a hole in the center with the push pin. • Place aside. • Look at a word on the wall. • Close your left eye and place your thumb of your right hand in front of your right eye about 6 inches away and focus your right eye on your thumb. • With your left hand, now place the index card immediately in front of your thumb and the word on the wall.

9. Activity 3: Building a Refracting Telescope • To understand that lenses cause rays of light to come to focus by building a refracting telescope. Safety Procedures: Extreme caution with cutting utensils (Xacto knife, PVC pipe cutter) Extreme caution with HOT glue gun

10. Determine the focal length of the PVC pipe by adding the focal lengths of the magnifying glass to the wall and to the table. • Cut PVC pipe. • Unscrew water bottle cap and cut a hole in the center. • Cut top rim off of water bottle, and insert PVC. Then, glue. Glue the back of the cap to the center of one magnifying glass. Glue the other end of the PVC to the center of the second magnifying glass. Screw the magnifying glass and cap on to the other end of the PVC.

11. Functions of External Structures • Eyelids • Lubricate the eye surface by distributing tears over the cornea • Eyelashes • Stop dust and sweat from getting into the eyes • Tear Fat Glands • Keep cornea moist • Wash foreign bodies out • Act as main supplier of oxygen and nutrients • Contain lysozyme

12. Extrinsic / Extraocular Muscles • Act to turn or rotate the eye • Conjunctiva • Lines the inner surfaces of the eyelids and continues to cover the front surface of the eyeball, except the cornea vertical horizontal torsion/twisting

13. Functions of Internal Structures • Fibrous Coat • Cornea • Primary and most powerful structure to focus light • Sclera • Provides protection to delicate structures, • Serves as an attachment for the extraocular muscles • Helps maintain the shape of the eyeball

14. Vascular Coat • Iris • Thin diaphragm composed mostly of connective tissue and smooth muscle fibers • Colored disc inside of eye which is an unique as a fingerprint • Divides the anterior chamber and posterior chamber • Pupil: regulates the amount of light passing through to the retina • Ciliary Body • Ciliary muscles serve as the chief agent in eye accommodation • Ciliary processes produce aqueous humor • Choroid • Absorb excessive light • Contain network of blood vessels to supply oxygen and nutrients

16. Retina • Innermost layer of the eye • Receives images, comparable to the film inside a camera • Photoreceptors • Rods • Cones that absorb long-wavelength light (red) • Cones that absorb middle wavelength light (green) • Cones that absorb short-wavelength light (blue) • Macula • Highest concentration of cones • Fovea centralis • Center of macula • Contains only cones and no rods

18. Lens • A transparent cystalline biconvex structure immediately behind the iris • Suspended from ciliary body by threadlike ligaments called zonules • Refracts light • Focuses light on the retina

19. Anterior Compartment • Composed of posterior chamber and anterior chamber • Aqueous Humor • Fluid that nourishes the lens and epithelial cells • Help refract light onto the retina • Posterior Compartment • Vitreous Humor • Clear gel • Helps refract light onto the retina

20. Optic Nerve • Transmits electrical impulses from the retina to the brain • The blind spot is located at the optic nerve head or optic disc where there are no photoreceptors • Retinal Blood Vessels • Arteries transmit oxygen and nutrients • Veins carry deoxgenated blood from the eye

21. Activity 5: How Absolutely Blind is Your Blind Spot? • Purpose: to demonstrate the extent of a student’s blind spot • If you really want to be amazed at the total sightlessness of your blind spot, do a similar test outside at night when there is a full moon. Cover your left eye, looking at the full moon with your right eye. Gradually move your right eye to the left (and maybe slightly up or down). Before long, all you will be able to see is the large halo around the full moon; the entire moon itself will seem to have disappeared.

22. Activity 6: Name That Structure

23. Disorders and Diseases • Astigmatism • Defect causing rays from a point to fail to meet in a single focal point • Cataract • A clouding of the lens • Color Blindness • Certain colors cannot be distinguished • Conjunctivitis • Inflammation or infected conjunctiva

24. Dry Eye • Deficiency of any of the three layers of tear film • Glaucoma • Damage to the optic nerve • Hyperopia • Visual images come to a focus behind retina • Iritis / Uveitis / Chorioretinitis • Inflammation of iris, entire uvea or choroid/retina • Myopia • Visual images come to focus in front of retina • More information • National Eye Institute, National Institutes of Health at http://www.nei.nih.gov

25. Building the Model • Model of bisected human eye • < 2 ft x 2 ft x 2 ft deep and > 1 ft x 1 ft x 1 in deep • Materials • Commercial models may NOT be used • Items that are not perishable • Required Structures • Cornea, sclera, iris, ciliary body, choroid, pupil, retina, lens, optic nerve, fovea centralis, vitreous chamber, central retinal artery, conjunctiva, and anterior chamber • Bonus points awarded for up to 4 additional structures other than the required structures

26. Building the Display • Dimensions not greater than 3 ft x 3 ft x 2 deep • Freestanding • Clearly labeled, hand-drawn or student’s original computer-generated diagram of the bisected human eye • Materials table

27. Questions • Component II of competition includes an understanding of the anatomy and physiology of the human eye • Randomly select 5 questions from assigned list of 22 questions (attachment to Model Science – The Human Eye Rules) • Each correct answer will be awarded up to 2 points; partial points may be awarded for partial answers • 5 unpublished tiebreaker questions available on day of the competition; each tiebreaker question will be worth up to 2 points each