Eye Structure and Seeing Light Presentation for lesson 5: Understanding the Structure of the Eye, in the Waves: The Three Color Mystery unit The slides are animated so you can click (space bar, mouse, etc.) to show the next item when the class is ready. Eye Structure and Seeing Light

The eye is like a camera: Light enters, is focused on a surface, and a picture is made. Light enters your eye through a clear portion of the sclera (the tough, white, outer covering of the eye), called the cornea.

The cornea is curved, so it slightly bends the light as it goes through. Light then passes through the aqueous humor (a clear fluid for eye nourishment, in the anterior chamber) and through the pupil. The pupil is simply a hole in the iris.

The iris is a muscle that controls the size of the pupil The iris is a muscle that controls the size of the pupil. The iris is the colored part of the eye. The color of the iris can be seen through the transparent cornea over it. What color(s) is your neighbor’s iris? In bright light, the iris expands and the pupil gets smaller In low light, the iris contracts and the pupil gets bigger

Directly behind the iris is the lens Directly behind the iris is the lens. This structure changes shape to focus the light so that we can see clearly. Its shape is convex, meaning it curves outward on both sides. The ciliary muscles above and below the lens control the shape of the lens.

Behind the lens is a clear gel called the vitreous humor Behind the lens is a clear gel called the vitreous humor. After moving through the vitreous humor, the light strikes the retina. The retina is the lining on the inside of the back of the eye that contains two types of light-sensitive cells: rods and cones.

Rods sense black and white and work in low light. Cones sense color and must have more light than rods to work. Three kinds of cones: L-cones sense long wavelengths in the red range M-cones sense mid-range wavelengths in green range S-cones sense short wavelengths in the blue range

The rods and cones send messages to the brain through the optic nerve The rods and cones send messages to the brain through the optic nerve. The brain makes sense of all the information it is receives. In your brain, the sight center is in the back, between your ears. This location explains why a blow to the back of your head might cause blindness, even though your eyes are fine.

Two Causes of Color Blindness Genetic (you are born with these types) Sometimes a cone is missing, or the cone does not recognize the correct wavelengths of light. L- and M-cone problems result in red-green color blindness, the most common. (left) Color Vision Test: If you see “5” = normal color vision; if you see “2” = red/green color deficiency (right) Color Vision Test: If you see “45” = normal color vision; if you see “spots” = red/green color deficiency Most people with color blindness are able to distinguish a small range of colors, so really, the term "color-deficiency" is more appropriate than "color-blindness."

2. Non-Genetic (these types occur after birth) Accidents that damage the vision center of the brain, cataracts, glaucoma, Parkinson’s Disease can cause S-cone problems, diabetic retinopathy can affect color vision

Eye Anatomy Review cornea pupil iris anterior chamber aqueous humor lens vitreous humor retina fovea choroid sclera optic nerve Let’s identify and describe the parts of a human eye: cornea: The clear portion of the eye through which light enters. pupil: A hole in the iris. iris: The colored part of the eye; a muscle that controls the pupil size. anterior chamber: The fluid-filled space inside the eye between the iris and the cornea; it is filled with aqueous humor. aqueous humor: The clear fluid behind the cornea; used for eye nourishment. lens: A structure that changes shape to focus light so that we can see clearly; located directly behind the iris. vitreous humor: A clear gel behind the lens, inside the eye. retina: The lining on the inside of the back of the eye that contains two types of light-sensitive cells: rods and cones. fovea: A part of the eye located in the center of the retina that is responsible for sharp central vision, so important in humans for reading, watching, driving or any activity in which visual detail is important. choroid: A vascular layer (containing blood vessels) of the eye containing connective tissue; it is located between the sclera and the retina. sclera: The tough, white, outer covering of the eye. optic nerve: A cranial nerve connected to the eye socket that transmits visual information from retina to the brain.

Image Sources 2004 Microsoft Corporation, One Microsoft Way, Redmond, WA 98052-6399 USA. National Cancer Institute at the National Institutes of Health http://www.cancer.gov/cancertopics/pdq/treatment/retinoblastoma/patient/page1/AllPages/Print MedLine Plus, National Institutes of Health http://www.nlm.nih.gov/medlineplus/ency/presentations/100206_1.htm MedLine Plus, National Institutes of Health http://www.nlm.nih.gov/medlineplus/ency/imagepages/9482.htm National Eye Institute, National Institutes of Health http://www.nei.nih.gov/healthyeyes/eyeexam.asp Wikipedia http://en.wikipedia.org/wiki/File:Schematic_diagram_of_the_human_eye_en.svg Federal Aviation Administration http://www.hf.faa.gov/webtraining/VisualDisplays/HumanVisSys2.htm Federal Aviation Administration http://www.hf.faa.gov/webtraining/visualdisplays/HumanVisSys6.htm Glaucoma, MedLine Plus, National Institutes of Health http://www.nlm.nih.gov/medlineplus/ency/imagepages/9349.htm