Human Eye A human eyeball is like a simple camera! Sclera: White part of the eye, outer walls, hard, like a light-tight box. Cornea and crystalline lens (eyelens): The two lens system. Retina: a thin film of rods and cones located inside at the back of eyeball. Anatomy of the Human Eye Iris: a colored muscle that acts like diaphragms to make the pupil larger or smaller depending on light conditions. Pupil: a hole in the eye that sllows light to pass through like a camera aperture. Eyelid: protects the eye like the lens cover of a camera. Sclera (The white/non-transparent tissue surrounding the cornea) Aqueous humor and Vitreous humor The Aqueous Humor is the clear liquid between the cornea and the lens. It helps regulate the pressure of the eye ball. The space that it inhabits is called the anterior chamber. The Vitreous Humor is the clear liquid between the lens and the retina. It is responsible for the shape of the eyeball. The space that it fills is called the vitreous body. Functions? Provides nourishment to the eye lens and cornea. Hold the shape of the eyeball. Focusing The cornea and eye lens form a compound lens system, producing a real inverted image on the retina. The eye has a limited depth of field. We cannot see things close and far at the same time. Accommodation The eye focusing is not done by change the distance between the lens and retina. Rather, it is done by changing the focal length of the eyelens! Ciliary muscles help to change the shape of the lens. Normal eyes can see 25cm to infinity, however, if the cornea bulges too much or too little. The accommodation does not help. (myopia or hyperopia) The Iris When it is open as much as it can be, there is low light level conditions. When the iris has a small opening, it can cut down the light intensity by a factor of 20. This happens during intense light conditions. Retina Structure Light sensitive layer is made of photoreceptors: rods (120 millions) and cones (7 millions) which absorb the light. Plexiform Layer: nerve cells that process the signals generated by rods and cones and relay them to the optical nerve. Choroid: carries major blood vessels to nourish the retina and absorb the light so that it will not be reflected back (dark pupil!) Rods and Cones Cones: for more precise vision, need strong light. They help to see colors. Mostly distributed in the center of the retina (fovea). Rods: for peripheral and night vision. Sensitive to light. Mostly distributed away from fovea. Fovea The fovea defines the center of the retina, and is the region of highest visual acuity. The fovea is directed towards whatever object you wish to study most closely - this sentence, at the moment. In the fovea there are almost exclusively cones, and they are at their highest density. Sensitivity Cones: slow, fine grain, like color film. Need high level of light (photopic condition, day) High density, high resolution. Rods: fast, coarse grain, black & white film Low level of light (scotopic condition, at night) No color is obvious. Adaptation: Changing of retina sensitivity. http://www.aoa.org/x6024.xml Vision begins when light rays are reflected off an object and enter the eyes through the cornea, the transparent outer covering of the eye. The cornea bends or refracts the rays that pass through a round hole called the pupil. The iris, or colored portion of the eye that surrounds the pupil, opens and closes. The pupil gets bigger or smaller to regulate the amount of light passing through. The light rays then pass through the lens, which actually changes shape so it can further bend the rays and focus them on the retina at the back of the eye. The retina is a thin layer of tissue at the back of the eye that contains millions of tiny light-sensing nerve cells. The images that we see are projected onto the retina upside down. Our brain quite simply, flips the images over so that we see things upright. The optic nerve transmits information to the brain. The vitreous body gives the eye its shape. What are lenses? Lenses bend light in useful ways. Most devices that control light have one or more lenses in them (some use only mirrors, which can do most of the same things that lenses can do). There are TWO basic simple lens types: Concave and Convex CONVEX or POSITIVE lenses will CONVERGE or FOCUS light and can form an IMAGE. CONCAVE or NEGATIVE lenses will DIVERGE (spread out) light rays Convex lens The correct name for farsightedness is Hyperopia. The shape of your eye does not bend light correctly, resulting in a blurred image. A convex lens is usually used to correct this problem. Concave lens The correct name of nearsightedness is myopia. Myopia occurs when the eyeball is slightly longer than usual from front to back. This causes light rays to focus at a point in front of the retina, rather than directly on its surface. A concave lens is usually used to correct this problem.