The dark circular muscular diaphragm that controls the amount of light entering the eye is

Vision The Mechanics of Vision The Eye The mammalian eye is a complicated structure, but it is not difficult to understand how it produces images, at least to first approximation. The human eye is fairly typical of mammahan eyes and is shown below. Light enters through the transparent cornea, whose form is approximately spherical. The light then passes through a chamber containing clear liquid called the aqueous humor. At the rear of the chamber is a muscular, ring-shaped structure called the iris. (It is the pigmented membranous cover of the iris that determines eye color.) The circular aperture in the ring is called the pupil. By contracting and relaxing under control of the autonomic nervous system, the iris regulates the diameter of the pupil and thus the amount of light passing farther into the eye. Just behind the iris is the crystalline lens, an elastic, transparent capsule. The lens is mounted in the ciliury muscle. This muscle can squeeze on the lens, making it bulge and thus reducing its focal length. Behind the lens is a chamber filled with a clear, jellylike fluid called the vitreous humor. At the rear of the chamber is a highly complex, light-sensitive structure called the retina. The retracting structures of the eye_the cornea and the lens_cast an image on the retina. The retina converts light into electrical signals that are transmitted and partially processed by the optic nerve. The final processing and interpretation takes place in the brain. THE STIMULUS ...

How Your Eye Works Much like a camera, the eye works by focusing light rays. Light entering the eye first passes through the transparent cornea, which has about 60 percent of the light-focusing ability of the eye. Light rays are then focused by the lens to make a clear image on the retina at the back of the eye. The image formed by the retina is transmitted to the brain as a series of electro-chemical impulses via the optic nerve. The Eye Think of the eye as a camera. The lids are the lenscap and the iris is the f-stop or diaphragm which lets in more or less light through the lens which focuses light on the retina. The retina is the film in the camera. The retina is connected by a coaxial type of cable called the optic nerve which travels to the brain. We’ll refer to this camera example again later. Cornea Anatomy of The Eye The cornea is the clear dome-shaped front window of the eye, which provides 60 percent of the light-bending (focusing) power of the eye. It is the only tissue in the human body which is transparent to light. In its normal state, the cornea has no blood vessels. Epithelial cells coat the outside of the cornea, providing both a smooth and a water-holding (tear) surface to allow clear vision. The lids not only are windshield wipers but, by spreading the tears around, also serve to keep the eye from completely drying out. Sclera and Conjunctiva This is the muscular outer coat of the eye which maintains the eye’s round shape. It is white and opaque to light...

Greg Foot demonstrates how the eye works and what happens if someone is long or short-sighted The eye is a sense organ containing receptors sensitive to light, intensity and colour. Structure Function Cornea Clear area of the sclera, it refracts light – bends it as it enters the eye. Iris Muscles which alter the size of the pupil, controlling the amount of light entering the eye. Lens Focuses light onto the retina. Retina Contains the light receptor cells. Optic nerve Carries impulses between the retina and the brain. Sclera White, tough outer coat. Choroid Pigmented middle layer with many blood vessels. It absorbs light to avoid reflection and nourishes the retina. Blind spot Where the optic nerve leaves the retina so lacks receptor cells. Pupil Small window at the centre of the iris through which light enters the eye. The retina Light passes through the eyeball to the retina. There are two main types of light receptors - rods and cones . Rods are more sensitive to light than cones so they are useful for seeing in dim light. There are three different types of cone cells which produce colour vision. A photograph of a human retina seen through the eye The pupil reflex The amount of light entering the eye is controlled by a reflex action . Rod cells on your retina help you see in low light levels, and the size of the pupil changes in response to bright or dim light. It becomes bigger to let in more light in dim conditions, and smaller in bright light to protect your retina. ...

MY= I have it Nearsightedness inability to see distant objects clearly two causes: 1. refractive: lens or cornea bends light too much and light focuses in the front of the retina 2. axial: eyeball is too long so point of focus falls in front of the retina far point is closer good vision= 20/20 so person who has this has 10/20 the region of the retina in which visual stimuli influence a neurons firing rate two dif types: 1. on-center- increase firing rate when a light is turned on i center, decreasing firing rate when light is turned on in surround 2. off-center- decrease in firing rate when a spot in the center of the receptive field is turned on, and increase when the spot is turned on in the surround

What Are the Eyes and How Do They Work? In a single glance, our eyes work with our brains to tell us the size, shape, color, and texture of an object. They let us know how close it is, whether it's standing still or coming toward us, and how quickly it's moving. Only part of the eye is visible in a person's face. The whole eye — the eyeball — is about the size and shape of a ping-pong ball. All parts of the eye are extremely delicate, so our bodies protect them in several ways. The eyeball sits in the eye socket (also called the orbit) in the skull, where it is surrounded by bone. The visible part of the eye is protected by the eyelids and the eyelashes, which help keep dirt, dust, and even harmful bright light out of the eye. Eyes are also protected by tears, which moisten them and clean out dirt, dust, and other irritants that get past the defenses of the eyelashes and eyelids. Tears also help protect against infection. With each blink, our eyelids spread a layer of mucus, oil, and tears over the cornea, which covers the front of the eye. The lacrimal (pronounced: LAK-ruh-mul) glands in the upper outer corner of each eye socket produce tears, which, after moistening the eyes, flow into canals in the eyelids. These canals drain into the lacrimal sac, a pouch in the lower inner corner of each eye socket. Tears then exit through a passage that leads to the nose. To see, the eye has to move. Six extraocular muscles surround the eyeball and act like the strings on a puppet, m...

To understand how the human eye works, first imagine a photographic camera—since cameras were developed very much with the human eye in mind. How do we see what we see? Light reflects off of objects and enters the eyeball through a transparent layer of tissue at the front of the eye called the cornea. The cornea accepts widely divergent light rays and bends them through the pupil—the dark opening in the center of the colored portion of the eye. The pupil appears to expand or contract automatically based on the intensity of the light entering the eye. In truth, this action is controlled by the iris—a ring of muscles within the colored portion of the eye that adjusts the pupil opening based on the intensity of light. (So when a pupil appears to expand or contract, it is actually the iris doing its job.) The adjusted light passes through the lens of the eye. Located behind the pupil, the lens automatically adjusts the path of the light and brings it into sharp focus onto the receiving area at back of the eye—the retina. An amazing membrane full of photoreceptors (a.k.a. the “rods and cones”), the retina converts the light rays into electrical impulses. These then travel through the optic nerve at the back of the eye to the brain, where an image is finally perceived. A delicate system, subject to flaws. It’s easy to see that a slight alteration in any aspect of how the human eye works—the shape of the eyeball, the cornea’s health, lens shape and curvature, retina problems—can ...