It's no secret that how we view the world heavily depends on one main organ; you guessed it, it's your eyes! Our eyes act as our window to view the world around us, allowing us to intake and make sense of the surroundings around us. But how exactly do they work? On the outside, all we see are two white ellipses with a colored circle on the inside of them. However, there is much more to them than meets the eye. The inner workings of the eye are much more fascinating than you think. Our eyes themselves are a marvel of biological engineering as they not only allow us to just capture light and color; they are instruments always at work, processing information, creating images, and connecting us to our environment. Inside our eyes, millions of cells, nerves, muscles, and lenses work together to help us see. So, without further adieu, let's explore how our eyes make vision possible.
How the Eyes Function
Our eyes primarily rely on, well, light to make sense of what's around us. You see, every object in the natural world has light reflected off of them, which allows us to see them. Our eyes make sense of patterns of light and then works with our brain to make those patterns into images. When light bounces off an object (e.g. a plant), the light first enters through our cornea. The cornea is shaped like a dome and bends light to help the eye focus. Essentially, this is the main entrance through which our eyes welcome light. The amount of light that is let in through the eye is controlled by the pupil, a black circle which is an opening in the middle of our iris. It expands during dark lighting conditions in order to optimize the amount of light that can pass through our eye, and contracts during bright lighting conditions due to the generous amount of light in our surroundings. These contractions are all controlled automatically by the iris, the colored part of the eye. Depending on your eye color, the iris might either be blue, green, hazel, amber, or brown (8-10%, 2%, 5%, 5%, and 70-80% respectively). Then, as light passes through the pupil, it passes through the lens (a clear inner part of the eye). The lens helps focus the light coming through your eye so that you can see clearly. The lens works in conjunction with the cornea to focus light clearly and correctly onto the retina. The retina is a light-sensitive layer of the eye with a collection of cells that line the inside of the back of the eye. The retina contains 130 million light-sensitive cells called photoreceptors that process light and convert that information into electrical impulses sent through the optic nerve to the brain (the optic nerve is essentially a direct highway of electrical impulse info from the eyes to the brain). There are two types of photoreceptors: rods and cones. Rods work at very low levels of light because they help us see everything in a grayscale (meaning they don't help with our color vision). On the contrary, cones require a lot more light because they are used to see color. We have three types of cones: blue, green, and red. There are over 100 million rod cells in the retina and about 6 million cone cells in the retina. The retina helps our eyes create a rough image of what we're looking at, but it sees the world upside down. So, when electrical impulses are sent to the brain, the brain is what actually turns the retina's image right side up. In addition to this, each eye has a slightly different perception of the world around us. So, not only does our brain convert the retina's image right side up automatically, it also merges the views of our right and left eye into one clear, concise view. These are the main parts of the eye that enable us to see, but there are a lot more additional parts of the eye that don't play a direct role in our sight. The conjunctiva, for example, is a clear thin tissue that covers the sclera and lines the inside of our eyelids. The sclera is the white part of our eye that surrounds the iris. The vitreous is a transparent gel that fills our entire eye and protects/maintains the shape of the eye. It also nourishes the eye's cells. The muscles of our eye control our eye's position and movement (for example, looking up, down, etc.). The macula is a small area of the retina that is responsible for central vision and helping us see fine details and color.
Visual Acuity and Common Vision Problems.
Visual acuity is defined as the clarity of the image seen by the eye. Ever heard of 20/20 vision? That's referring to visual acuity, or VA for short. VA has been tested by using an eye chart at a distance of 20 feet. Over the course of testing many patients with an eye chart, eye doctors have determined what an average VA is for most people when standing 20 feet away from an eye chart. This measurement is called "normal" vision. Having a VA score of 20/20 refers to the fact that when standing 20 feet away from the eye chart, a normal-sighted person can see line 20 of the eye chart clearly, or "20/20". This means that a person has normal vision, not perfect vision. Now, visual acuity can be affected by a lot of eye-related problems (duh). Sometimes, the lens can't focus light clearly or the shape of the eyeball slightly isn't round, so the image the viewer sees is blurry. If such an error occurs where nearby objects look blurry, a person usually has farsightedness (hyperopia). However, if a person is unable to see faraway objects, that person is usually nearsighted (myopia).
Not all eye conditions are a result of lens/eyeball problems. Some eye conditions affect the retina. If some people have the edges of their retina impaired, they develop tunnel vision, which hinders their peripheral vision. The same goes for the middle of the retina; blind spots can develop, therefore blocking some areas of view (scotomas). Cloudiness in the lens (cataract) may also occur, increased eye pressure (glaucoma), damage to the cornea, or problems with the eye muscles.
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