The Journey of Light into the Eye - Project NEURON



The Biology of How We SeeFigure SEQ Figure \* ARABIC 1: A drawing of the anatomy of the eye. (“Drawing of the Eye” by the National Eye Institute, National Institutes of Health. Public domain.)The Journey of Light into the EyeIn order for an animal to survive, it needs to be able to detect food and find food and mates or avoid predators. Animals have evolved many different types of sensory organs which help them detect objects or characteristics in the environment, and the eye is an organ sensitive to light.Light is continuously travelling through the air, bouncing off of or being altered by various objects. Your eye captures some of that light and “translates” it into information that the brain uses to help you interact with the world. Seems like a difficult job—how does the eye do that?If light bounces toward your eye, the first thing it meets is your cornea—a clear outer layer at the front of the eye (see figure 1). Light passes through the cornea and “bends” or changes direction so that it enters the eye instead of bouncing off in a different direction. Therefore the cornea focuses light into the eye.Get some air! If you wear contact lenses excessively, blood vessels will start to grow across your cornea. Gross! Why does this happen? The cornea is the only part of the entire human body that does not contain blood vessels, because otherwise the capillaries would be so dense that you would not be able to see. However, all living cells need oxygen to survive, so the cornea receives oxygen that diffuses across the cell membrane directly from the air. If there is not enough oxygen, your body will try to compensate by growing new blood vessels into the cornea. So do your eyes a favor and give them air!After being focused by the cornea, light passes through the pupil, a hole whose size is controlled by a muscle called the iris. In a dark environment, your pupil will expand to allow as much light in as possible; in a bright environment like a sunny day, your pupil will constrict to limit the light entering the eye and prevent your eye from being overwhelmed.Next, light passes through the lens, which focuses light onto the back of the eye. Small muscles adjust the shape of the lens, allowing you to see distant and nearby objects. If you are reading a book and then look up to view a cat across the room, the lens flattens slightly to bring the cat into focus. When you look back down at the book, the lens squishes and becomes rounder to bring the text into focus. As people get older, the lens becomes less flexible and able to focus light from close objects, which is why older individuals may start using reading glasses or bifocals. While it is traveling to the back of the eye, light passes through the vitreous humor, a clear gel that fills the eye and helps keep its shape.Floaters! Have you ever seen little specks of dust or hairs in your field of vision when you look at a bright, blank background like the sky? These “floaters” are proteins in the vitreous humor that have clumped together to form visible globs. You’ll only notice them when they are moving because your brain edits them out when they are stationary. Floaters will eventually settle to the bottom of your eye over time and are not harmful.At the very back of the eye, light is focused onto the retina. The retina is the first stage of image processing and is made up of several different layers of cells. Light passes through several layers of transparent nerve cells before encountering the photoreceptors, special cells that detect light, at the back of the retina. At the center of the retina is a little divot or pit called the fovea. In this area, the layers of nerve cells of the retina are shifted aside, allowing light to strike the photoreceptors directly. This makes the center of the image extremely sharp.A Closer Look at PhotoreceptorsPhotoreceptor cells are specialized so that some are better at detecting certain types of light than others. Humans, like most other vertebrates (fish, amphibians, reptiles, birds, and mammals), have two general types of photoreceptors: rod cells and cone cells. There many times more rods than cones in the human eye: approximately 120 million rods compared to 6 million cones.Rods are sensitive to low levels of light and are especially useful for nighttime vision. However, they are easily activated and will all be turned “on” in bright light. When you walk from a bright area into a dark room, you might only see black for a while and then, after about 20 minutes, gradually make out shapes and objects in the dark room. What you are experiencing are your cones gradually being reset to “off” and then afterward detecting subtle levels of light within the dark room.Red lights at sea: Although rods are sensitive to low levels of light of all colors, they are mostly insensitive to deep red colors. This fact is exploited by sailors working on boats in the ocean at night who use red lights while they are working in the cockpit. The red light allows sailors to simultaneously use their red cones to see the controls inside the boat and use their rods to see other vessels on the dark waters of the ocean.Cones are not activated in low light conditions but are useful for color detection when there is a lot of light. In a typical human eye, there are three types of cones: long-, middle-, and short-wavelength cones which are also called red, green, and blue cones, respectively. Input from each of these three cone types are combined by your nervous system to see every color on the spectrum, from dark brown to bright orange or light purple!Rarely, people may be missing one or more cones, or the cone(s) may have a defect and do not work very well. These abnormalities may cause individuals to have trouble distinguishing, or telling the difference between, particular colors. This is commonly known as color blindness, although most individuals who are color blind could be better described as “color deficient.”Integration of SightPhotoreceptors in the retina are activated by light and send signals to nearby nerve cells. These nerve cells integrate and process some of that information and then send it to the optic nerve. The optic nerve passes out the back of the eye ball and connects directly to the brain, where the information is processed as what we call “sight.” Now the brain can interpret the image and determine if it there is, for example, a predator that needs to be avoided or a piece of fruit that can be consumed! ................
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