![]() ![]() The lens absorbs it before it can reach the retina. People don't see ultraviolet light, but the human retina can detect it.If you wear special goggles that make you view everything upside down, after a few days your brain will adapt, again showing you the "correct" view. When the brain translates the image, it automatically flips it. The eye acts exactly like a camera in the sense that the image formed on the retina is inverted (upside down).The functioning of the eye is fairly simple, but there are some details you might not know: The vitreous humor is a transparent watery gel that supports the eye and allows for this distance. Vitreous Humor: A certain distance is required to focus light.Ciliary muscles surround the lens, relaxing to flatten it to image distant objects and contracting to thicken the lens to image close-up objects. Lens: While most of the focusing of light is done by the cornea, the lens allows the eye to focus on either near or distant objects.As the pupil dilates (gets bigger), more light enters the eye. The size of the pupil is determined by the iris, the contractile ring that is associated with eye color. Iris and Pupil: Light passes through the cornea and aqueous humor through an opening called the pupil.The aqueous humor helps to shape the cornea and provides nourishment to the eye. Aqueous Humor: The fluid beneath the cornea has a composition similar to that of blood plasma.The eyeball is rounded, so the cornea acts as a lens. Cornea: Light enters through the cornea, the transparent outer covering of the eye.For example, the color yellow results from green and red cones being stimulated while the blue cones have no stimulation.To understand how the eye sees, it helps to know the eye structures and functions: But it is the overlap of the cones and how the brain integrates the signals sent from them that allows us to see millions of colors. Since the three types of cones are commonly labeled by the color at which they are most sensitive (blue, green and red) you might think other colors are not possible. We have three types of cones. If you look at the graph below, you can see each cone is able to detect a range of colors. Even though each cone is most sensitive to a specific color of light (where the line peaks), they also can detect other colors (shown by the stretch of each curve). Now that we know how these photoreceptor cells work, how do we use them to see different colors? Because the RPE is right next to the discs, it can easily help reload photoreceptor cells and discs with the molecules they need to keep sending signals. This requires the movement of lots of molecules, which they need to replenish to keep working. Rods and cones are similar, but instead of running, they are constantly sending signals. In order to keep muscles in the body working, the runner needs to eat special nutrients or molecules during the race. Think about someone who is running a marathon. While there are many other reasons having the discs close to the RPE is helpful, we will only mention one more. Light can also have damaging effects, so this set up also helps protect your rods and cones from unnecessary damage. This means that your vision is a lot clearer. By having the discs right next to the epithelial cells (retinal pigmented epithelium: RPE) at the back of the eye, parts of the old discs can be carried away by cells in the RPE.Īnother benefit to this layout is that the RPE can absorb scattered light. Image by HuBoro.įirst of all, the discs containing rhodopsin or photopsin are constantly recycled to keep your visual system healthy. Animals that have to see in the dark have many more rods than humans have.Ĭell orientation makes it easier to recycle parts. Other animals have different numbers of each cell type. Many of these are packed into the fovea, a small pit in the back of the eye that helps with the sharpness or detail of images. The human eye only has about 6 million cones. We have three types of cones: blue, green, and red. The human eye has over 100 million rod cells.Ĭones require a lot more light and they are used to see color. ![]() Rods don't help with color vision, which is why at night, we see everything in a gray scale. We use these for night vision because only a few bits of light (photons) can activate a rod. There are two types of photoreceptors involved in sight: rods and cones. The retina also contains the nerves that tell the brain what the photoreceptors are "seeing." If you think of the eye as a camera, the retina would be the film. This is where the photoreceptors are located. You can see in the drawing on the left that the back of the eye is lined with a thin layer called the retina. Click to enlarge and for more information. ![]()
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