Retina, Rods and Cones The students were given something to look at in the dim light and then in the light. Then they shared their observations. The retina contains two types of photoreceptors (light sensors), rods and cones. There are around 120 million rods in each eye. They are most sensitive to light and dark changes, shape and movement and contain only one type of light-sensitive pigment. Rods are not good for color vision. In a dim room, however, we use mainly our rods, but we are "color blind." Rods are more numerous than cones in the periphery of the retina. Next time you want to see a dim star at night, try to look at it with your peripheral vision. There are about 6 million cones in each eye. The cones are not as sensitive to light as the rods. However, cones are most sensitive to one of three different colors (green, red or blue). Signals from the cones are sent to the brain which then translates these messages into the perception of color. Cones, however, work only in bright light. That's why you cannot see color very well in dark places. Source | Jack S asked "How do eyes make color?" |
The cornea, a transparent dome, sits in front of the colored part of the eye. The cornea helps the eye focus as light makes its way through. It is a very important part of the eye, but you can hardly see it because it's made of clear tissue. Like clear glass, the cornea gives your eye a clear window to view the world through. The lens works much like the lens of a movie projector at the movies. Next time you sit in the dark theater, look behind you at the stream of light coming from the projection booth. This light goes through a powerful lens, which is focusing the images onto the screen, so you can see the movie clearly. In the eye's case, however, the film screen is your retina. Source | The blind spot is the area on the retina without receptors that respond to light. Therefore an image that falls on this region will NOT be seen. It is in this region that the optic nerve exits the eye on its way to the brain. |
Chloe wanted to know "How do glasses work?" We can better understand how the cornea and lens work by understanding how glasses correct vision. Take a look at this TEDEd video. | |
Autorefractors Your eye doctor may use an autorefractor or aberrometer to automatically estimate your eyeglass prescription before fine-tuning it using a manual refractor. This is also how they test young children's eyes. | Manual Refraction with a Phoropter During a refraction, the doctor puts the instrument called a phoropter in front of your eyes and shows you a series of lens choices. He or she will then ask you which of the two lenses in each choice looks clearer. Based on your answers, your eye doctor will continue to fine-tune the lens power until reaching a final eyeglass prescription. Source |
Colin asked "If your eyes are like 90% water, why don’t they drip out?” Between the cornea and the iris and between the iris and the lens are two small cavities. These cavities are filled with a clear watery fluid known as aqueous humor. This fluid aids good vision by helping maintain eye shape, providing support for the internal structures, supplying nutrients to the lens and cornea, and disposing of the eyes' cellular waste. The large cavity in back of the lens (the center of the eyeball) is filled with a clear gel-like substance called vitreous humor. Light passing through the lens on its way to the retina passes through the vitreous humor. The vitreous humor is 99 percent water and contains no cells. It helps to maintain the shape of the eye and support its internal components. Source | The sclera provides the outer protective covering that gives structure to the eye. It's like the rubber part of a ball. |
Chloe asked "Why do you tear?" Here's a fascinating TEDEd that describes 3 different kinds of tears that we shed each with their own characteristics. | |
Jack C asked “What are the muscles in your eye?” Jake asked “How do your eyes turn?”
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