The Human Eye

Anatomy of the Eye, Vision: Light & Color, Normal Vision and Vision Problems.

Eyes are organs that detect light. The human eye is wrapped in three layers of tissue. The first layer is called the sclerotic coat. This tough layer creates the "white" of the eye except in the front where it forms the transparent cornea. The cornea admits light to the interior of the eye and bends the light rays to that they can be brought to a focus. The surface of the cornea is kept moist and dust-free by secretions from the tear glands. The next layer of the eye is called the choroids coat. This middle layer is deeply pigmented with melanin. It reduces reflection of stray light within the eye. The choroids coat forms the iris in the front of the eye. This, too, is pigmented and is responsible for eye "color". The size of its opening, the pupil, is variable and under the control of the autonomic nervous system. In dim light (or when danger threatens), the pupil opens wider letting more light into the eye. In bright light the pupil closes down. This not only reduces the amount of light entering the eye but also improves its image-forming ability. Finally, there is the most well known layer of the eye: the retina. The retina is the inner layer of the eye. It contains the light receptors, called rods and cones. The retina also has many interneurons that process the signals arising in the rods and cones before passing them back to the brain. All the nerve impulses generated in the retina travel back to the brain by way of the axons in the optic nerve. At the point on the retina where the approximately 1 million axons converge on the optic nerve, there are no rods or cones. This spot, called the blind spot, is thus insensitive to light.





In the human eye, light enters the pupil and is focused on the retina by the lens. Light-sensitive nerve cells called rods (for brightness) and cones (for color) react to the light. They interact with each other and send messages to the brain that indicate brightness, color, and contour. Although cones operate only in relatively bright light, they provide us with our sharpest images and enable us to see colors. Most of the 3 million cones in each retina are confined to a small region just opposite the lens called the fovea. So our sharpest and colorful images are limited to a small area of view. Because we can quickly direct our eyes to anything in view that interests us, we tend not to be aware of just how poor our peripheral vision is. Rods are extremely sensitive to light. A single photon (the minimum unit of light) absorbed by a small cluster of adjacent rods is sufficient to send a signal to the brain. So although rods provide us with a relatively grainy, colorless image, they permit us to detect light that is over a billion times dimmer than what we see when its sunny.




Most common vision problems occur in the lens. The lens is located just behind the iris. One common problem is Farsightedness. If the eyeball is too short or the lens too flat or inflexible, the light rays entering the eye — particularly those from nearby objects — will not be brought to a focus by the time they strike the retina. Eyeglasses with convex lenses can correct the problem. Farsightedness is called hypermetropia. Another common problem is called Nearsightedness. If the eyeball is too long or the lens too spherical, the image of distant objects is brought to a focus in front of the retina and is out of focus again before the light strikes the retina. Nearby objects can be seen more easily. Eyeglasses with concave lenses correct this problem by diverging the light rays before they enter the eye. Nearsightedness is called myopia. The term color blindness is something of a misnomer. Very few (~1 in 105) people cannot distinguish colors at all. Most "color-blind" people actually have abnormal color vision such as confusing the red and green of traffic lights. As high as 8% of the males in some populations have an inherited defect in their ability to discriminate reds and greens. Abnormal blue sensitivity occasionally occurs in humans but is much rarer than abnormalities in red-green vision. The gene for the blue-cone opsin is located on chromosome 7. Thus this trait shows an autosomal pattern of inheritance being found in females as often as in males.

Work Cited

Montgomery, Ted. “Anatomy, Physiology, Pathology of the Human Eye.” Ted Mongomery.com. 2000. 25 February 2008. <>.

“Eye Anatomy.” St. Lukes. St. Lukes Cataract and Laser Institution. 1998-2008. 26 February 2008. .

“Human Eye.” The Journey. The Journey Home. 2002. February 26 2008. .

Rainbows

Light is an important part of daily life. Growing plants use photosynthesis and light is a key component in this process. In order to hunt their prey or to escape a predator animals also need light. Some say that light has helped humans evolve into the complex beings that we are today. When we wake up in the morning until we fall asleep we are surrounded by light. Everything we look at in our surroundings is light. Do we ever stop and think about what we are actually seeing? Our eyes can only really see light, so are we really connected to these objects? On earth we can see one of the “most spectacular light shows.” This light show is a rainbow. How do we see the wide array of colors? It is all because of light (Freudenrich)!
A traditional rainbow is all different colors of the spectrum spread out and then diverted into the eye by small drops of water. Each raindrop contributes only one color to the rainbow that you see! Where is the sun when you see a rainbow? It was probably behind you while the rain was probably somewhere in front of you. Because the “sun is behind you and the rain is in front of you the sunlight must have been bouncing off the raindrops and reflecting into your eyes.” This is confusing because sunlight is primarily white light and that is what is reflecting off the raindrops, how do we see all of the colors of the rainbow? I found out that “these colors are present because the sunlight is not only reflecting off of the raindrops, but it is also refracting and dispersing in the raindrops.”
Rene Descartes was the first person to clearly discuss the formation of a rainbow by raindrops. In his studies he simplified the studies on rainbows by “reducing it to one rain droplet and how it interacts with the light falling upon it.”
He writes:"Considering that this bow appears not only in the sky, but also in the air near us, whenever there are drops of water illuminated by the sun, as we can see in certain fountains, I readily decided that it arose only from the way in which the rays of light act on these drops and pass from them to our eyes. Further, knowing that the drops are round, as has been formerly proved, and seeing that whether they are larger or smaller, the appearance of the bow is not changed in any way, I had the idea of making a very large one, so that I could examine it better.
Descartes describes how he held up a large sphere in the sunlight and looked at the sunlight reflected in it. He wrote "I found that if the sunlight came, for example, from the part of the sky which is marked AFZ

and my eye was at the point E, when I put the globe in position BCD, its part D appeared all red, and much more brilliant than the rest of it; and that whether I approached it or receded from it, or put it on my right or my left, or even turned it round about my head, provided that the line DE always made an angle of about forty-two degrees with the line EM, which we are to think of as drawn from the center of the sun to the eye, the part D appeared always similarly red; but that as soon as I made this angle DEM even a little larger, the red color disappeared; and if I made the angle a little smaller, the color did not disappear all at once, but divided itself first as if into two parts, less brilliant, and in which I could see yellow, blue, and other colors ... When I examined more particularly, in the globe BCD, what it was which made the part D appear red, I found that it was the rays of the sun which, coming from A to B, bend on entering the water at the point B, and to pass to C, where they are reflected to D, and bending there again as they pass out of the water, proceed to the point.”
This quote shows how the shape of the rainbow is explained. You can simplify this analysis by considering the path of monochromatic light through one raindrop. If you imagine how light is refracted through the raindrop, reflected inside the raindrop, curved, mirror-like surface of the raindrop, and last but not least how it was refracted as it emerges from the drop. We can apply the results for a single raindrop to a whole collection of raindrops in the sky. By applying these results we can visualize the shape of the bow.
What makes the colors in the rainbow? A traditional rainbow is made up of seven colors. ROY G. BIV (red, orange, yellow, green, blue, indigo, and violet). But despite that these seven colors are the ones visual to our eyes, a rainbow is actually a continuum of colors from red to violet and beyond these are colors that we can see. A rainbows color comes from two basic facts:
· “Sunlight is made up of the whole range of colors that they eye can detect. The range of sunlight colors, when combined, looks white to the eye. This property of sunlight was first demonstrated by Sir Isaac Newton in 1666.”
· Light of different colors is refracted by different amounts when is passes from one medium (air, for example) into another (water or glass, for example).
Two scientists, Descartes and Willebrord Snell, made a discovery. This discovery was how a ray of light is bent, this is also known as refraction, as the waves travel through different densities or materials. When light paths are traced through a raindrop, for example red and blue light, a person would find that the angles of deviation would differ. The blue light is refracted more than the red light.

The Picture below shows a faint secondary rainbow above then a bright primary rainbow. There are also several pastel-shaded rainbows inside the primary rainbow. While the additional rainbows can not be explained by geometric optics, they have their own term which is called “supernumerary.” But we do have explanations for the primary rainbow and the secondary rainbow. The primary rainbow comes from a “single internal reflection of refracted light inside a raindrop.” While the secondary rainbow results from “a double internal reflection (About Rainbows)”




In order to see a rainbow we need sunshine and falling rain. Rainbows are very rare. Rainbows are much rarer than we think. Halos are seen more frequently. The best times to see rainbows are in the late afternoon and the early morning because in order for us to see rainbows the sun cannot be too high in the sky. The lower the sun is in the sky the higher the bow is. A rainbow is not just a set of colored rings, the sky inside of the rainbow is bright as well because raindrops direct the light there as well (Bianco).










Works Cited

Bianco, Carl. "how stuff works." How Vision Works 02/21/2008 . http://www.howstuffworks.com/eye.htm.

Freudenrich, Craig. "How Light Works." How Stuff works 02/24/2008 .
The National Centor for Atmospheric Research & the UCAR Office of Programs. "About Rainbows." 2/29/2008 .

Rainbows, Mirages and the Color of the Sky and Sunset

There are many spectacular phenomenons in nature including rainbows, mirages, and the color of a beautiful sunrise or sunset. Often, these beautiful images seem unexplainable when actually they can be quite simply elucidated by the natural laws of physics. Rainbows, mirages, and the color of the sky and sunset are examples of how physics can make possible what seems impossible.
It is known that a prism is an object used to break down white light into different spectrums ("Rainbows"). The different spectrums that appear on the other side of the prism are the colors that make up a rainbow: red, orange, yellow, green, blue, indigo, and violet. The prism allows the colors to split up because each spectrum reacts to the class of the prism in a different way ("What Causes a Rainbow"). The glass forces each spectrum to bend, or refract, light at different angles which explains the separation of the colors ("Rainbows").
The idea of rainbows reinforces the concept of prisms. In a rainbow, each raindrop acts as a prism ("Rainbows"). Light enters a raindrop and is refracted through the other side and dispersed into different spectrums ("Rainbows"). Rays of sunlight that are refracted in a raindrop are the rays that create a rainbow ("What Causes a Rainbow"). Every raindrop that adds to the formation of a rainbow is responsible for only one color ("Rainbows"). For example, one raindrop might contribute to the blue ray of a rainbow, while another raindrop may add to the indigo ray. Although each raindrop does act as a prism and splits the sun’s light into all the colors in the spectrum, they are so small; only one beam of light is emitted that contributes to the overall rainbow ("Rainbows"). Raindrops that contribute red light are highest in the sky while those that contribute violet light are lowest. A point called the antisolar point, which is 180 degrees from the sun, determines where a rainbow is formed ("Rainbows"). Consequently, although a rainbow is a beautiful image, it can easily be explained by the laws of physics.
A mirage is another example of a phenomenon that physics helps us to understand ("Mirages"). Basically, mirages are created when there are two separate layers of air that are at different temperatures. The border between warm and cold air can bend light, similarly to a prism because cold air is denser than warm air ("Mirages"). This is essentially how a mirage is created. Often when the boundary between layers of warm and cold air is bent or curved, mirages act as mirror images of images far away ("Mirages"). They can also magnify these distant places and make them look much closer than they really are. A Fata Morgana is a mirage that occurs above water ("Mirages"). Before mirages were identified and explained, people saw them at sea and believed their destination was much closer than it really was. Mirages were even once thought of to be the work of witches ("Mirages").
There is also a physics explanation for the color of the sky at sunrise and sunset ("Blue Skies and White Clouds"). It is often wondered why the sky appears blue in the middle of the day, but red in the morning and evening. This can be explained by the laws of the scattering of light in the atmosphere ("Blue Skies and White Clouds"). During the middle of the day, or noon, the sun’s light passes through a thinner layer of atmosphere than it does during morning or evening. Not much of its light is scattered, which is the reason why it appears to be practically white ("Blue Skies and White Clouds"). Molecules in the atmosphere help to scatter the sun’s light. Humans see the sky as blue because the light being scattered is from the blue end of the spectrum.
This is not true of clouds, because water droplets that make up clouds are of a greater size than the molecules in the atmosphere that scatter the sun’s light. Because these droplets are larger than the molecules in the atmosphere, light is allowed to scatter color equally. Because color is being scattered equally, clouds are seen as white ("Blue Skies and White Clouds").
It is clear that even natural phenomenons such as rainbows, mirages, and the color of sunsets, sunrises, and the sky can be explained by physics’ simple laws. Because physics can help explain these natural occurrences, people are less fooled by mirages that create false images; astounded by the appearance of a rainbow, and puzzled over why the sky is red sometimes and blue at others. By looking at these examples, it is plain to see that physics can explain what was believed to be unexplainable.

Works Cited:

Department of Physics and Astronomy, Arizona State University. "Rainbows." Patterns in Nature: Light and Optics. 1995-2000. 20 Feb 2008. http://www.acept.id.asu.edu/PIN/rdg/rainbow/rainbow.shtml#top

Department of Physics and Astronomy, Arizona State University. "Blue Skies and White Clouds." Patterns in Nature: Light and Optics. 1995-2000. 20 Feb 2008.
http://www.acept.id.asu.edu/PIN/rdg/sky/sky.shtml.

Kryslek, Lee. "Mirages in the Sky." 1996, 1998. 20 Feb 2008. http://www.unmuseum.mus.pa.us/mirage.html.>

"What Causes a Rainbow?" How Stuff Works. 20 Feb 2008. http://www.howstuffworks.com/question41.htm.

Young, Andrew T. "An Introduction to Mirages." 1998-2008. 20 Feb 2008. http://www.mintaka.sdsu.edu/GF/Mirages/mirintro.html.