The Human Eye
Normal human eye sight is made up of cones and rods with a greater number of cones around the central region than in the peripherals. The eye is also characterized by having a region of dense photoreceptors in an area of the retina known as the fovea. It is here that the lens in the eye focuses the majority of light upon. In and around this space is where focused vision takes place. The region where this takes place is know as the macula and within it no rods are present. In the eye there are three type of cones and a single type of rod. While rods are responsible for black and white light as well as luminosity, it is the cones that are responsible for colour. As light enters the eye and is focused by the lens, the image is inverted and the information absorbed by the cones and rods is sent upside-down through the optic nerve. The optic nerve is also a blind spot in the eye where no information is gathered. Once the occipital lobe of the brain gets the information, the image is once again inverted restoring the object to the upright position. Additionally, the missing information from the blind spot is filled in by using information collected from the second eye.
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Below are two videos on inverting eyesight and the blind spot within your eye.
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The Cones and the Rods
The Structure of the cones and rods is a bit odd and seems almost nonsensical upon first inspection. The photoreceptor cells are placed backwards or so it would seem with the absorption site located beneath the axons and the body of the cells. However there is an important reason for this, the ganglion cells before the rods and cones act like fiberoptic cables further concentrating the light to where it needs to be. [1] |
The cones and rods pick up various wavelength of light and combine them to create the colours we see. In a normal human eye the wave lengths of light are picked up by the following: Rods from 4-600 nm in black and white, and cones from 350-800 nm in colour. The three types of cones are named short, medium, and long cones. This is not for the physical length of the cone's structure but rather detonating the wavelength of light they can pick up. The short cones focus on blue light, while the middle cones absorb green light, and the long cone gather red light. By combining the light absorbed by our eyes we can see other colour like cyan and yellow. In total the human eye can perceive up to 10,000,000 colours. Although this number is constantly changing, as it is an estimate, the lowest among estimates is still 7 million colours. [2]
The image on a black background is an actual representation of the colours we can observe and the sensitivity we can observe them at. Looking above at the troughs in the waves, this is where we have lesser sensitivity to the colours we see. This is the primary reason why in every colour chart cyan, teal, and turquoise take up so little space while greens seem to dominate. |
The King of Sight
Many animals have superior vision to us humans, just like the cephalopods mentioned in the blind spot video. Some of these animals may see rather obvious, like an eagle or owl. But the most interesting and exotic eyes come to us from a rather unassuming place - The mantis shrimp.
This shrimp's weird looking eyes have an astonishing number of cone photoreceptors. In humans we have three, the mantis shrimp has twelve! Not only does it have far more photoreceptors than us, but its eyes move independently from one another scanning the environment separately. This shrimp sees all visual light, a bit more infrared light and a lot more ultraviolet light. In the centre of the eye you will notice a clear separation by a specialized structure, this is known as the midband and is where the majority of photoreceptors are located. |
The shrimp's eyes have at least five photoreceptors functioning in the ultraviolet (UV) range but interestingly, they only contain two UV opsin. An opsin is a pigment that absorbs light optimally around a specific range. In the mantis shrimp, these UV opsin are tuned to 384 and 334 nm. This means there must be another tuning device involved in the shrimp's sight, and as it turns out, there is. Within the midband that bisects the eye, there are six horizontal rows of ultraviolet coloured filters. These colour filters are normally invisible to us but can be observed by their weak ultraviolet simulated autofluorescence. These filters block specific wavelengths of light diversifying the UV sensitivity of the midband UV photoreceptors. This process produces the full range of visual sensitivity we expected the shrimp to have. [3]
Below is the UV photoreceptors with and without the UV colour filters present within the midband of the eye.
Human, mantis shrimp eye comparison with the full range of photoreceptors.
To learn about colour in our environment click below.
References:
[1] http://www.dailymail.co.uk/sciencetech/article-3000797/Why-eyes-wired-BACKWARDS-Strange-retina-structure-enhances-vision-study-reveals.html
[2] http://hypertextbook.com/facts/2006/JenniferLeong.shtml
[3] https://www.youtube.com/watch?v=9aNc3o4GvAw
Images:
References are beneath every image and here in order from left to right, top to bottom.
https://commons.wikimedia.org/wiki/File:Poison_Green_Forest.jpg
http://www.dralexerdie.ca/your-eye-health/how-the-eye-works
https://www.sciencenews.org/article/how-rewire-eye
https://upload.wikimedia.org/wikipedia/commons/9/94/1416_Color_Sensitivity.jpg
http://photo.net/learn/optics/edscott/vis00010.htm
http://phys.org/news/2013-09-mantis-shrimp-world-eyesbut.html
https://www.youtube.com/watch?v=9aNc3o4GvAw
[1] http://www.dailymail.co.uk/sciencetech/article-3000797/Why-eyes-wired-BACKWARDS-Strange-retina-structure-enhances-vision-study-reveals.html
[2] http://hypertextbook.com/facts/2006/JenniferLeong.shtml
[3] https://www.youtube.com/watch?v=9aNc3o4GvAw
Images:
References are beneath every image and here in order from left to right, top to bottom.
https://commons.wikimedia.org/wiki/File:Poison_Green_Forest.jpg
http://www.dralexerdie.ca/your-eye-health/how-the-eye-works
https://www.sciencenews.org/article/how-rewire-eye
https://upload.wikimedia.org/wikipedia/commons/9/94/1416_Color_Sensitivity.jpg
http://photo.net/learn/optics/edscott/vis00010.htm
http://phys.org/news/2013-09-mantis-shrimp-world-eyesbut.html
https://www.youtube.com/watch?v=9aNc3o4GvAw