Why Series: Why Do We have Different Eye Colors?

Eye color is a polygenic phenotypic character determined by these distinct factors: melanin and collagen deposits, and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris. The color of your eyes depends on how much of the pigment melanin you have in your iris—the colored part of your eyes, and how much melanin and collagen deposits you have in your stroma. Shortly, the more pigment you have in your eyes, the darker they will be. This means that green, blue and gray eyes are lighter because they have less “turbid medium in the eye”. Why, and in which sense, the amount of melanin and collagen deposits determines the color of an eye? The fact here is that…your eyes aren’t blue (or green) just because they contain pigmented cells. There is actually more physics involved here than you may think! The eye color is actually structural. Our beautiful colored part of the eye is called the iris, and it’s made up of two layers. One is called “epithelium”, at the back of the eye, and the other is the “stroma”, at the front. The epithelium is very very thin, (it is only two cells thick) and contains black-brown pigments. Sometimes, you can notice some people have some dark specks in their eye. It is, in fact, the epithelium peeking through. The stroma is made up of colorless collagen fibers. But it can also contain melanin. Fascinatingly, the amount of collagen over the amount of melanin it’s actually what controls our eye color. This is exactly how things work: Brown eyes Brown eyes are eyes rich in melanin in their stroma, containing a huge amount of it, which is able to absorb most of the light entering the eye regardless of collagen deposits, giving them their dark color. I often hear brown-eyed people complaining about their eye color. Don’t be silly! You got rich-melanin eyes! This should be included on your strengths list. Green eyes Green eyes don’t have much melanin in them, but they also have no collagen deposits. This means that, as in brown eyes, some of the light entering them is absorbed by the pigment. But at the same time, the particles in the stroma scatter light as a result of something called the Tyndall effect, which creates a blue hue. Combined with the brown melanin, this results in the eyes appearing green. Last but not least, let’s talk about blue eyes. People with blue eyes have a completely colorless stroma. There is no pigment at all in it, and it also contains no excess collagen deposits. This means that, while brown and green eyes absorb light, all the light that enters in a blue eye is scattered back into the atmosphere and, as a result of the Tyndall effect, creates a blue hue. This is interesting because it means that blue eyes aren’t always “that blue”. It actually depends on the amount of light available when you look at them! We know all of this is just mind-blowing! Could you imagine that your eye color actually depends on melanin, collagen and their response to light? Could you imagine that all of this is regulated by the laws of physics? Ok, but as we’ve seen, the scattering of light, when it happens, is kinda directed toward blue. It’s like he has a preferred color (or, as scientists would say, a preferred wavelength). Why? “We’re gonna answer the question, but let us know what you think about the video so far! Leave us a like if you’re enjoying it, and a dislike if you think we can do better!. And If you would like to help improve the quality of our content, please check us out on patreon, we would like to say a big thank you to all those who show their support.” Because the Tyndall effect, also called the Tyndall phenomenon, is the scattering of a beam of light by a medium containing small suspended particles—e.g., smoke or dust in a room, which makes visible a light beam entering a window. In Tyndall scattering, short-wavelength blue light is scattered more strongly than long-wavelength red light, just as the more famous Rayleigh Scattering. However, Rayleigh scattering occurs from particles much smaller than the wavelength of light, while the Tyndall effect occurs from particles roughly the same size as the wavelength of light. Also, Rayleigh scattering it’s responsible for the color of the sky. So our last why is: why does the Rayleigh scattering determine the blue color of the sky? And our last “because” will include some math. So this is just for you, nerdy guys! If we have a molecule and we send an electric field of light towards it, the electric charges of the molecule will move in response to the oscillating electric field. In this case, it turns out that the Rayleigh scattering Intensity for a single molecule is given by the following formula: The strong dependence of diffusion on the inverse of the wavelength (1/lambda^4), implies that blue light is scattered much more than red light. In fact, blue light is characterized by a shorter wavelength than red light, making the term (1/lambda^4) bigger in the case of blue light, which results in a major intensity of the scattered blue light over the red scattered light. In the atmosphere, therefore, the “blue photons” are scattered when the wave crosses the sky, and this is the reason why you can see blue light coming from all regions of the sky while the other photons derive more directly from the sun. The Rayleigh diffusion is also responsible for the red color that objects, (for example clouds), take on at dusk or dawn. In fact, in these conditions, the solar rays cross a greater thickness of the earth’s atmosphere and therefore encounter a greater number of diffusing centers, so that not only the blue photons but also the yellow ones are diffused. The result is that sunlight is deprived of all components of the spectrum except red. However, the sky remains blue due to the large number of blue photons always scattered in the upper atmosphere. As we can read on Wikipedia: “At this point, it is natural to ask why the sky is blue instead of purple since, according to Rayleigh’s law and the inverse dependence on the fourth power of the wavelength, it would be natural to expect a sky of this color. One of the factors is given by the fact that the human eye is more sensitive to the wavelength corresponding to blue light rather than violet, having photoreceptors that have a greater sensitivity for this color. Furthermore, the light from the sun is composed of more photons in blue than in violet. The “celestial” color we see therefore derives from the superimposition (a ” weighted average”) of the colors that come to us from the sky, especially purple, blue and, to a lesser extent, green.” “This video ends here! Thanks for watching everyone! What’s your eye color? Do you like this “why series”? Is there anything more you want to hear? Let us know in the comments below, be sure to subscribe, and I’ll see you next time on the channel!”

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