What is it with these color combinations (red-green and blue-yellow) that a person with a certain type of color blindness make them hard to distinguish and differentiate?

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What is it with these color combinations (red-green and blue-yellow) that a person with a certain type of color blindness make them hard to distinguish and differentiate?

In: Biology
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Our eyes have three different receptors to perceive colour. Specific pigments that change something (a protein signal) when a particular set of wavelengths hit them. Someone with red-green colour blindness has a “faulty” pigment that doesn’t react to the wavelengths that make red and green look different.

Visible light exists on a spectrum from around 380 to 700 nanometers of wavelength. Your eyes have three types of color receptors, each centered on different points on that spectrum.

However, these color receptors don’t just detect light precisely at their designated wavelength. They merely detect light most brightly at that wavelength. At wavelengths close to their point, they detect the light but more dimly.

So if you’ve got incoming light that is midway between your red and green color receptor, the information received by the eye is “a bit dim” on both receptors – and your brain can guess that the light is midway between those two colors (because the blue color receptor is registering ‘very dim’).

What happens with most colorblindness is that these central points for the color receptors are too close together. For example, your ‘red’ receptors might not actually be ‘red’ but more ‘orange’. Having these receptors too close together makes it more difficult to determine colors in that range.

So, what we perceive as colour is actually our eyes detecting a quality of the light we see called “wavelength”. Light behaves like a wave, and the wavelength is the distance between the crests of each individual wave. We can see light in a range from about 380nm to 700nm. nm is nanometers, a unit of measurement so small that your brain can’t even comprehend how small it is. Short wavelengths, towards the 380nm end of this “visible spectrum” we call blue light, and long wavelengths, towards the 700nm end, we call red light. Light can also come in wavelengths longer and shorter than this, such as infrared and ultraviolet, as well as X-rays and radio waves.

When light collides with molecules in our eyes called Opsins, energy is transferred to those opsins and this causes the opsin to release an electrical signal (in simple terms). That electrical signal gets sent as a message to the brain saying “Cell #1842 has just detected light!” (again, in simple terms). This is how we see light. Now, the thing about opsins is that they only respond to specific ranges of wavelengths. If light of another wavelength hits them, they don’t do anything. This makes them specific. Opsins also respond to different parts of their bands at different strengths. The same opsin will have a bigger response to say, 380nm (very blue) than it would have to 500nm (quite green).

We have four kinds of opsins relevant to colour perception. The first is rhodopsin, which is located in rod cells in the eye. This is extremely sensitive: It has a big response even to just small amounts of light. However, it’s also not very selective. It responds to a wide band of colours (pretty much the entire visible spectrum). Rhodopsin can’t detect colour because of this, only the presence or absence of light. This makes it great for seeing in the dark, and is also the reason that everything looks black and white at night: All light gets interpreted as white. The other three are called L, M and S, for Long, Medium and Short – Long corresponds to long wavelengths, Short to short, and as you may have guessed, Medium also corresponds to long, but slightly less long than Long (there’s an interesting reason for that I’ll describe later).

These opsins are best described by something called their “absorption maxima”. That’s the wavelength at which they respond to light the most. They still respond to longer and shorter wavelengths, but not as much, and they respond less the further away in either direction you get. This creates curves of wavelengths they respond to, which overlap with the other opsins, best displayed using a chart that looks something like [this](https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Fwww.researchgate.net%2Fprofile%2FZachary_Aidala%2Fpublication%2F279528962%2Ffigure%2Ffig2%2FAS%3A669510981922823%401536635265851%2FDifferences-in-wavelengths-of-maximum-absorbance-of-the-three-cone-opsins-S-M-and-L.jpg&f=1&nofb=1). Note that if you were to pick any given wavelength on the X axis along the bottom, you could draw a line up from it and it would cross over at least one of the curves, with the curves representing how much light gets absorbed by a particular opsin.

Each line you drew in this way would have a unique number for its absorbance on the S, M and L curves. The absorbance on each curve is used by the brain to calculate the wavelength of the light, based on how opsins respond to it. Lets say then that I shine a light at your eye that’s specifically 500nm only. We consult the chart and draw the line up. First our line crosses the S curve, and it’s very low on the S curve – the S opsin doesn’t absorb this very well. In your eye, your S opsins aren’t doing much. They’re sending a small response to your brain, but not a big one. Next it crosses the L line at around 50% absorbance, and just above that, it crosses the M line at at around 75% absorbance. In your eye, your M opsins are giving a big response, and your L ones a medium response. Your brain sees the information coming from each opsin and figures out “Ah, this light must be 500nm!” Or well, it doesn’t specifically do that, but it goes “Ah, this light must be green!”, because this is light your brain would interpret as green. A longer wavelength, say 600, wouldn’t trigger the S opsins at all, would trigger the L opsins a lot more, and trigger the M opsins a lot less. To your brain, this would look like a yellow or orange.

Now, in partial colourblindness, one of these colour-sensitive opsins doesn’t work properly. it can break in different ways, but for sake of example, we’ll say that you have [protanopia](https://upload.wikimedia.org/wikipedia/commons/a/af/Color_blindness.png) on this chart. What this means is that you can’t properly distinguish between red and green. Why? Because your L opsins don’t work. In protanopia, the L opsin is non-functional. To properly detect colour, at least two different opsins need to be triggered, but for you, light with wavelengths of around 530nm to 700nm only trigger one opsin. Your brain has to figure out the colour based only on the amount of response that the M opsin is giving, and can’t compare it to the amount of response an L opsin is giving, so it interprets light of these wavelengths as a kind of yellowy blur that gets less intense towards 700nm simply because the M opsin is having a lesser response here, without an L opsin’s higher response, as would be normal.

So as for why M is really just shorter-Long, this is because most animals actually have *five* opsins, sometimes even more, with four opsins that can detect colour, rather than three, and these are pretty evenly distributed across the wavelengths, creating something like a Very Short, Medium-short, Medium-long and Very Long band (for many, the Very Short goes into the ultraviolet – wavelengths humans can’t see). However, in the Triassic period, our mammalian ancestors became nocturnal, and in the darkness lost the two medium opsins, retaining only the Short and Long. When mammals emerged back into the daytime, many mammals continued to possess only two opsins, which is why dogs are colourblind. Primates re-evolved a third opsin, but this opsin was inferior. It was the result of duplication and subsequent mutation of the gene that codes for the Long opsin. It mutated towards the middle of the visible spectrum, but it didn’t do a great job of it, so in humans today it’s still closely related to its Long ancestor, leaving us not too great at distinguishing colours in the middle of the spectrum compared to the ends. There are many animals who would consider us colourblind, as they can distinguish colours we can’t thanks to having more and better distributed opsins.