eli5: If the human eye can only see 10 million colors, how do we know there are 18 decillion colors in the universe in total.
In: Biology
The human eye is not perfect enough to see all colors and visions, almost like the camera is not as good as the human eye.
Due to experiments with animals, it is determined that some animals can see colors that the human eye cannot detect. For example, the eyes of insects have UV vision, while snakes, some fish, and frogs see with infrared vision.
This means that those animals cannot see the colors that the human eye can detect. Therefore, it is assumed that these animals can see other colors that the human eye cannot detect.
Color is just the name humans have given to wavelength of light, those that humans can see. Thought physics we know that they are more wavelength than those, notably the edges of our vision: infrared and ultraviolet.
Colour is strange, as it isn’t solely linked to physics but to the biology of our eyes.
I have no idea how you could put a number on the possible colours in the universe.
More detail: we have, in our eyes, rods (sensitive to light and dark) and cones (sensitive to either red, green or blue). As such a particular colour doesn’t need to correspond to a single wavelength of light – purple, for example, is when both a red and a blue cone are equally active. There cannot be a pure beam of purple.
Other creatures have different cells in their eyes, so could have crazy numbers of overlap colours – purplish green that to us would be white, for example.
I have no idea where you got your 18×10^33 number from. A “color” is just a language convention for a certain part of the light spectrum. Since EM spectrum is continuous, you can have an arbitrary number of colors fit within it. In English, light between 570 and 590nm is called “yellow”. There’s no reason we can’t break that up and say “570-580” is “yellow” and “580-590” is “murgle”. Some color names like “pink” or “brown” don’t exist on the light spectrum; they only exist when our eyes react to two or more different wavelengths and our brains combine those signals and attribute a name to them. And given that our eyes are limited to only a part of the EM spectrum, we know there are wavelengths outside that some other animals can perceive, or whose effects we can empirically prove, so we have given them names as well like “ultra violet” (btw, people with cataract surgeries often can perceive far into the violet spectrum, beyond normal eyes). So it’s pretty arbitrary, and you can attribute any number to your color count, as long as you have the words for it, as it’s more of a language than physics/biology question. Heck, there are languages who only recognize three or four colors in total. Japanese to this day isn’t clear on green-blue split, and there are languages that have different words (and thus consider them separate colors) for what English would call dark and light blue. Irish makes a distinction between what in English is “green” and Hungarian has two terms for “red” (and no, they are not light and dark red). And there are languages with three terms only – light, dark and red.
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