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This has a pretty interesting answer I think!

It is because of our brains and language.

You see, colors do not actually exist. What we perceive as colors are just different wavelengths of light.

We don't have to go into what wavelengths are, just see them as a quality light has that can differ.

Eyes evolved to see a certain spectrum of light. That means a certain slice of all wavelengths light can have, the rest is invisible to us; like radio waves or x-rays!

The human brain evolved to divide this visible light so we can make out subtle differences, most likely like seeing a tiger in the grass, this is the sensation of color.

So actually, rainbows are just a gradient of wavelengths, but we perceive that as individual colors.

But the weird thing is, just how individual they are perceived to be seems to be closely linked to language.

Some languages only have three words for all colors: red, black and white. What seems like blue to you, a native speaker of that language might say is black. What you say is purple they might say is red.

Some languages have four colors and so on.

English, not counting hues, generally has 6 main colors. Red, orange, yellow, green, blue, (indigo), violet. Does that ring a bell?

That's the colors the rainbow is said to have in English (and many other languages). Isn't it strange and kind of awesome?

The brain and how it perceives colours is an interesting thing. Take magenta for example, in reality that colour doesn't exist but it's our brains filling it un to make sense for us. Also the colour yellow is seen by everyone differently as our eyes only have red, blue and green rods so it takes information from the green and red cones and fills in the blanks. I'm sure I read somewhere that goldfish have yellow rods un their eyes so can truly see what the colour yellow us. Also look up about impossible colours, it's fascinating what our brains do and how we perceive what we "see" is just the brains interpretation of data/information.

and the impact of language is shown by the notion that there are 7 colours in the rainbow. Whilst a rainbow is actually a spread across colour, if you were to divide it, it would better fit into ROYGBV and skip indigo.

To continue the question why does the rainbow have different wavelengths in different areas and why is it always in the same order? Is the rainbow always the same ratio of size of every “colors“ field compared to the total width? This is so fascinating.

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The idea that the rainbow has seven colors actually comes from Isaac Newton, who greatly advanced our understanding of light. He understood that it was a continuous spectrum, but made many analogies to musical notes. The wavelengths of the “seven colors” are proportional to the wavelengths of the notes of one octave of a major scale.

The brain doesn't fill anything in. It has 3 possible inputs from the 3 rod types. What we call colors are just combinations of those inputs.

Different wavelengths are bent at different angles, which is why you always see them in the same order.

Yes, every rainbow looks the same. Rainbows are an illusion caused by the way light bounces around inside spherical drops of water. The angle light is bent as it passes from air to water and back again depends on wavelength.

The brain fills in all kinds of information. Not just colours.

For instance, our eyes have multiple blindspots. But the brain fills in the details - by making em up.

Want to test that? Hold both your thumbs straight out. Look at the left one. Slowly move your right one to the right. And OMG did it disappear? Surely not? No, thats the blindspot where the optic nerve meets your eye.

This is more than just artistic. Blue and red are close to an octave apart (a factor of 2 in frequency).

Fun fact. The Japanese don’t have a separate word for ‘green’ and ‘blue’. They named the part of the visible spectrum that, to English speakers, is between what we call ‘green’ and what we call ‘blue’. What we call a ‘green light’ (on a traffic light) they call ‘blue light’, what we call ‘green apple’ they call ‘blue apple’ etc.

There used to be 5 colors until Newton

They are frequencies, it's the same order because red is the lowest frequency and violet is the highest frequency. When light hits water its path is bent based on it's frequency.

yeh, and 7 was a sacred number

Perhaps saying "interprets" rather than "fills in" is more accurate when talking about colours. Either way the brain processes that information and what we "see"is the result. Also I think it's cones that are more to do with colour rather than rods so I got mixed up with that.

That's not exactly true. The word 'midori' means green, and has been in use since the heian period (8th century) but was considered a shade of 'ao' ( blue) until early twentieth century, where midori has been more frequently used to describe what English speakers would call green. You are correct about traffic lights, and about certain green plants being called 'ao'.

I tried, the right one never disappeared.

A better test. With both eyes open you do not see your nose. Close one of your eyes and you nose appear.

This one is kind of hard to explain without pictures... or in a timely manner. Basically when the visible light goes through the medium that splits it into the rainbow it changes angle, and part of what determines how much the angle changes is the wavelength. The reason the order and ratio is the same is because the rainbow is basically the colors ordered from longest to shortest wavelength spead a based on the change in wavelength. For instance, if you pointed a light that contained all wavelengths but yellow at a prism the rainbow would look the same as it would with all colors with a missing band where the yellow would be because the absence of yellow light does not affect the wavelengths of the other colors. That last bit is actually super important in a handful of scientific fields.

Is there a reason why a camera would pick up the same view as a person? Or is that our interupertation of the picture or what we are seeing in the camera/photo?

the reason we perceive the spectrum as bands is primarily a function of our eyes and how they detect light at three main wavelengths. Whether the person considers the intermediate colors as unique or part of the dominant RGB bands, though, is generally socio-linguistic.

When you look carefully at a spectrum, there is no segregation between colors, the colors transition instead. We humans divide it into "bands" (unique colors) by choice, not because such bands are actually there. But ask someone where the line between colors actually and specifically happens, and I bet you would get lots of different answers.

The seeing of bands of color is actually more about the human retina and cones. As such the cones are more sensitive to particular wavelengths of electromagnetic energy (for brevity we call these RGB, the perception of yellow is the RG both being stimulated) and less sensitive to other wavelengths. At this level it does not involve the brain/language as implied by this post. Obviously categorizing and naming colors is the brain activity; as is the opponent process, but that's not particularly relevant to the retinal activity that "creates" the bands.

The language bit here is about how people “respond” rather than “perceive.” They might use the same word for two colors, that doesn’t mean they cannot “see” or “understand” that green and blue have differences. It is also about the question you ask them: “what color is this?” vs “which of these is darker?”

This is a good ELI5 because of how you wrote it. You clearly love this topic and it makes me love it.

Moving on from Newtons work, James Clerk Maxwell developed the understanding that all light we detect is a combination of red, green, and blue (excluding tetrachromats). Even thought we see the rainbow, that's all made up by our brain using RGB data.

Both film and digital cameras, by no mere coincidence, respond to the same basic three colors the cells in our eyes do - red, green and blue - so they essentially see the same thing our eye does.

When those images are displayed back to us, our vision system responds to certain proportions of red green and blue as all of the colors of the rainbow, so to speak.

When you look at something emitting a pure yellow frequency, your eyes don’t actually record yellow, it records a certain amount of green and a certain amount of red (and technically probably a certain amount of blue because there is a lot of overlap between the three kinds of receptors and red even actually wraps around and has a little hump in the blue spectrum giving you purple) and your vision processing center interprets that as yellow. If you display red and green light together at the same proportions, your vision system will see that as the same yellow and not know any better.

That’s what the screen you’re looking at right now does - it’s just a bunch of tiny red, green and blue lights shining at different places at different proportions to recreate all the colors you’re seeing.

The light in a rainbow is made up of many more wavelengths than red, green, and blue. Indeed, every color and shade in the rainbow is a distinct wavelength. Our eyeballs have three different kinds of color receptors. Each receptor responds to visible light, but more strongly to certain wavelengths. You can trigger any color that we can perceive using 3 wavelengths such as red, green, and blue (some other combinations can also be used). We can perceive many more wavelengths than that; we just can't distinguish them.

Technically we're all seeing in XYZ , which can then be mapped onto RGB. Mostly.

If an optical illusion tells you "you're going to see a colour that doesn't exist!", It's because by staring at a colour for long exhausts some of the detectors, and then the remaining detectors will send a colour in XYZ space that doesn't properly map to RGB.

One possibility is that Newton's "indigo" was what we call blue, and his "blue" was what we call cyan.

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Counter question: are you sure they're in separate lines?

It looks like a continuous gradient to my eyes, I've never seen "bands".

There's more yellow and red than others to me, but I'm pretty sure that's largely because the middle is the most intense (you can easily see this with black and white photography) and red stands out more noticeably against the blue sky than the bluer half.

This is so far from an ELI5 answer.

Light gets refracted every which way in a rainbow thru water droplets in the air - depending on the wavelength of light and the angle it passes thru a water droplet. But, in the center of the rainbow, refracted wavelengths or colors combine with refracted colors from other droplets to make white light again. That's why the center of a rainbow is often brighter than the surrounding sky. The bands of colors we think of as a rainbow are just the edge where colors are refracted at angles that don't combine with other colors.

Say what?

No. Colour spaces are complicated and make me cry.

I've only ever seen separate lines in artwork/recreations of rainbows. Rainbows in real life, or photos of them, look like the color gradients you see in color pickers (like in Photoshop).

Well a rainbow isn’t 6 or 7 colors inherently, but I perceive it as 6 distinct colors- others who use only 4 words for colors would be able to distinctly separate where their 4 perceived colors begin and end. There are fascinating studies about different cultures and their ability to actually tell the difference between colors.

i couldn’t find the video but here’s the article

Then congrats you are the first of your kind biological anomaly and researchers would love to dissect your eye...or you did it wrong.

Have you ever seen that dress that some people see it white and gold and others blue and black? If not, just google (the dress colors) you will find it, it is actually crazy how my brother see it blue and black and I see it white and gold I just can't understand how.

That’s one of those envisioning things at play. Most people can’t detect slight variations in color even when they can see them, so while you do indeed see a gradient your memory tends to trim it down to less variance. It’s estimated that we can discern around a million distinct colors, but remembering that is difficult for all but a very small number of people. Most of just remember the idea of a rainbow and then reconstruct the imagery later.