How does color work. I know how light gets absorbed and reflected depending on the color of the thing, but that does not explain HOW things have color.
Like. I can have a red house, red clothes and my blood is red. But all those things are VERY different things. What properties do all of them have that makes them red? How does my red look red ? Molecules? And we can mix colors too. What specificly is mixed?
What quality in red paint is also present in my blood?
I am not the best at explaining, but what I want to know is what do same colored things have in common that makes us see them as the same color despite being very different kinds of things.
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Color is based on the wavelength of light that your eye detects. Red is at the long end of light our eyes can detect, with violet at the short end.
https://upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Linear_visible_spectrum.svg/660px-Linear_visible_spectrum.svg.png
This is for “pure” monochromatic colors, colors made of only 1 wavelength of light, but we can blend together colors to kind of trick our eyes into seeing “inbetween” colors. A combination of a red and blue would make magenta, a color that *has no single wavelength*. There is no single source of light, dye, or pigment which can produce magenta. It requires a combination of both red and blue.
The thing common to blood, paint, and markers is either:
A: the light they reflect is in the red region of wavelength
Or B: the light they reflect is a combination of light which separately look similar to that of red light, and so together our brains process it as being red light.
>light gets absorbed and reflected depending on the color of the thing
That’s kind of backwards, it’s the inherent chemical properties of things that cause the balance of absorption and reflection of light that results in those things reflecting/emitting a spectrum of light that our brains interpret as a certain color.
Color itself isn’t exactly a property of things – a white piece of paper can still look very red if you just shine red light on it…
When light is emitted, it’s because an electron is transitioning from a high energy state to a lower energy state, and the difference between those determines the wavelength. More energy = shorter wavelength.
When light hits an object, if an electron can jump by the same amount of energy, the photon can be absorbed. If the electrons need a different amount of energy, the light either reflects off or passes through. Different materials have electrons that need different amounts of energy, so they absorb different wavelengths.
When the light hits our retina, we have three types of cone cells that are more sensitive to different wavelengths, and our brain interprets the relative amount of each type of cell as color. Mixing color on your computer monitor is about selectively activating those cone cells using red, green and blue light.
Print and paint work the opposite way, you have pigments that selectively absorb light, so you see the different colors when illuminated with a white light(which contains all the colors)
So for red: With additive color, you need a red light, with subtractive color you need to absorb everything except red light, so you would mix magenta(to absorb green light) and yellow(to absorb blue light).
Pure light contains all colors. Most objects absorb only some of those colors and reflect the rest. The combination of colors that is reflected is the color that you perceive the object to be.
Let’s leave colour mixing aside for a moment, because it’s something that happens in the eye and the brain. It has quirks and often depends on context of what you’re seeing (ex. there’s no such objective thing as “brown”).
So, light can interact with atoms in two ways:
It can get absorbed and/or emitted by electrons jumping between energy levels. This is VERY selective for colour, because only specific wavelengths* can interact with specific transitions, as they have specific energies*. This means they literally carve slices out if white light and emit monochrome light*.
*It’s more of a “very narrow band around the theoretical ideal”, because in the quantum world nothing is exact, but that takes more words and makes little difference.
Light can interact with with charges, usually just electrons but potentially whole molecules, without getting absorbed. Light is just a wave of electromagnetism. Electromagnetism is the thing that interacts with electric charge. A changing electromagnetic field makes charges move, and moving charges change the electromagnetic field.
That’s a complicated way of saying that when a passing EM wave (light) makes a charge (like an electron) wiggle, the charge wiggles the EM field back. Depending on the exact material (metals vs. molecules, single crystals vs. polycrystals etc.) this can result in light getting refracted (like glass) specular reflected (like a mirror) or diffuse reflected (scattered, like white paint).
The thing is, almost no materials interact with all wavelengths equally. Only a small subset, like silver or aluminium (so stuff you’d coat a normal mirror with) reflect most light equally. Even other metals like gold or copper take on a hue because some wavelengths interact differently.
Plus, refraction bends different wavelengths differently, because this interaction with light changes its speed (as in, literally the light moves slower, there’s no “bouncing” involved) in that medium differently depending on wavelength. That’s how you get a prism.
TL;DR Things can have colour because they absorb/emit specific wavelengths, and/or because the light that’s refracted/reflected is separated like in a prism and different wavelengths take wildly different paths. It’s most often a combination of the two, light gets refracted, diffused, some gets out, some gets absorbed. The exact mixing of these gives you the range of wavelengths that can then make it to your eye.
Now the eye works is a funky way. We have three colour detectors, which peak primarily in blue/violet, green and red. That’s what makes the “visible spectrum” visible, as opposed to infrared or ultraviolet.
But the two key words here are “peak” and “primarily”. Their sensitivities are spread fairy widely around a peak wavelength, similarly to a bell curve, but blue/vioet does extend a bit inti green, green stretches both ways through to blue, and through yellow and orange to red, and red actually stretches all the way back to blue, where it actually has a smaller secondary peak.
That last one is important, because that’s what tricks our brains into seeing a colour “between” blue and red, when both get activated. That’s why we end up with a colour WHEEL in RGB. That’s where other colour mixing comes from, too. The brain doesn’t see electromagnetic waves. It sees the levels of activation of our receptors. And it can’t tell if that activation is caused by one wavelength activating multiple receptors to a specific level, or multiple wavelength doing the exact same thing. So we can trick it to see colours that “aren’t there” (wavelength wise) my mixing RGB (for emissive displays) or YCM (for absorbing pigments) in different ratios.
But other colours are more subjective than even colour mixing. Brown just straight up doesn’t exist in RGB, for example. You will not find it on the colour wheel. It is a purely subjective interpretation of orange coming from the contrast between the “brown” object and it’s background. On a bright background, dark orange looks brown. Bit the exact same hue (say, if you take it’s hex code) on a black background will look like the orange it is.
TL;DR Vision is complicated and confusing.
While others have explained the physics well, the other half is your brain.
Your brain makes everything up, and often lies to you. Color is *entirely* subjective. There is no objective truth to color perception, despite being able to break the light down into different intensities of different wavelengths. My red and your red might “look” completely different to us, or they might be similar. We only have other red things that we call red, and it’s all relative and things we learn as a kid.
Similarly, you can’t imagine a new color that you or anyone else has never seen before. It will *only* be a mix of colors we already have. People who are colorblind can be told that this is red and that is green, but they have no way of understanding the difference or even what it “should” look like.
Your brain lies about what your eyes see in lots of ways, in fact. It’s a fascinating delve.
So I can’t give you a good explanation of why a certain wavelength comes from a surface of a certain substance/material, but I can tell you a bit about the light itself and how your eyes deal with it ^^and ^^rant ^^a ^^lot ^^about ^^color ^^theory.
So first off, your eyes have special sensitive cells called photoreceptors which respond to light. There are two kinds of photoreceptors: rods and cones. Rods are not color-specific and are more sensitive to low levels of light than cones, and this is why in really low-light situations it’s hard to distinguish colors. Cones are what you’re more interested in, and there are three kinds. Each kind detects a range of light, peaking in red, green, or blue depending on which type of cone cell. [Here’s a picture of the wavelengths each cone is sensitive to](https://upload.wikimedia.org/wikipedia/commons/thumb/0/04/Cone-fundamentals-with-srgb-spectrum.svg/810px-Cone-fundamentals-with-srgb-spectrum.svg.png).
So now we have three primary colors we can work with and mix together. Different amounts of red, green, and blue can appear as the different colors on the spectrum. This is how the screen you’re looking at now works: lots of little glowing dots in red, green, and blue vary in brightness to appear as all the colors in the RGB color space. *It should be noted that they can’t produce all the colors your eyes can see*, but it’s complete enough for realistic-looking photos and videos. White light contains all the colors, so when a pixel on your phone looks white, each red, green, and blue part is emitting at 100%. Likewise, black is RGB(0,0,0) – ideally this would be zero light emitted, but your screen still glows a bit. We’ll just say it’s zero.
This is what we call the “additive” color model, because we’re adding different color elements together. There’s also the *subtractive* color model, which is more useful for describing mixing pigments, paints, or other things that *reflect* instead of *emit* light.
Pigment mixing, or “subtractive” color, is the same thing as the additive model, but in reverse: the things like your house, clothes, and blood actually *absorb* some wavelengths of light. You know how your black shirt gets hotter than your white shirt in the sun? It’s absorbing more light. Remember how on your screen, black is RGB(0,0,0)? Well it’s kinda like that. All the wavelengths (ok not all but for our purposes “all”) are being absorbed, and none reflected, so your eye basically gets RGB (0,0,0) from your black shirt. Whereas the white shirt reflects all the wavelengths (ok not all but, again, ya know…) and your eye receives RGB(100%,100%,100%).
So when we’re mixing paints, pigments, etc, we want to know how much of each to use, right? Let’s use a printer as an example. It uses four colors of ink. Cyan, magenta, yellow, and black.
This is where I think it gets really cool.
If we consider cyan, magenta, yellow, and black the “primaries” in subtractive color mixing, *they’re actually the secondaries in RGB!* Check it out. Mixing 100% red and 100% blue in the RGB model makes magenta, red and green gives you yellow, and blue and green gives you cyan.
[Seriously, check it out.](https://www.csfieldguide.org.nz/en/interactives/rgb-mixer/) I know it sounds weird that red and green give you yellow, but that’s probably because you’re used to the red-yellow-blue model of the color wheel. I’ll get to that.
So just like before, the *additive* model and the *subtractive* model are the reverse of each other! The primaries in one are the secondaries in the other. When your printer wants to print red, it’ll use yellow and magenta. Because we’re *subtracting* light instead of adding it. This might not be very intuitive but if you think about it, it kinda does make sense in a really cool way (at least I think so but I’m a nerd so 🤷🏻♂️). Think about it: Which color do yellow and magenta have in common in the other model? Red!
[Again, check it out.](https://www.w3schools.com/colors/colors_cmyk.asp)
More pigment makes things darker. Subtractive. Technically, mixing cyan, magenta, and yellow will give you black, but your printer adds black for more defined dark colors, and to save ink (you’d use a lot of ink printing black text if you’re mixing ALL the colors).
So what’s the deal with the color wheel? I grew up thinking that the primary colors were red, yellow and blue. In these new models, that doesn’t make sense, right? Well, if you squint real hard, a red-yellow-blue palette kinda looks a bit like a magenta-yellow-cyan palette. Kinda. I’m comparing these two because they’re both subtractive. When you’re working with red, yellow, and blue paint, you can make a lot of the colors you can with cmyk. Probably enough for most people. And of course, red, yellow, and blue give us the color wheel we’re all familiar with – Red, orange, yellow, green, blue, purple.
There’s still a problem here. So far I’ve been assuming we’re mixing colors on a white surface. Since subtractive colors are, well, subtractive, we’re assuming we’re starting with all the colors (white) and subtracting from there. If we’re starting with a dark surface, we’re not going to get any brighter colors than the pigments we’re working with. you could make gray with cyan, magenta, and yellow, but you’re not getting white. Simple solution: white pigment. White paint is almost like “cheating” in subtractive color. You’re essentially subtracting your subtraction. Which is just adding with extra steps.
Anyway I think I’ve covered most of it. I’ve probably forgotten something or worded something poorly or maybe gotten something technically or completely wrong; if that’s the case I invite corrections in the interest of giving OP the most correct answer we can. I also don’t feel like going back through this and editing it right now so I’m sorry if this is kind of a stream-of-consciousness wall of text or presented poorly (grammar, punctuation, spelling, etc included).
[EDIT: There’s more. There actually are more color models, like HSL and HSV. We’ll cover that next semester when you take the 200 level course]
OP if you still have any questions, lay it on me and I’ll try to shed some light on it.
Light “come from” an object and go into your eye, with various wavelength. The mechanism of how light could “come from” an object vary a lot, depending on the object.
Once light get into your eye, it hit a cone cell, which send a signal to the brain though an optical nerve; each cone cell is given its own optical nerve (note that I’m being deliberately vague about what exactly is this signal, the mechanism is quite complicated). The precise mechanism is something that is pretty deep into biology. But in summary, there are up to 4 possible types of cone cell (but usually 3, and some people have less), which determine how sensitive it is to certain wavelength. The signal sent by each cone cell depends on the wavelength of light it receives and what type of cone it is.
Finally, the brain receives all these signal and interpret them. The precise mechanism for this is still highly mysterious. Unfortunately, part of the difficulty is that how the brain interpret these visual signals depends on tons of additional context, many of which are not visual-based. But as a first order approximation, the brain combine the signals from individual cone cell into aggregate signals that allows color to mix together. Since most human have 3 types of cone cells, there are 3 types of signal, and since cone cells are extremely densely populated and different types are mixed with each other, the brain interpret aggregated signals from many cone cells near each other into a single color.
——————
Now, let’s go back to the part we understand the most: how light of various wave length can come from an object. There are many mechanism, but 2 most common mechanisms are: reflection and spontaneous emission.
Reflection: light reflected off an object. A lot of the wavelengths are absorbed, or transmitted through. The light you see coming from the object is the remaining light that get reflected.
Spontaneous emission: electrons absorbed energy somehow which raise its energy level, then at some point, it loses energy, producing a photon in the process.
>How does color work. I know how light gets absorbed and reflected depending on the color of the thing, but that does not explain HOW things have color.
This is where you’re wrong. Things do not have colour at all. Light typically posses multiple wavelengths of light in the visible spectrum. Certain objects absorb certain wavelengths while reflecting others, in essence removing certain colors of light. The resultant wavelengths that DO reflect are perceived by the eyes and translated into color by the brain. Simple as that.
Things don’t have an inherent property of colour per se. They have reflective properties which allows us to see them as a certain colour when light strikes the object and enters our eyes.
To put things in a different perspective, prototypes for invisibility cloaks currently work by redirecting light so that it doesn’t reflect back to your eyes.
So if the light never reflects off the object, you don’t even see it, let alone know what colour it “is”. Unless it lets out its own light, of course, like say a flame.
In short, we see things by capturing the light that comes from them with our eyes. The colour of the light is determined by the wavelength of the light. We then recognise where the light comes from, and only then can we recognise that lump of colour as a visible object.
If two things look to have the same colour, they have the same reflective properties. If they produce their own light, then they have they have the same luminescent properties.
And how do we know that my red is the same as your red? We don’t. We can’t experience what it’s like to be each other, so we just take common consensus as the rule. Of course, this consensus might arise from the fact that we’re genetically and structurally very similar to each other, but that’s not yet rigorously founded.
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