Stupid color question

This morning’s paper explained why snow appeared white, stating that certain substances absorbed/reflected different wacelengths of light. It gave the example of a blue shirt that absorbs all of the wavelengths except for blue, which it reflected making it appear blue.

Fine. I’ve known that since grade school. But what I don’t know is what is it that makes a certain object absorb/reflect certain wavelengths?

Using the example of a shirt, you can have identical shirts dyed any number of colors. Which makes me figure it has to be an aspect of the dye that does the absorbing/reflecting, rather than the material of the shirt itself. So, is it a physical aspect of the dye that essentially works as a little prism? Or something else?

How about flowers? Why is a red rose red and a yellow rose yellow? Do they contain a substance analogous to clothes dye?

And I know my brain always starts to hurt when I try to compare aspects of light with paint…

I feel so silly not really understanding something so basic, but would appreciate it if any of you more learned folks could dispel this particular aspect of my ignorance.

Let’s start with an anology. Assume you had a bucket of mixed spheres, ranging from tiny (BB Sized) up through Marble, golf ball, tennis ball and softball sized. If you wanted quickly sort these, the easiest way would be to dump them through a series of sieves, each containing holes which would allow the smaller balls to pass through, but retain the larger balls.

The balls are various wavelengths of light, and the holes are the sizes of the various dye molecules. If a large ball ( long wavelength) hits a sieve with a small holes, it will bounce off of it. A smaller ball (shorter wavelength) will go into a hole (Be absorbed)

What is happening is that wavelengths of light have to hit something the right size to be absorbed, if not they are reflected. A dye molecule is essentially something that is the right size to absorb one colour of light, while reflecting the rest. (Technically, the full explination on this mechanism is a lot more complex, involving quantum effects)

Flowers and other naturally coloured items contain pigments which do exactly what was described above. Remember, many dyes and pigments were (and are) made from natural occuring materials, including flower petals.

I hope this helps

Regards
FML

All right. I appreciate the analogy. But I guess I really want to get down to the quantum level. What is it that makes a particular dye/pigment molecule the right “size” to absorb one color of light?

And then, is it really a matter of a particular molecule absorbing one color of light, and an item has a whole array of molecules absorbing different colors, or does a particuar molecule absorb an entire spectrum of colors except for the ones it reflects? For example, is it that red dye is the right size to absorb all colors except for red? Or does red dye contain a variety of different molecules that absorb green, blue, indigo, violet… - everything except for red?

Or are all molecules essentially similar in their ability to absorb colors, but they differ with respect to the spectra they reflect?

It really bugs me to even have to ask such stupid questions about something so basic to our very perception of the world.

I have the feeling any explanations you attempt are going to make my puny lawyer-brain hurt. :slight_smile:

This raises another question:

If I examine red paint on a molecular level, will I be able to detect molecules that are, in fact, red; or would I detect something that is a marker for “redness,” without actually being red, itself?

And if I mix red paint and yellow paint and examine it on a molecular level, will I detect “redness” and “yellowness,” or will I detect “orangeness”?

In a theoretical dye, you could indeed have one type of molecule which would absorb all but (say) one red wavelength, and it would be a very, very red dye.

In the real world, most dyes are blends of different chemicals, with different atoms and molecules. That’s because “red red” is ass ugly. You want “red” with a teeny, tiny bit of “blue” or “yellow” or “black” - nearly infinite combinations of slightly different dye molecules which each absorb and reflect slightly different wavelengths, which make the eye see an aggregrate color of “red” that usually isn’t, technically speaking, the reddest red it’s possible to make.

This is why your garments turn different colors after many washes - some of the dye rinses out every time. If you have a dye with lots of little red dye molecules, they tend to literally slip out of the fabric easier than larger molecules - which is why reds tend to bleed. More of the yellows or blues will be noticeable after a time, and your once vibrant red shirt becomes closer to an orange. This is less noticeable these days with fiber-reactive dyes (dyes that go inside the fibers of the fabric, instead of sitting on top of them), but it still happens to some extent.

The most noticeable change is in your blacks. There’s really no such thing as a “black” dye, even though we call it that. Black dye is really really concentrated mixes of all the other colors, which each absorb their spectrum of light. If you look closely, you can sort your blacks into green-blacks, blue-blacks and purple-blacks, correlating to the predominant dye molecule in each. As they are repeatedly washed, again, dye molecules will escape, first the tiny reds, then the yellows, leaving dark dark blue behind. (Blue is a large enough molecule that it’s always the last to leave.)
(Note: for “red” read “magenta”, which is the actual third primary color of solid pigment blending, not red like we learned in kindergarten.)

It’s going to come down to the energy levels of the electron orbitals of the dye molecules. You may know how in a single atom, there many electron orbitals. When all the electrons have filled the lowest energy states they can, the atom is said to be in its ground state, but those upper electron orbitals are still there, they’re just empty. If one of those electrons is in a higher energy orbital, the atom is said to be in an excited state.

If the electron falls down from a high energy orbital into a lower energy orbital, the atom will release a photon of a certain wavelength, dependent on the energy difference between the ground and excited states. Likewise, if you hit an atom in the ground state with a photon of that wavelength, there is a high probability the atom will absorb the photon, kicking the electron up into the higher state.

If you look into spectroscopy used by astronomers, the star’s spectrums will have bright lines corresponding to where the atoms that make up the star are in excited, states, and thus emitting the wavelengths corresponding to the bright lines. There may also be dark lines, where light from background stars are being absorbed by interstellar gasses. This is analogous to how dyes work.

I’ve never worked with dye molecules, but I expect they are chosen because they have particularly good properties of absorbing across a wide range of frequencies, instead of at little sharp lines like individual atoms do. They probably also have to have to favor releasing the energy through other channels (alternate sets of energy transitions that work at different wavelengths), or else they’d just be reradiating the wavelegnths of light they’re supposed to be absorbing. Optical brighteners in laundary detergent, and flourescent colors do this also, except they absorb UV wavelengths, and reradiate in the visible.

I think I remember something about chemical bonds on “dye” molecules and how their length determined they colours they transmitted. Also, that stressing that bond (by placing different functional groups at either end) affected their transmissivity. Excuse my vagueness, just hoping that this will prompt a real chemist to jump up and elaborate on this.

As for actual dyes being mixes of several colours, make a fat blot with a coloured marker on an end of a strip of a paper towel and dip that end on alcohol. Makes for a fun little show of what mixes on each particular shade.

I can’t answer the question directly (or, at least, not any better than other posters have!) but I just wanted to suggest a fun little experiment so you can see the mixtures used to obtain certain colours. It’s pretty qualitative, but this is the kind of stupid stuff I like about chemistry, so here goes…

Sharpies. Or other markers. Actually, if you can get “equivalent” markers from two brands, that could prove interesting. So, for example, say you have a “black” from Sharpie, and a “black” from Crayola. (You can, of course, do this with any colour, but lets just stick to these two markers to make it easy on me!) Get a large coffee filter and cut out a rectangle, say 1x3 inches (or as wide as you want, depending on the number of markers you have). About 1cm from the bottom of the strip, make a dot with each marker, a short distance apart from each other. Into a mug with a very small amount of water, dip the bottom of the coffee filter into the water, and use a clothes pin or something to hold the strip in place. Make sure the strip is basically perpendicular to the water, so that the water will absorb up in a straight line across the strip. Take the strip out before the water line reaches the top, and mark that line so you know how far it is.

You should be able to see spots of different colours come out of your “black” spots. Usually a mix of blues, reds and yellows, all combined to make “black”. Your Sharpie spot and Crayola spot are probably different mixes of dyes, so your red might have gone farther in one than in the other, or two other colours might be inversed. Different dyes “like” the water more than others, and will travel with it, while others will “dislike” the water and tend to stick to the paper more. The ones that traveled further “liked” the water. If you have two spots of “blue” for example, both at about the same distance, then divide the distance of “blue” from your start spot by the distance of water from the start spot and compare them to each other. Similar ratios (Rf) suggest similar or same molecules.

You can try this with different papers (coffee filter, tissue paper, printer paper) but I don’t know if you’ll see much difference (your Rfs might change, which means that the affinity for the papers change for each molecule). Better yet, you can try it with different solvents. Water, Vodka (40% ethanol), nail polish remover (acetone) maybe some sort of oil, preferably colourless. The level of “like” for the solvent will change depending on the molecule, and some may travel less or travel further.

This, is chromatography :smiley: I get paid for this.

:confused: Huh?
I always thought it was the opposite - red pigments are the largest. They are the hardest to adhere so they dislodge the easiest.

Man, this is obviously something I can’t just read and understand. :confused:
I’m gonna have to get back to this when I can concentrate on it for a while, and then come back with a bunch more boneheaded questions.
Thanks so far, though!

:smack:
That’s right!
We are talking about dyes, not solid pigments like that found in paints!! :stuck_out_tongue:

Okay. I’ve actually been thinking about this more than my babysteps would suggest, but here’s what I understand/question so far:

So let’s say I have a natural fiber cotton shirt. I assume it would appear some kind of off-white, because of the manner in which the natural cotton (cellulose?) molecules reflect and absorb light.
Am I correct in assuming the light is coming in photons - little “packages” that contain all of the various frequencies. The extent to which something has color depends on the extent to which these photons interact with the substance’s molecues: each individual photon could either pass between different molecules, pass through a single molecule, or collide with a part of a molecule.

Dye actually affects the cotton molecules, either by sitting on top of the cotton molecules, or with more advanced dyes, binding with them.

So is it that the addition of the dye molecules to the natural cotton molecules changes the way the photons interact with the shirt at a molecular level? How exactly? Are the collisions more “complex” within more “complex” molecules, with light bouncing around inside and among them before being reflected back out?

You’ve said that a particular dye will rarely be a single color - the various hues of dye molecules will loose their bonds with the cotton on different schedules, resuting in fading over time.
If I dye a shirt red - which actually includes a little blue and yellow, will some red, blue, and yellow molecules attach to each cotton molecule, or will there be some red cotton molecules, some blue ones, and some yellow ones?

How about panache’s question - if I could see these dye molecules sitting on top of or bonded with the cotton molecules, would they appear red, or would they just appear as a structure that reflects red?

And how is it different if I dye the shirt, or if I paint it? How do the red paint molecules - which I assume primarily sit on top of the cotton fabric as a whole, differ from the molecular bonding of dye? I intended my OP to be about color, not dye, and used a shirt merely as an example.

Here’s a goofy thing I’m wondering - assume a certain color of “red” - which I assume would be most accurately described as a combination of various spectra. Are identical molecular reactions taking place in all objects that appear that color - whether made of fur, rubber, paint, plastic, etc.? Or are there multiple molecular “ways” that different objects can appear the same color? Does that even make sense as a question?

Another similar question: I assume a dye can be made out of either natural or manmade pigments. I also assume, however, that manmade vs natural dyes can be distinguished upon laboratory examination. But I assume the extent to which they appear to be the same color depends on the extent to which they absorb/reflect photons in the same manner. Let’s say I dye a piece of cotton and a piece of wool the same color, so that in a close-up photo they might appear indistinguishable. How would the “redness” differ at a molecular level?

Then I guess what we haven’t addressed is how I perceive certain wavelengths as color. I guess we have 3 color receptors in our eye like a TV (in the rods - or is it the cones?) I seem to recall that those color receptors don’t exactly correlate with the 3 primary colors, but that your brain “tricks” yourself into perceiving a certain combination as “blue.” And primates evolved such that we lost the ability to see things in UV and/or infrared, and that birds and insects see objects as appearing vastly different than we…

Final question - what the heck is going on with shiny metallic colors, like polished steel or gold?

Thanks again. I really appreciate your attemtps to help me make sense of this.

You need to get yourself a copy of Kurt Nassau’s classic book The Physics and Chemistry of Color. There are many ways that objects “decide” which colors to absorb or reflerct, and Nassue (who was a researcher at Bell Labs) goes into them in his book. I’ve never met him, but I’ve talked to him on the phone.

metal colors look different because their surface is reflective (as a mirror). If you sand them, they don’t look “metallic”. Even a polished metal, if perfectly flat and lit by a uniform light source, will look a solid colour. What looks “metallic” is the environmental light reflecting in the surface. (this is why metallic markers look so sucky, they can’t make a good surface)

as an aside to further confuse you. Blue is very uncommon in nature and more so among animals. Most blues you see in the animal world (fish scales and bird feathers) are refractive. That is they don’t have a blue dye but work like a tiny prism refracting light.

I you mix red and yellow (and they don’t react), you will detect “redness” and
“yellowness” (think absorption spectra, the little rainbows with the black lines). That said there is also “orangeness” that looks different and has a different little striped rainbow.

Thanks - I’ll give it a looksee.

The problem often is, however, that even when I read supposedly dumbed-down explanations, I have a hard time understanding them. I remember being totally confused by what I thought was going to be an entertaining book on the physics of golf. And about a quarter of the way through the mass-audience directed Brief History of Time, I had to call it quits. I’ll give your recommendation a shot, tho.

Sapo - probably one of the few things I remember from grade school was a kid doing a science project showing how blue jay feathers weren’t really blue.

Dins, take a glass of water.

Actually, make that DI or distilled water (there is a technical difference but not much on a molecular level).

Now add salt to it and stir it.

After a while, you have a glass of water that looks exactly like it did before but tastes and smells different (well, I do notice the smell, others don’t); not only do you now have Cl- and Na+ ions in the water, but the presence of the ions affects the water around each ion. It does not affect it in a way that we can perceive visually, though.
Take another glass of water.

Add CuSO4 to it, stir enough.

Now you have a clear glass of… bright blue water! Unless there is something seriously wrong with your sight, you wouldn’t drink it by mistake, as you might the salty water.

The interactions between the Cu++ and the SO4= and the water aren’t so different from those of the saltwater, though; the color is coming exclusively from the Cu++.

Now an extra twist of lemon: you can change the color of the CuSO4 water by adding to it salts that, by themselves, would not change the color of water. This is because the Cu++ gets attached to the ions in these other salts, forming what’s known as “complexes”. The Cu++(H2O)n complex is a certain shade of blue, but the Cu++(CN-)n complex is a brighter shade. *I’m using n because right now I’m not sure what’s the right “coordination number” for Cu++; different metals have different numbers.

Now look at your shirt. Think of it as a solid solution. The white cotton threads are the colorless molecules of water; the dye, which in most dye jobs should be spread evenly throughout the fabric, is like the ions in the liquid solutions.

Usually in the context of solutions and of organic molecules, it’s not an individual atom that captures the photons; it’s a bond. That bond can be part of the dye or of the new molecule formed when dye molecules interact with substrate molecules… a photon doesn’t care about how “a bond of the same length as the photon’s wavelength” (aka “a bond able to absorb the photon”) was formed, only that it exists and they found each other.

Whether it’s a bond or an atom that absorbs it, the absorber goes to a higher energy state and comes back down after a while. In general, it comes back down to the same ground state it started from, but there are examples of reaction started through light. In those reactions (which, when they happen in a dye, mean that the color will eventually fade if left in the sun for too long, and I’m sure you’ve seen this happen), the light provides enough energy for the molecule(s) to get energetic enough to perform a reaction that they wouldn’t be able to do in their ground state. So, you may have a reaction along the lines of this “light-triggered breakup”:

M1 + hv --> M1* --> M2 + M3
Again: photons don’t care whether the “stuff” absorbing them is organic, inorganic or an alien from outer space. All they care about is that it’s got an energy hole the right size.

(MS in Quantum Chem, what?)

If you have two objects that are the exact same shade, it may or may not be due to the same “molecular reactions”.

The two objects can have completely different chemical compositions. But hey, they could still have “energy holes” of the exact same size and in the exact same ratios, yes?

Yes, they could. But it’s not compulsory. You can get the same exact shade by mixing different sizes of “energy holes” (different molecules) in different ratios.

Mind you: if the two objects are of the same nature (ex, car paint), they’re highly likely to have the same chemical composition to a very accurate degree. In the specific example of the car paint industry, the amount of substances used as dyes isn’t so large and every factory I’ve seen had several people dedicated to “reverse engineering and reproducing” their competitor’s products.

And molecules don’t have a label saying “manmade” :slight_smile: Some manmade molecules have not been found in nature so someone with enough knowledge of biochemistry will say “gee, it’s got Cl atoms, betcha that didn’t come straight from a plant”.

But the big impulse to chemistry in the XIX century came from some guys figuring out how to make, from coal, molecules that until then were only available in minute amounts, at cheap-by-comparison prices. And undistinguishable from the “natural” original.
(Sorry about the triple post)

Dinsdale, before you get a copy of Nassau’s book, you might want to have a look at thjis website:

http://webexhibits.org/causesofcolor/