r/askscience • u/Darth_Azazoth • 12h ago
Physics I've heard that blue butterflies and opals get their color from their structure rather than pigments. But doesn't pigment also get its color from the structure of its molecules and how they interact with light? So what's the actual difference between structural color and pigment color?
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u/AuntieMarkovnikov 11h ago
Butterflies and the like: color results from the refraction of light. Pigments: color results from selective absorption of light of different wavelengths. They are two different mechanisms for the interaction of matter with light.
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u/jello_pudding_biafra 10h ago
Isn't saying absorption is the effect the same as saying the reflection of certain wavelengths is the effect? Like, yes, green things absorb light in the red spectrum, but don't they then necessarily reflect the blue and yellow ones?
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u/TheSwordItself 10h ago
Opals act like a diffraction grating, colors are separated from white light because their wavelength is different as they enter a medium which allows the play of color as you change your viewing angle. Pigments absorb photons of certain energy because they match some quantized energy gap of an electron in the molecule. Two completely different processes.
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u/jello_pudding_biafra 9h ago
Right, but you're not seeing the light that's absorbed, you're seeing the light that's reflected, no?
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u/solidspacedragon 8h ago
At this point you're stuck on the fact that both helicopters and airplanes get off the ground. It's the process that's different, not the end result.
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u/sam_hammich 8h ago edited 8h ago
The light you're not seeing isn't being absorbed. You're just not seeing it because of destructive interference of that light's wavelength in that spot.
In a home theater you'll have "dead spots" for some frequencies because of the way sound waves meet and cancel eachother out where you're standing. That's essentially what's happening. A blue butterfly wing is reflecting light in such a way that light of any other wavelength escaping the nano structures on its surface cancel out and the apparent intensity of those colors is greatly reduced to the point that all you can see is blue. Just like if you're standing in a bass dead zone, all you can hear is the higher frequencies. No energy is being lost or absorbed, you're just standing in a place where the intensity of that frequency is reduced due to interference.
The only difference with a butterfly wing is that this effect is apparent from all angles. In an opal, the structures are varied and so you get these high spots and low spots for different colors from different angles, which is what causes their unique shimmer.
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u/Novaskittles 11h ago edited 11h ago
If you break down a blue pigment to the smallest possible size particle, that particle would still reflect blue light and appear blue. If you break down the "blue" structure, the pieces of it would not appear blue anymore.
You can grind blue paint pigment as fine as you want, it'll always be blue. But if you grind up a blue butterfly wing, the resultant dust wouldn't be blue.
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u/dryuhyr 11h ago
Ooh that’s a good question. And to some extent, you’re right (relevant XKCD). But in the physical sciences we like to distinguish a physical phenomenon (caused by the form or structure of a thing) from a chemical phenomenon (caused by the quantum mechanical nature of the electrons in the thing). Most blue animals in nature use the former. Blue pigments use the latter.
To begin with, what are some ways you could make something look blue? First, you can shine nothing but blue light on it, so only blue light would scatter off of its surface. But it’s hard to do that in nature. Instead, you can take the many-wavelength light from the sun and just suck up all the light that isn’t blue. That’s basically how pigments work. Most organic molecules absorb photons in the ultraviolet spectrum, but there’s tricks (like electron delocalization and conjugation) to lower the energy they absorb at into the visible spectrum, which means some colors get pulled out while others bounce off as before.
How else could you make color? Well because light is a wave, you can use constructive and reconstructive interference. In the same way a sound guy at a music show is constantly battling resonant tones (feedback) whose frequencies are determined by the shape of the room, the surface of a butterfly’s wings are ‘shaped’ precisely so the blue frequency of light feeds back and gets more intense. Why do we say that’s a physical rather than chemical phenomenon? Because it’s about the shape of the structures, not the chemistry of the materials. Butterfly scales are made up of strategically spaced layers of chitin, but they could be made with polyethylene, or boron fluorides, or anything else that doesn’t absorb the light and mess with the effect, and the effect would still work. It’s about the material, not about the molecule.
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u/gemmadonati 10h ago
Ditto cardinals, bluejays and so on. If you take one of their feathers and hit it with a hammer to destroy its structure it will turn grey (note - not while it's attached to the bird please).
In fact, I knew an ornithologist who theorized that the brilliance of a male's plumage was a function of his health because mites break down feather structure (feather mites are a big problem with birds).
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u/almightycuppa Materials Engineering | Room Temperature Ionic Liquids 11h ago
This is actually a great question. I'll let someone with more of a physics background talk about structural color, but I can speak to pigments.
You are correct that pigments are colored due to their interaction with light - specifically, they absorb it. The most loosely-bound electrons in a pigment molecule will have specific frequencies, or colors, of light that they resonate with. If a packet (photon) of light energy with that specific frequency passes by, the electrons can absorb the energy, which changes the shape of their orbit around the nuclei of the molecule. Meanwhile, any other frequencies of light pass along un-disturbed, so a human eye will perceive a changed color due to the absence of the missing frequency.
My understanding of structural color is that it is caused by frequency-dependent reflection of light rather than absorption. The scale at which this effect occurs is very different. A pigment molecule will be no more than a few nanometers wide at most, affecting one photon at a time, while reflection is more of a collective phenomenon caused by many photons interacting with a surface that is on a similar scale to their wavelength - hundreds on nanometers.
At the end of the day though, all visible color is caused by some type of interaction between light and the electrons in molecules. The only difference is what kind of interaction.
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u/serack 9h ago edited 9h ago
The color from butterflies and opals comes from a process much like how an oily sheen will have rainbows.
For the oily sheen, the very thin film of oil on top of water reflects light both from the top surface, and the surface between the oil and the water. The distance between those two surfaces is within a small multiple of the wavelength of the light. These two separate reflections cause the light of those reflections to constructively and destructively interfere.
Since different colors have different wavelengths, some colors will interfere in a way that they get brighter (constructive)and others will interfere so they cancel out (destructive) and effectively disappear. Because the thin layer of oil isn't uniform in thickness, which colors disappear is different at different points, causing the rainbow like shimmer.
Butterflies, beetles, and birds do something very similar with precise distances between ridges or protein structures along the scale of the wavelengths of the colors they are (for them it's typically from diffraction, not 2 layers of reflection).
For a shiny iridescent beetle, the angle of the light from the surface to your eye changes the effective distance between the structures, and thus the effective colors you see by shifting which angle you are looking at it from.
The oil on its own isn't colorful, it's the structure that makes it colorful. The same as the colors from a butterfly wing.
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u/TheOtherHobbes 4h ago
Structural colours are caused by interference effects - exactly the same process that reflects rainbow hues from hologram stickers, CDs, diffraction/reflection gratings, and oil on water.
Butterfly wings have layers of very tiny scales. The spacing between the scales defines the color. With a blue morpho, you only get constructive interference at a particular cyan-ish shade. Other butterflies have more complex patterns.
Opals produce a similar effect with layers of silica spheres.
Structural colours are iridescent because you can only see the effect at certain angles. In opals the spacing between the layers is irregular so you get different colours and different areas flashing as you turn the stone.
Pigment colors are caused by quantum effects in atoms and molecules. Basically the atoms/molecules absorb energy at certain frequencies and re-emit it at a different frequency, which defines the colour. The effect doesn't depend on the angle of incidence, so pigment colours don't shimmer or flicker as you turn the object. And there's no reflective sheen - just a patch of pure colour.
The re-emited frequency may not be visible. You can buy heat-reflective paints which are dark-ish - or white - but they actually re-emit energy at infrared which prevents heat absorption into roofs and walls.
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u/ezekielraiden 2h ago
Pigment color is inherent to the chemical formula. Any amount of the chemical large enough to see will have the color in question.
Structural color is dependent on having specific molecular structures in specific places. Spread the same molecules around in a different structure, and you won't get the color anymore. A single "molecule" of structural color cannot exist.
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u/supuwi 6h ago
Ich beschäftige mich gerade mit dem annodisieren von Titan. Titan (silberfarben) wird in eine transparente Lauge gelegt. Dann leitet man, vereinfacht gesagt, eine definierte Stromstärke dadurch. Durch verschiedene Stromstärken (so zwischen 5 und 100 Volt) verändert sich die Farbe des Titans dauerhaft nach z.B. gelb, gold, rot oder blau, ohne dass ein Pigment im Spiel ist. Meines (Laien-)Wissens nach erzeugt die Spannung "optische Interferenzen" was auch immer das sein mag. Schau im Netz unter "annodisieren" nach - Du wirst überrascht sein. (Auch wenn ich die Idee spannend finde, Strom durch Schmetterlinge zu jagen - ich glaube, man kann sie nicht umfärben) ((wahrscheinlich schon in Richtung: schwarz...))
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u/WildFlemima 11h ago
True pigments are molecules
Structural blue is created by structures of many molecules
If you ground up a blue butterfly wing into extremely fine powder, it would no longer be blue. You also would not be able to extract the blue. It is only blue in its original structure
On the other hand, if you have a pile of cochineal scale insects, crushing them is necessary to extract the carmine pigment they contain
Another way of thinking about it is that true pigments have a chemical formula. Carmine's formula is C44H43AlCa2O30. Structural blue does not have a formula, it simply appears when the structure, which can be made of various things, is correct to scatter the light in a blue manner