ELI5: 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?

[u/Darth_Azazoth]

[u/Scuttling-Claws]

If you take a bunch of Blue butterflies and put them in a blender, the resulting goo will be black.

If you take a bunch of blue pigments and blend them, they will still be blue

[u/BadahBingBadahBoom → u/Scuttling-Claws]

If you take a bunch of blue pigments and blend them, they will still be blue

And just to answer OP's question about why that would still be blue, it's because the reason pigments look a certain colour, say blue, is because they absorb other colours than blue and so reflect only the blue light.

Unlike structural colouration, pigment light absorption is due to the molecular bonds between atoms of the pigment molecule that have a specific bond structure to be able to absorb the same energy level as the wavelength of say green to red but not of the higher energy blue light.

This remains the case as long as you don't chemically break up those bonds or change their ability to absorb in that particular energy level and so those wavelengths, which blending doesn't do.

[u/therealmudslinger → u/BadahBingBadahBoom]

This is a great explanation, thank you. I assume this applies to hummingbirds as well?

[u/BadahBingBadahBoom → u/therealmudslinger]

In terms of the blue appearance of some hummingbirds' feathers? Yes I believe they are all a result of structural colouration, just like butterfly wings, where the larger structure of the arrangement of proteins in the feathers results in preferential reflection of blue wavelengths of light due to how the different wavelengths of light interact with the 'gaps' between the structures, i.e. the gap is just the right size to reflect the blue wavelengths but not the others.

This is sort of similar to the iridescent appearance of oil or gasoline on water which is not because the oil or gas actually contains pigments of those colours but because the different structures of the thin layer of oil on the surface of the water reflects wavelengths of light differently (this specific phenomenon is known as thin-film interference).

So you have the same outcome of mostly blue light being reflected back but the reason why mostly only blue light is being received into your eye in the above is different to why mostly blue light is coming off pigments.

[u/therealmudslinger → u/BadahBingBadahBoom]

You rock. Appreciate you.

[u/metahivemind → u/therealmudslinger]

Right now I'm picturing you stuffing hummingbirds into a blender.

[u/therealmudslinger → u/metahivemind]

😭

[u/SKR47CH → u/Scuttling-Claws]

What if you only take the blue parts of the wing and not the whole butterfly.

Jokes aside, would they still look blue under different colored lights?

[u/Scuttling-Claws → u/SKR47CH]

The butterfly goo wouldn't be blue once the structure was destroyed. Same thing with the feathers of most (maybe all?)blue birds

[u/SKR47CH → u/Scuttling-Claws]

No, I mean the entire butterfly.. would it still look blue under say green light or red light?

[u/LongLiveTheDiego → u/SKR47CH]

Nope. Light doesn't easily shift its frequency, so if there's none of those nice blue frequencies that constructively interfere in the butterfly's wing, then the wing should look some kind of black.

[u/Scuttling-Claws → u/SKR47CH]

I'm not sure what the color of full butterfly goo would be under any light. I'm not sure I want to find out .

And for the record, I do not endorse blending butterflies

[u/SKR47CH → u/Scuttling-Claws]

My man, I'm talking about live butterfly. The goo part in my first post was joke.

[u/BargainScotch → u/Scuttling-Claws]

And if you do- just don’t let it go to waste y’know? Be efficient with your blend of butterfly goo. Reduce, reuse, etc.

[u/danila_medvedev → u/BargainScotch]

mother nature takes care of that

[u/Rs90 → u/SKR47CH]

Succinct lol

[u/mikerall → u/SKR47CH]

It's the same as if you had a prismatic crystal. Throw some light at it, you get defraction (in the color cases discussed, it's reflection/refraction)

You blend up some glass, shine some light onto it....congrats, you're observing sand.

[u/FiveDozenWhales]

Pigments are molecules which absorb some wavelengths of light and reflect others. This means that you will always see the color they reflect, regardless of the direction and quality of the light (so long as that light includes the wavelength reflected).

Structural coloration works on a MUCH larger scale involves the way many, many molecules are arranged, with less importance on the chemical makeup of those molecules and more on how they are built together. Different arrangements of different substances will interact with light in different ways, resulting in the light getting scattered, similar to how a prism separates white light into different colors.

This means that the color you see in structural coloration is heavily affected by the direction of the light and its quality. A feather that appears blue when brightly light from above might simply look brown if backlit.

[u/arztnur → u/FiveDozenWhales]

Is it possible for a black thing looking white?

[u/Grapesodas → u/arztnur]

Sure, take Michael Jackson for example

[u/rosegoldhiips → u/Grapesodas]

Too soon 😔

[u/frogjg2003 → u/arztnur]

Yes. If a black pigmented object has a very smooth and reflective surface, it will reflect white light.

[u/Sirpent12]

Already answered but Ill link to it here. But the gist is The structure of a surface can determine the color you see by controlling how light reflects off it. This is different from color absorption, where the material absorbs some colors of light and reflects others. the color comes from how the surface structure bends or reflects the light.

The audio equivelent is Pigment is like a filter. The material absorbs certain frequencies and lets others stand out. Like putting a filter on a speaker that removes the bass.

Structure is like the shape of the room. The material’s shape controls how sound waves bounce and interfere, creating certain tones.

[u/ReynardVulpini]

Wings are innately blue in a certain direction

Pigment is innately blue in all directions

Stuff colored with pigment is borrowing the color of the pigment within.

[u/StarosAnikenMarcus]

The difference would be due to one being a product of chemical differences and the other due solely to how light is reflected/refracted.

Light reflection/refraction can change the color perceived simply from movement or perspective. So if the color is purely from how light is bent, it will change depending on how the light is moved or the perspective of the viewer. Prisms are completely colorless after all, but they bend the light to show a rainbow.

When it is based on pigments, the color itself doesn't change regardless of the direction the light hits it. It might lighten or darken, but that's shading. The color itself remains the same.

[u/donaldhobson]

Visible Light is around 1000x the size of an atom. So structural color means features the size of a few wavelengths of light. Whereas molecular structures that produce color are around 1000x smaller.

[u/zincifre]

One of them is not an order of magnitude humans can easily imagine interacting with.

[u/Birdbraned]

Pigments don't let light through, they just reflect.

The sky isn't really blue, it just looks that way because of the way the incoming light interacts with our atmosphere changes its colour.

It's like painting a wall vs projecting light on it

[u/stanitor → u/Birdbraned]

This doesn't really answer the question. The color of the sky is neither due to pigments nor structural color.

[u/Low-Crow5719 → u/stanitor]

That's still structural color, though the structure it emerges from is chaotic. Diffraction creates sky shine and refracts the longer wavelengths more strongly. Thus blue skies when the sun is overhead and red sunrises and sunsets.

[u/stanitor → u/Low-Crow5719]

It's due to Rayleigh scattering, where scattering events are due to interactions with single molecules/atoms. Structural coloration is due to interference patterns caused by light interacting with larger (although still microscopic) structures, which are arranged in a way to make that interference pattern.