The difference between being blue vs. scattering blue light

How is the color of pigments different physics-wise than blue objects that are blue because they scatter light? What is the difference between a glacier that absorbs red wavelengths and emits blue vs. blue pigment that absorbs red wavelengths and emits blue?

There are lots and lots of things that look blue, but it is patiently explained that there is no blue pigment; this things are blue because they scatter blue light, or have structural coloration. In the case of glaciers, for example, the explanation is that they absorb the red wavelengths so emit the blue.


A blue pigment absorbs red & green, and only reflect (scatter) blue light. And in general, a pigment is an opaque solid substance, and the color is an intrinsic property of that substance.

Air, water & ice transmit red & green. Actually they transmit all colors. They scatter a tiny bit of blue light, and scatter even less green, and even less red.

Structural coloration is not an intrinsic property of the substance, which is why it’s usually distinguished from pigments. If you melt down a peacock feather and let it solidify, you don’t get a green lump of material.

Ore are you are asking the physics behind what makes blue pigments blue?

XKCD has made the argument that the sky is blue because the air is blue. But the problem with that is that, by the same standard, the answer to “why are sunsets red?” is “because the air is red”.

Usually, when we say that an object or substance is a particular color, it’s a statement about what wavelengths that thing is absorbing. When color is due to absorbtion, things will look the same color in reflected or transmitted light. But when something has color due to differential scattering, it’ll look different in reflected and transmitted light.

The conflict between these answers and the XKCD answer looks to me like a semantic distinction over what “being a certain colour” means. One says that since there are different mechanical reasons behind things looking a certain way, there should be separate categories. Basically that one is an illusion and the other is inherent. The other says that it’s all illusion, and that the so-called inherent colours are just a different type of illusion. Am I wrong?

What do you mean by “illusion”? Air is blue when you look at a volume of air illuminated by a white light (and not looking though the air at the light source). You can take a spectroscope or a take a photo through calibrated color filters and confirm it. I wouldn’t call that an illusion.

As an interesting side note to this: our liquid oxygen lines were submerged in insulated troughs filled with liquid nitrogen to keep the LOX super cooled. You could spot a small LOX leak because when it mixed with the LN2 in that area the result was a sky blue liquid.


And you could spot a large LOX leak because the area around it was a glowy red-orange plasma.

Well, ultimately I guess the answer is going to have to go into the physics.

IIRC from high school chemistry, when light strikes an atom it excites the electrons which move to a higher energy level. The electrons will emit photons and settle back down. The frequencies of the photons it emits may not be the same as the incoming ones, so cobalt might just emit photons in the blue range. The substance is said to “absorb” the other colors but I believe that is layman’s language and it is not absorbing anything at all in the sense that a black hole absorbs light. The energy doesn’t just disappear into an atom never to be seen again.

But how is scattering different than this process? The explanation of glaciers I mentioned can be found on many web sites, and it says that the ice “absorbs” red and so you see blue. And once we know the difference, how do know that ice isn’t really blue, that it just scatters blue light?

Does this mean that the answer to the OP is “nothing, or at least nothing significant”?

I’ve tried to explain what exactly the difference is. Whether or not that’s a significant difference is a matter of opinion and/or semantics, and outside my area of expertise.

Sorry, gotta switch from blue to yellow for simplicity. What about the following scenario:

You have an object that reflects basically pure red and pure green and absorbs everything else. It will look yellow to the human eye. Additive colors and all that.

So is this a yellow object or not? It is not putting out any pure yellow wavelengths at all.

Similarly the color on this page. That’s actually red and green you’re looking at.

To humans “yellow is yellow” but in terms of the Physics (and Biophysics), things are entirely different.

Don’t get me started about the Land Effect. Okay, I’ve started. Look at the examples here. Red+gray = blue to the eye.

Yeah, human vision (and that of other mammals and other animals) doesn’t tell the whole story about the spectrum of light. But that’s not really relevant, here. Any given spectrum has some color that it will be perceived as, and that’s all that matters for this question.

Consider, say, copper sulfate. If we shine a light on some copper sulfate crystals, or a solution of copper sulfate, and look at it from the side, we’ll see a spectrum that we describe as blue. If, instead, we shine the light through the crystals or solution, and look at the light that’s coming through, we’ll still see light that we describe as blue. No matter how we look at copper sulfate, it always looks blue. So we might as well just say “Copper sulfate is blue”.

But now consider air. If we take a bunch of air, and shine a light at it from the side, and look at the air, we’ll see light that we describe as blue. But, if we shine the light through the air, and look at the light coming through, we’ll see light that we describe as red. Does this mean that air is red, or that it’s blue? Or both at once? I don’t think anyone would say that it’s purple. Air behaves very differently from copper sulfate, in this regard.


We had a leak like that once in an industrial plant. The asphalt road exploded.