One128 wrote:There are, however, numerous misconceptions in your response. I'll try to address them one by one.
Heh, and fail!
First, red illumination is not an analogy for the sky. Instead, shine white light in a fog-filled room with black-painted walls, and observe a bright blue color. The fog is the origin of that color. The fog "is blue." The blue color may be a consequence of pigments, or of interference, photonic crystals, rayleigh effect, etc., but the color is located in the fog, not in the light.
One128 wrote:First of all, what hits our eyes when we look at the sky isn't reflected light - it's scattered light.
Yes, get our terms correct: scattering is a subset of reflection: scattered light is always reflected light, but reflected light isn't necessarily scattered light.
One128 wrote:In the most generic case of diffuse white lighting - one that we generally use as a standard for evaluating color - we would simply see white light. There is no basis for calling the air blue - any more than calling it red, yellow or white.
All your misconceptions hinge on your error above.
On the contrary: for light scattered by air, the scattered color is blue, regardless of source diameter. In other words, for diffuse white lighting, the perceived color of air is blue, not white. This may be difficult to demonstrate using a 10KM cube of atmosphere, but we can use a miniature version which displays Rayleigh color: use a piece of aerogel.
Aerogel under collimated illumination is blue, aerogel under diffuse illumination is blue. Only under transmitted light does aerogel appear red-orange. But note well: to perceive the blue color, the aerogel (and the atmosphere) must be observed against a dark background. If somehow we should combine the transmitted and the scattered light, for example by placing the aerogel against white paper, then the observed colors will vanish. In "the blue sky" case, this doesn't occur, since the layer of air is backed up by a black background.
One128 wrote:Air is indeed not a pigmented material, but it is also not a colored material. Not any more than you can say a CD is made of colored material
But a CD *is* a colored material! Like soap bubbles and glass interference filters, the color of these materials is structural. Google: structural colors. The CDROM aluminum itself is colorless broadband reflecting. The rainbow colors are a consequence of the diffraction grating. Like cdroms and holographic foil giftwrap, dichroic inks are colored materials, and their color varies with observation/illumination angle.
One hangup I detect in most people: equating pigment with "having color." So, if it's not pigmented, therefore it cannot "have color?" This is wrong. Materials with structural colors (such as soap films, CDs, bluejays, human irises, aerogel, the sky...) all "have color" without having pigments. Destroy the structure, grind up the photonic crystal layer or the thinfilm stack, and unlike with pigments, the colors vanish. Tear the iris muscle out of a blue eye, place it on a white surface, and you'll not detect any blue at all. There was no blue pigment: the color was entirely structural.
Perhaps this will illuminate the problem: water films, soap bubbles, oil films, they are rainbow-colored materials, yet water itself is colorless.
The word "water" doesn't imply "thin film." Only water with the thinfilm structure is a colored material. An oil slick is rainbow colored, while the oil itself is not. On the other hand, the word "aerogel" does
imply a structure: although silica is colorless, yet silica aerogels are blue materials (except when viewed with transmitted light.) The color isn't in the silica molecule, it's in the gel structure at larger scale.
One128 wrote: just because you can see colors on it when you light it in a particular way. Under diffuse white light, air doesn't have any color (and neither does a CD, discounting the sometimes fancifully dyed plastic).
Yes, good point: the colors of CDs and of reflection holograms require collimated light. But the blue color of the sky does not. If we place air (or a hunk of aerogel) against a black surface, then illuminate it with a diffusing hemisphere, it will still appear blue. But, if we replace the black background with white, then the blue color disappears, same as with bluejay wings and blue irises in human eyes. The blue structural color requires that we prevent the orange transmitted light from getting back to the observer.
One128 wrote:You can verify this next time you are under a completely overcast sky (which gives out diffuse white light). The air won't appear blue
Might that be because 10-20KM distances are required? Maybe not, because many other structural colored materials
behave that way too. In general, if a material splits light into brightly colored transmitted and reflected/scattered beams, and then if we then set up a situation in which we recombine the transmitted and scattered beams, then the observed colors will vanish. The structure was changed. Peel the photonic crystal layer off a black-pigmented bluejay feather, and paste it on a white feather instead, and the feather appears white. The split blue and orange rays were recombined. For bluejays to "be blue," the black background is required, it absorbs the transmitted orange light. I conclude: bluejays are blue. Apparently you conclude that bluejays are colorless, because the color requires specific illumination, and the materials involved have no pigments?
One128 wrote:This should have rung a bell - clearly, if the air appears red when we shine light on it in a particular way, there is no basis for calling it blue, as opposed to red. What reason is there for calling it blue, and not red? Because we like shining from the side more than we like shining from behind?
Air is blue. Also, air is red. Air is a dichroic material, much like color-changing inks and paints. Or like rainbow-colored soap bubbles.
Find a car with dichroic paint. The car looks bright red. What there has the red color? Not the illumination. The color is produced by the dichroic paint. Now change your angle, and the car is bright green. What material there is green? The dichroic paint.
One128 wrote:Under diffuse white light (shining uniformly from all sides),
Ah, that's different.
If we observe the transmitted light combined with the reflected light, then all structural colors vanish, and only pigments remain. Your recipe for light shining uniformly from all sides is a good way to detect pigments, and separate them from structural-color materials such as bluejays and Morpho wings and solutions of suspended nanodots.
If you then use this to deny the existence of structural colors, then I think you've left the path of simple truth and scientific thinking. Bluejay feathers really "are blue," even if the color requires a black absorptive background, it isn't based on pigments, and hence disappears under certain types of illuminations.