Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Why is the sky blue?

explainers.blog

Post reply on HN
Print view View original

131–140 of 279 posts

Re: Why is the sky blue?

#131

Post by jxdxbx »

If you say that air isn’t “really” blue because the reason it is blue is different than the reason that other things are blue (e.g. most blue things in daily life absorb frequencies other than blue), this is equivalent to saying that birds with blue feathers due to structural coloration—where instead of pigment, microscopic structures interfere with light—don’t “really” have blue feathers.

This is just silly. Some birds have blue feathers. There are various ways to be blue. Similarly, air is blue.

Re: Why is the sky blue?

#132

Post by mncharity »

zehaeva wrote: post #66
oxag3n wrote: post #60

I have a related but deeper question about sun and colors: Sunlight in space is considered white. When it reaches earth surface, it's considered a warmer color. Why human eyes that never (during evolution) saw sunlight without the atmosphere, consider it true white, and not colder color?

I think at this point you need to consider how the human eye see color. It's not like each wavelength gets picked up and then communicated perfectly. (I'm going to skip over some basic stuff, and use some generalities) Each Cone in the eye responds to a range of frequencies. This means that things that unless it's on the extreme low, or high, end of the frequencies that the human eye can discern you are going to have…

IIUC, saturation is a (not uncommon) distractor here. As you get the same observation when desaturated by a neutral filter. Even on the "ground" with low air mass (Sun vertical, at altitude, etc).

Re: Why is the sky blue?

#133

Post by __MatrixMan__ »

KellyCriterion wrote: post #5

Interesting here is: Actually, for most blue butterflies, it’s not even a pigment-it’s just a trick of the light. Since blue is so rare in the biological world (hardly any plants or animals can produce real blue chemicals), they evolved structural colors. Their wings have these microscopic ridges that reflect blue light while canceling out other colors. It’s basically the same reason the sky looks blue, just built in…

I wonder if the interference-based-blue of the morpho butterfly evolved because it's difficult to make blue pigment for some reason having to do the chemistry of our biosphere, or if it's an evolutionary response to humans who may have captured the blue ones and ground them up for pigment (much like we did with https://en.wikipedia.org/wiki/Tyrian_purple snails).

I'm not aware of any record of us having done so, but it's absolutely the kind of thing we would do, and there's much more pre-history than history when it might've happened.

Re: Why is the sky blue?

#135

Post by rcxdude »

jxdxbx wrote: post #128
thoughtlede wrote: post #103

I think we can simplify the answer to this question for most audience and say "the air is blue". If they say, the air appears to be clear when I stare at something other than sky, the answer is you need more of air to be able to see its blue-ness, in much the same way that a small amount of murky water in your palm appears clear, but a lot of it does not. If they ask, why don't I see that blue-ness at dawn or dusk, t…

Yes, I came here to say this. The whole topic drives me crazy. Air is just blue. Everything is a color because of some physics reason. Some birds have blue wings due to microscopic structures and how light interacts with them, rather than pigment. If you took a large column of air into space and shined white light through it, it would be blue.

No, it would look red. The weird thing about air is that it's not reflection or absorption that gives the color, but scattering, and that means the color is strongly dependent on what direction you are looking at it from in a way that most transparent mediums aren't.

Re: Why is the sky blue?

#137

Post by nostrademons »

jet98 wrote: post #92
smegger001 wrote: post #85

Earlier quoted context omitted.

because they are the frequencies that pass though water most readily, and we are made of mostly water

that's interesting. I thought it was because our sun's spectrum has the most energy in visible light band - therefore we evolved to see the light which can give us the highest SNR. Can you be more specific?

The sun's spectrum doesn't have the most energy in the visible light band, though it's close. Most of the energy is in the infrared band:

https://sunwindsolar.com/blog/solar-radiation-spectrum/?v=0b...

Both the "because that's what the sun emits" and "because we are mostly water" explanations are incomplete. There are plenty of other animals [1] that can "see" infrared.

The real reason is simply because that's how we evolved. That's how the "because those are the frequencies that pass through water" explanation comes into play: vision first evolved in aquatic animals, so frequencies that don't penetrate water wouldn't have been all that helpful to their survival and reproductive success, and so wouldn't be selected for. But that's incomplete too: salmon are one of the top IR-sensing animals and they live in water, so when there's an evolutionary need to select for IR vision, it happens. The reason we "see" in the visible light range is simply that that's how we've defined "visible".

There are some physics reasons as well, notably that most mammalian body structures emit heat, which would blind an animal that relies on infrared to see (notice how most of the animals that can see infrared are cold-blooded reptiles, fish, and insects), and that most of the high-resolution biochemical mechanisms that can convert electromagnetic waves to electrochemical nerve impulses operate in the visible light range. Structures that convert infrared radiation to nerve impulses are more complex and more costly to support, so unless there's a clear survival benefit for the species, they tend to get selected away.

[1] https://a-z-animals.com/animals/lists/animals-that-can-see-i...

Re: Why is the sky blue?

#138

Post by mncharity »

thoughtlede wrote: post #103

I think we can simplify the answer to this question for most audience and say "the air is blue". If they say, the air appears to be clear when I stare at something other than sky, the answer is you need more of air to be able to see its blue-ness, in much the same way that a small amount of murky water in your palm appears clear, but a lot of it does not. If they ask, why don't I see that blue-ness at dawn or dusk, t…

As confusion elsewhere on this page illustrates, one also needs to clarify absorption. "It's just blue" sky and "it's just blue" stained-glass have quite different behavior. Both side scatter some blue, but while one mostly transmits the rest, the other mostly absorbs the rest, for very different experiences peering through it.

So perhaps "clear with a blue tint"?

Re: Why is the sky blue?

#139

Post by jxdxbx »

rcxdude wrote: post #135
jxdxbx wrote: post #128

Earlier quoted context omitted.

Yes, I came here to say this. The whole topic drives me crazy. Air is just blue. Everything is a color because of some physics reason. Some birds have blue wings due to microscopic structures and how light interacts with them, rather than pigment. If you took a large column of air into space and shined white light through it, it would be blue.

No, it would look red. The weird thing about air is that it's not reflection or absorption that gives the color, but scattering, and that means the color is strongly dependent on what direction you are looking at it from in a way that most transparent mediums aren't.

Ok, so the air would be red from one angle, blue from another. In each case, that is what color the air “really” is, in the same sense that a butterfly’s wings are blue (but not from every angle)

Re: Why is the sky blue?

#140

Post by mncharity »

yawpitch wrote: post #77

The sky isn’t blue. It’s transparent. That’s why you can see stars that aren’t blue at night. When struck by sunlight at the right angles it appears blue, but saying it is blue is like saying the ocean is green when a bucket of it clearly isn’t.

Perhaps "transparent with a blue tint"?
Post reply on HN