The slider in the first demo under "What's so special about blue" is broken; the colors are correct when the slider is all the way to the left or right, but not when it's in the middle.
Why is the sky blue?
251–260 of 279 posts
Re: Why is the sky blue?
#252At least there is an answer, a long answer, but we can explain it. Try answering to corollary question: why is the night sky black? We dont have any good answers to that one.
The big bang ... expansion ... something something ... dark energy....
Re: Why is the sky blue?
#253Earlier quoted context omitted.
Well, no; what's coming into the cloud is sunlight, filtered through the atmosphere. It scatters that light and so it has that color. The same like a heap of powdered salt in the palm of your hand, held up to the sun. Clouds illuminated by the setting sun aren't white.
> Well, no; what's coming into the cloud is sunlight, filtered through the atmosphere. It scatters that light and so it has that color. The model the article describes is: 1. The sun puts out a bunch of light. 2. Light that is lower-energy than blue light fails to be effectively scattered by the atmosphere, and follows a path from the sun to you. 3. Light at the levels of blue and up is effectively scattered by the a…
The cloud is getting light from both the sky background and direct sunlight.
Re: Why is the sky blue?
#254[deleted]
Re: Why is the sky blue?
#255Earlier quoted context omitted.
It also needs a bit of biology. Our eyes don't have a flat response over frequency, they're more sensitive to blue than violet. Violet gets scattered even more than blue, and the violet light does shift our perception of the color. But it does so less than it would if we had photoreceptors more sensitive to violet , so the resulting perceptual color depends not just on the intensity of the light at different frequenc…
There are differences in receptor behavior across species, but they are understandably clustered around the parts of the spectrum in which sol is most luminous. An earth-like planet orbiting a different star would likely have evolved photoreceptor arrangements which match that star instead. So after scratching the biology itch we'll probably need to talk about fusion byproducts in sol-like stars.
> An earth-like planet orbiting a different star would likely have evolved photoreceptor arrangements which match that star instead.
No, not really- the limitation is chemical, not evolutionarily-driven. Earth is very well lit in infrared, but it's very difficult to make a chemical that is biologically useful for seeing infrared because the wavelengths are just too long. Its very challenging to do more than the most primitive kinds of sensing in infrared. If our sun was much dimmer, we would probably be blind, but if not our eyes would still not see in far infrared. Same goes for ultraviolet- the energy is too high and molecular bonds are too weak. Seeing in visible light is a reversible reaction, but ultraviolet wouldn't be.
What you're saying is true of ocean animals, especially in the deep sea. They don't see red very well or at all, but the evolutionary pressure against seeing red is not terribly high except very deep where food is very limited.
There also is evolutionary pressure on our vision, but it has nothing to do with the sun. We're twice as sensitive to green since it is so common and important, but green comes from photosynthesis and not from the color of the sun. In a way, we are most sensitive to the least important color of light- the color that is not absorbed by plants. The wasted, useless byproduct of sunlight is what lets us identify food.
Plus, we actually basically only see in blue and green. The overlap between rods and red/green cones is huge. "red" and "green" as we perceive them are mostly fabrications of our neural circuits- if we were seeing them how our photoreceptors actually receive light, all shades of green/red would be very strongly mixed together. All shades of red would look significantly green except for the very farthest reds, which would look very dark because of low sensitivity.
Re: Why is the sky blue?
#256Earlier quoted context omitted.
Well, no; what's coming into the cloud is sunlight, filtered through the atmosphere. It scatters that light and so it has that color. The same like a heap of powdered salt in the palm of your hand, held up to the sun. Clouds illuminated by the setting sun aren't white.
> Well, no; what's coming into the cloud is sunlight, filtered through the atmosphere. It scatters that light and so it has that color. The model the article describes is: 1. The sun puts out a bunch of light. 2. Light that is lower-energy than blue light fails to be effectively scattered by the atmosphere, and follows a path from the sun to you. 3. Light at the levels of blue and up is effectively scattered by the a…
> s sending you more blue light than the background level.
The background level is black! No air, no scattering. d > A cloud that isn't between you and the sun is getting its light from the sky background, which is blue. Why is the cloud not blue?
The cloud is bathed in intense, direct sunlight which is slightly yellow, and it is exposed to a small amount of scattered blue light. It could be that this mixture whitens the color, essentially reconstituting the light.
Why does white paint look pure white? Or snow on the ground? Same reason. It's reflecting the sky blue in a small amount, plus the yellowish sunlight.
If you are in a dark enclosure like a cave or barn, and sunlight is streaming in through a small aperture, if you hold some white object up to that light, it doesn't look the same as if you do that outside because it's not illuminated by the blue sky, only by direct sunlight.
Moreover, the shadowed parts white object sitting outdors, exposed to sunlight. tend to be tinged with blue.
Re: Why is the sky blue?
#257I 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…
Isn't it the water that is blue? Water is mostly clear, but it is very very slightly blue.
Re: Why is the sky blue?
#258Earlier quoted context omitted.
There are differences in receptor behavior across species, but they are understandably clustered around the parts of the spectrum in which sol is most luminous. An earth-like planet orbiting a different star would likely have evolved photoreceptor arrangements which match that star instead. So after scratching the biology itch we'll probably need to talk about fusion byproducts in sol-like stars.
> they are understandably clustered around the parts of the spectrum in which sol [sun] is most luminous That would be understandable but it’s not what’s going on: http://hyperphysics.phy-astr.gsu.edu/hbase/Chemical/watabs.h...
I didn't know that about water, thanks.
Re: Why is the sky blue?
#259Earlier quoted context omitted.
maybe in 100M years we'll get a 4th cone or rod. Probably from nuclear mutation...
There's some evidence that tetrachromacy already exists in a few humans. If so we have the gene already. But why would it spread?
this is true of many human mutations with the development of medical care, its not like we face evolutionary pressure for many things
Re: Why is the sky blue?
#260Earlier quoted context omitted.
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)
Except that one is transmissive and the other reflective. They're not the same kind of thing. TBH I feel like a demo like this is the only way to get an intuitive feel for it: https://www.youtube.com/watch?v=-Xx7sPPTu3Y .
(I agree that just going on about Rayleigh scattering is probably overly obtuse: at least not without explaining that scattering is part of how color is formed in the first place. But it's also not just a case of 'well air is blue like apple juice is orange')