I wonder if any of the more scientifically minded people on here could help me, please.

[Horsa]

Basically "particles" that are responsible for keeping some other, "larger" sub -atomic particles together. But at those scales things are not "black and white", the existence of such particles is different from the existence of the macroscopic bodies that we know in day to day life. At the end of the day you have a lot of complicated equations and you interpret some solutions to those equations as a particle (when some conditions are met).

Thank you very much for the information.

 

[mrzz]

In the Rio Open thread an interesting topic appeared: it seems that there are some claims that Rio de Janeiro has the "bluest sky" in the world. It does not make that much sense to me, but this is an interesting topic so I will try a few words on it. I have a feeling that a certain "Polaussie" signal analysis scientist will have a lot to add.

First, from a purely physical point of view, the color of the sky is a phenomenon mostly driven by what happens in the outer layers of the atmosphere. What we call "color" is related to the wavelength of the electromagnetic waves that constitute light. The thing is that "color" is the name that we give to what we see, therefore to what is the result from the interaction from this wave with our brain. So at the end of the day we need to consider at least three "layers":

1) The wave itself and phenomena related to it;
2) What happens in our eyes;
3) What happens in our brain.

The sky is blue mainly to what happens, as I wrote above, in the atmosphere, in an effect called "light refraction". In a nutshell, light changes its direction when it changes the medium on which is traveling. Just think of the famous cover of Pink Floyd's "Dark side of the moon" (very far from my favorite Floyd albums, btw). Sunlight enters our atmosphere in a non perpendicular angle and things happen just like in the album cover. We down here on the surface are on the "blue region" (that is, most of the light that is refracted on the sky is blue. What comes straight from the sun is still white, though). This is basically why we travel by airplane and get real high in the sky, it goes to a darker blue tone. Astronauts even on very low orbits don't see blue, even fighter planes that go really high (Russian ones!) can see a dark sky (not enough atmosphere above to refract light), and why the sky turns red in sunset and dawn (different refraction angle due to different relative position of observer and sun regarding the atmosphere).

So if we were only to consider that, it would hardly be true that any city could claim to have a "bluest sky". Some geographic considerations could be made, and in that case I would say that cities in the Netherlands (bellow sea level and at reasonable latitude) would get the prize (albeit for the slimmest margin, probably impossible to discern with naked eye).

But would other physical effects come in to play? Humidity has a role, yes, as it changes the refraction angle. Not sure if it helps or disturbs the "blueness" (would need to check some numbers and do some calculations), but anyway if it is the case there are a multitude of sea level/high humidity area cities in the world, not just Rio, which is in fact not particularly humid. It could be on some sort of sweet spot, but so would be a lot of other places. I guess clean air would help, in that case Rio is also in a decent position but surely there are thousands of similar places with cleaner air.

When we consider the "what happens in our eyes" part, we basically thicken the plot above, because, among other things, different stimuli can lead to the exact same effect on the eye cells.

Light can be mono-chromatic, that is, just one wave-length, but in most (natural) cases it is poly-chromatic, as we have wave packets. Now, roughly speaking, the eye cells send to the brain the "average effect" of the incoming light, so (roughly speaking again) if you have two pulses of, say, 480 and 520 Nanometers of wavelength, and one with the combined intensity of both, but with 500 nm of WL, you basically see the same color (the math is not that simple, things are not exactly linear, but the point is that there are various combinations of wave-lengths and intensities that would produce the same effect).

Why this is important? Because different wave-lengths will refract differently. So it adds some complexity to the analysis. At the same time, it is responsible for the beautiful, subtle changes in tone we see in a lot of situations. Complexity can be beautiful in simple ways too.

But what really messes up everything is what happens in our brain (not exactly a surprise, isn't it?), because our brain takes things in to context (this is true for the "our eyes part" too, in fact). We all know that if you put two colors side by side, and you change one of them, the way that you see the one which was fixed changes. So the same sky will look different in different places. Let alone the fact that different people have different eyes... last but not least, psychological effects will probably have a giant drive here (a lot of subjects on this thread would benefit from psychologist's eyes, @El Dude). I have an impression that people in a good mood will have tendency to see the bluest skies (in a beautiful but somewhat contradictory piece of word play, thinking of my friend @Horsa).

So, put it all together and I really think that the blueness of the sky is in the eye of the beholder. My friends from Rio (cariocas) maybe won't like it, but if you give a second thought, it is not a bad thing, as it might tell more something about their eyes rather than their city.

And, of course, that they brag like hell.

 

[Horsa]

In the Rio Open thread an interesting topic appear: it seems that there are some claims that Rio de Janeiro has the "bluest sky" in the world. It does not make that much sense to me, but this is an interesting topic so I will try a few words on it. I have a feeling that a certain "Polaussie" signal analysis scientist will have a lot to add.

First, from a purely physical point of view, the color of the sky is a phenomenon mostly driven by what happens in the outer layers of the atmosphere. What we call "color" is related to the wavelength of the electromagnetic waves that constitute light. The thing is that "color" is the name that we give to what we see, therefore to what is the result from the interaction from this wave with our brain. So at the end of the day we need to consider at least three "layers":

1) The wave itself and phenomena related to it;
2) What happens in our eyes;
3) What happens in our brain.

The sky is blue mainly to what happens, as I wrote above, in the atmosphere, in an effect called "light refraction". In a nutshell, light changes its direction when it changes the medium on which is traveling. Just think of the famous cover of Pink Floyd's "Dark side of the moon" (very far from my favorite Floyd albums, btw). Sunlight enters our atmosphere in a non perpendicular angle and things happen just like in the album cover. We down here on the surface are on the "blue region" (that is, most of the light that is refracted on the sky is blue. What comes straight from the sun is still white, though). This is basically why we travel by airplane and get real high in the sky, it goes to a darker blue tone. Astronauts even on very low orbits don't see blue, even fighter planes that go really high (Russian ones!) can see a dark sky (not enough atmosphere above to refract light), and why the sky turns red in sunset and dawn (different refraction angle due to different relative position of observer and sun regarding the atmosphere).

So if we were only to consider that, it would be hardly be true that any city could claim to have a "bluest sky". Some geographic considerations could be made, and in that case I would say that cities in the Netherlands (bellow sea level and at reasonable latitude) would get the prize.

But would other physical effects come in to play? Humidity has a role, yes, as it changes the refraction angle. Not sure if it helps or disturbs the "blueness" (would need to check some numbers and do some calculations), but anyway if it is the case there are a multitude of sea level/high humidity area cities in the world, not just Rio, which is in fact not particularly humid. It could be on some sort of sweet spot, but so would be a lot of other places. I guess clean air would help, in that case Rio is also in a decent position but surely there are thousands of similar places with cleanest air.

When we consider the "what happens in our eyes" part, we basically thicken the plot above, because, among other things, different stimuli can lead to the exact same effect on the eye cells.

Light can be mono-chromatic, that is, just one wave-length, but in most (natural) cases it is poly-chromatic, as we have wave packets. Now, roughly speaking, the eye cells send to the brain the "average effect" of the incoming light, so (roughly speaking again) if you have two pulses of, say, 480 and 520 Nanometers of wavelength, and one with the combined intensity of both, but with 500 nm of WL, you basically see the same color (the math is not that simple, things are not exactly linear, but the point is that there are various combinations of wave-lengths and intensities that would produce the same effect).

Why this is important? Because different wave-lengths will refract differently. So it adds some complexity to the analysis. At the same time, it is responsible for the beautiful, subtle changes in tone we see in a lot of situations. Complexity can be beautiful in simple ways too.

But what really messes up everything is what happens in our brain (not exactly a surprise, isn't it?), because our brain takes things in to context (this is true for the "our eyes part" too, in fact). We all know that if you put two colors side by side, and you change one of them, the way that you see the one which was fixed changes. So the same sky will look different in different places. Let alone the fact that different people have different eyes... last but not least, psychological effects will probably have a giant drive here (a lot of subjects on this thread would benefit from psychologist's yes, @El Dude). I have an impression that people in a good mood will have tendency to see the bluest skies (in a beautiful but somewhat contradictory piece of word play, thinking of my friend @Horsa).

So, put it all together and I really think that the blueness of the sky is in the eye of the beholder. My friends from Rio (cariocas) maybe won't like it, but if you give a second thought, it is not a bad thing, as it might tell more something about their eyes rather than their city.

And, of course, that they brag like hell.

I was actually going to ask a question about the same statement "the sky being bluer in Rio than elsewhere" but thought I'd be nice to you & not ask so many questions especially as I've been under the influence of some sweet & fruity rose wine for the last 3 days left over from my Birthday last Tuesday. I thought the same as you did & remember quite a lot of the information you come up with.

I agree our eyes are different. Mine are a perfect example with 1 eye doing a lot more work than the other so much so that although I've got perfect spectacles at the moment anyway I don't always see as well as I could. The funny thing is I often notice mistakes others don't & read very well anyway. I don't even notice 1 eye is doing more work than the other until I have eye tests. People attempting to use paralanguage on me don't get on very well as instead of my eyeballs going up & down & to the right & left depending which part of my brain I'm accessing mine just go up & down as my optician told me. She also told me that that was because of squint surgery I had as a little girl. People who know me know I normally tell the truth anyway & there are other ways of telling whether someone is lying or not. They're not 100% accurate anyway.

I agree. This thread could do with more people who see things through all different perspectives & from all different backgrounds. We do have fascinating conversations here though.

Thank you very much for the word play.

 

[Horsa]

Right guys, what do you think of this?

https://www.msn.com/en-gb/news/worl...around-neptune/ar-BBTOOfn?ocid=spartanntp_edu

I've noticed they've called it something with the Greek prefix meaning horse.

 

[Chris Koziarz]

In the Rio Open thread an interesting topic appeared: it seems that there are some claims that Rio de Janeiro has the "bluest sky" in the world. It does not make that much sense to me, but this is an interesting topic so I will try a few words on it. I have a feeling that a certain "Polaussie" signal analysis scientist will have a lot to add.

First, from a purely physical point of view, the color of the sky is a phenomenon mostly driven by what happens in the outer layers of the atmosphere. What we call "color" is related to the wavelength of the electromagnetic waves that constitute light. The thing is that "color" is the name that we give to what we see, therefore to what is the result from the interaction from this wave with our brain. So at the end of the day we need to consider at least three "layers":

1) The wave itself and phenomena related to it;
2) What happens in our eyes;
3) What happens in our brain.

The sky is blue mainly to what happens, as I wrote above, in the atmosphere, in an effect called "light refraction". In a nutshell, light changes its direction when it changes the medium on which is traveling. Just think of the famous cover of Pink Floyd's "Dark side of the moon" (very far from my favorite Floyd albums, btw). Sunlight enters our atmosphere in a non perpendicular angle and things happen just like in the album cover. We down here on the surface are on the "blue region" (that is, most of the light that is refracted on the sky is blue. What comes straight from the sun is still white, though). This is basically why we travel by airplane and get real high in the sky, it goes to a darker blue tone. Astronauts even on very low orbits don't see blue, even fighter planes that go really high (Russian ones!) can see a dark sky (not enough atmosphere above to refract light), and why the sky turns red in sunset and dawn (different refraction angle due to different relative position of observer and sun regarding the atmosphere).

So if we were only to consider that, it would hardly be true that any city could claim to have a "bluest sky". Some geographic considerations could be made, and in that case I would say that cities in the Netherlands (bellow sea level and at reasonable latitude) would get the prize (albeit for the slimmest margin, probably impossible to discern with naked eye).

But would other physical effects come in to play? Humidity has a role, yes, as it changes the refraction angle. Not sure if it helps or disturbs the "blueness" (would need to check some numbers and do some calculations), but anyway if it is the case there are a multitude of sea level/high humidity area cities in the world, not just Rio, which is in fact not particularly humid. It could be on some sort of sweet spot, but so would be a lot of other places. I guess clean air would help, in that case Rio is also in a decent position but surely there are thousands of similar places with cleaner air.

When we consider the "what happens in our eyes" part, we basically thicken the plot above, because, among other things, different stimuli can lead to the exact same effect on the eye cells.

Light can be mono-chromatic, that is, just one wave-length, but in most (natural) cases it is poly-chromatic, as we have wave packets. Now, roughly speaking, the eye cells send to the brain the "average effect" of the incoming light, so (roughly speaking again) if you have two pulses of, say, 480 and 520 Nanometers of wavelength, and one with the combined intensity of both, but with 500 nm of WL, you basically see the same color (the math is not that simple, things are not exactly linear, but the point is that there are various combinations of wave-lengths and intensities that would produce the same effect).

Why this is important? Because different wave-lengths will refract differently. So it adds some complexity to the analysis. At the same time, it is responsible for the beautiful, subtle changes in tone we see in a lot of situations. Complexity can be beautiful in simple ways too.

But what really messes up everything is what happens in our brain (not exactly a surprise, isn't it?), because our brain takes things in to context (this is true for the "our eyes part" too, in fact). We all know that if you put two colors side by side, and you change one of them, the way that you see the one which was fixed changes. So the same sky will look different in different places. Let alone the fact that different people have different eyes... last but not least, psychological effects will probably have a giant drive here (a lot of subjects on this thread would benefit from psychologist's eyes, @El Dude). I have an impression that people in a good mood will have tendency to see the bluest skies (in a beautiful but somewhat contradictory piece of word play, thinking of my friend @Horsa).

So, put it all together and I really think that the blueness of the sky is in the eye of the beholder. My friends from Rio (cariocas) maybe won't like it, but if you give a second thought, it is not a bad thing, as it might tell more something about their eyes rather than their city.

And, of course, that they brag like hell.

I like your conclusion that blue sky is "in the eye of the beholder" and how u came to it. It's such an ephemeral quantity that it's impossible to measure it objectively.
However, your description of the physical phenomena leading to the perceived blue colour of the sky is somewhat imprecise and needs clarification.
You mention "light refraction" in the atmosphere, and give the picture of "Dark side of the moon" cover. That picture of a light beam dispersion due to difference of speed of light in air vs. glass, resulting in angled separation of light spectrum is an equivalent of a rainbow phenomenon:
https://en.wikipedia.org/wiki/Rainbow
The "blue sky" perception is due not not quite to that phenomenon, but to the phenomenon of electromagnetic wave scattering. In particular, a type of scattering when the particle sizes are less than 1 tenth of the wavelength, which is the case of N2 O2 particles that make up the air and 400-700nm wavelength of visible light. This type of scattering is called Rayleigh scattering:
https://en.wikipedia.org/wiki/Rayleigh_scattering
In short what we see as "the sky" is the mass of O2 N2 particles taking the energy from sun's radiation and then "giving off" the radiation in all directions, like a large diffuser. I don't quite understand the precise mechanism (Rayleigh did for what he received Nobel Price) but apparently no quantum energy excitement is involved as in infrared radiation absorption by greenhouse gasses.
But whatever the precise mechanism of Rayleigh scattering, It happens that shorter waves (blue) are scattered more than the longer ones (red). That's why we see the sky as "glowing blue". The sunset is red, because when you look at the sunset, you look at the sun light filtered by the atmosphere, i.e. what is left of the sun rays after the scattering process. If the process removes some of the blue spectral component, what's left appears to be red. Even if you look directly into the sun overhead, it still appears not perfectly white but slightly yellow (no coincidence we always paint the sun in yellow) because some small amount of blue scattering is still removed.
Perhaps by "dispersion" you meant "scattering", yet I wanted to clarify it here. Rayleigh scattering happens around O2 N2 particles. Dispersion, refraction, and other ray tracing phenomena happen around the particles much bigger than the incoming electromagnetic wavelength. Raindrops are such particles, with resulting rainbow phenomenon. But when there is no diffraction/dispersion of the light and there is no rainbow (when observer is at most angles) the clouds appear white with various intensities (dark when intensity is very low due to blocking of the sun-rays) but no blue. That's because light scattering by large particles such as water droplets (or even pollution smog, still much larger than O2 N2) is the same for all wavelengths (red or blue). The dependency of scattering on wavelength shown nicely on this picture:
https://en.wikipedia.org/wiki/Diffuse_sky_radiation#/media/File:Rayleigh_sunlight_scattering.png
is for O2 N2 particles only.

 

[Horsa]

I like your conclusion that blue sky is "in the eye of the beholder" and how u came to it. It's such an ephemeral quantity that it's impossible to measure it objectively.
However, your description of the physical phenomena leading to the perceived blue colour of the sky is somewhat imprecise and needs clarification.
You mention "light refraction" in the atmosphere, and give the picture of "Dark side of the moon" cover. That picture of a light beam dispersion due to difference of speed of light in air vs. glass, resulting in angled separation of light spectrum is an equivalent of a rainbow phenomenon:
https://en.wikipedia.org/wiki/Rainbow
The "blue sky" perception is due not not quite to that phenomenon, but to the phenomenon of electromagnetic wave scattering. In particular, a type of scattering when the particle sizes are less than 1 tenth of the wavelength, which is the case of N2 O2 particles that make up the air and 400-700nm wavelength of visible light. This type of scattering is called Rayleigh scattering:
https://en.wikipedia.org/wiki/Rayleigh_scattering
In short what we see as "the sky" is the mass of O2 N2 particles taking the energy from sun's radiation and then "giving off" the radiation in all directions, like a large diffuser. I don't quite understand the precise mechanism (Rayleigh did for what he received Nobel Price) but apparently no quantum energy excitement is involved as in infrared radiation absorption by greenhouse gasses.
But whatever the precise mechanism of Rayleigh scattering, It happens that shorter waves (blue) are scattered more than the longer ones (red). That's why we see the sky as "glowing blue". The sunset is red, because when you look at the sunset, you look at the sun light filtered by the atmosphere, i.e. what is left of the sun rays after the scattering process. If the process removes some of the blue spectral component, what's left appears to be red. Even if you look directly into the sun overhead, it still appears not perfectly white but slightly yellow (no coincidence we always paint the sun in yellow) because some small amount of blue scattering is still removed.
Perhaps by "dispersion" you meant "scattering", yet I wanted to clarify it here. Rayleigh scattering happens around O2 N2 particles. Dispersion, refraction, and other ray tracing phenomena happen around the particles much bigger than the incoming electromagnetic wavelength. Raindrops are such particles, with resulting rainbow phenomenon. But when there is no diffraction/dispersion of the light and there is no rainbow (when observer is at most angles) the clouds appear white with various intensities (dark when intensity is very low due to blocking of the sun-rays) but no blue. That's because light scattering by large particles such as water droplets (or even pollution smog, still much larger than O2 N2) is the same for all wavelengths (red or blue). The dependency of scattering on wavelength shown nicely on this picture:
https://en.wikipedia.org/wiki/Diffuse_sky_radiation#/media/File:Rayleigh_sunlight_scattering.png
is for O2 N2 particles only.

That's fascinating & very informative. Thank you very much, Chris. Some of what you had to say transported me back to High School science lessons where we learnt why things are the colour they are & how light disperses as well as looking at glass prisms & the colours which it gives off. We learnt the Mnemonic Richard of York gave battle in vain to remind us of these colours. I did, however, learn a lot from the rest of your post.

 

[Chris Koziarz]

Right guys, what do you think of this?

https://www.msn.com/en-gb/news/worl...around-neptune/ar-BBTOOfn?ocid=spartanntp_edu

I've noticed they've called it something with the Greek prefix meaning horse.

I think the name given to this moon is a little frivolous: it probably does not make any sense to most people,including myself. And the name may be confusing to some because it clashes with well established name of hippocampus (part of brain). Perhaps the author of the new moon name is a horse lover like yourself. Otherwise, why would s/he choose such name?

 

[Horsa]

I think the name given to this moon is a little frivolous: it probably does not make any sense to most people,including myself. And the name may be confusing to some because it clashes with well established name of hippocampus (part of brain). Perhaps the author of the new moon name is a horse lover like yourself. Otherwise, why would s/he choose such name?

I was thinking something similar. Not all horse lovers know all the origins of these words though so maybe the person who named the new moon isn't just any old horse lover but a horse lover who is aware of the origins of words. I've no idea why s/he would choose the name otherwise.

Maybe they were into Greek mythology because Hippocamps were mythological sea horses. I've just took some advice you gave me a bit since & looked for myself. I found this:-

https://www.theoi.com/Ther/Hippokampoi.html

I love the picture included in the link.

 

[Horsa]

What is "two-photon" in-vivo microscopy, please?

 

[Horsa]

Why is the scientific name for the puffin fratercula arctica when they live in Britain which is nowhere near the Arctic?

 

[mrzz]

I like your conclusion that blue sky is "in the eye of the beholder" and how u came to it. It's such an ephemeral quantity that it's impossible to measure it objectively.
However, your description of the physical phenomena leading to the perceived blue colour of the sky is somewhat imprecise and needs clarification.
You mention "light refraction" in the atmosphere, and give the picture of "Dark side of the moon" cover. That picture of a light beam dispersion due to difference of speed of light in air vs. glass, resulting in angled separation of light spectrum is an equivalent of a rainbow phenomenon:
https://en.wikipedia.org/wiki/Rainbow
The "blue sky" perception is due not not quite to that phenomenon, but to the phenomenon of electromagnetic wave scattering. In particular, a type of scattering when the particle sizes are less than 1 tenth of the wavelength, which is the case of N2 O2 particles that make up the air and 400-700nm wavelength of visible light. This type of scattering is called Rayleigh scattering:
https://en.wikipedia.org/wiki/Rayleigh_scattering
In short what we see as "the sky" is the mass of O2 N2 particles taking the energy from sun's radiation and then "giving off" the radiation in all directions, like a large diffuser. I don't quite understand the precise mechanism (Rayleigh did for what he received Nobel Price) but apparently no quantum energy excitement is involved as in infrared radiation absorption by greenhouse gasses.
But whatever the precise mechanism of Rayleigh scattering, It happens that shorter waves (blue) are scattered more than the longer ones (red). That's why we see the sky as "glowing blue". The sunset is red, because when you look at the sunset, you look at the sun light filtered by the atmosphere, i.e. what is left of the sun rays after the scattering process. If the process removes some of the blue spectral component, what's left appears to be red. Even if you look directly into the sun overhead, it still appears not perfectly white but slightly yellow (no coincidence we always paint the sun in yellow) because some small amount of blue scattering is still removed.
Perhaps by "dispersion" you meant "scattering", yet I wanted to clarify it here. Rayleigh scattering happens around O2 N2 particles. Dispersion, refraction, and other ray tracing phenomena happen around the particles much bigger than the incoming electromagnetic wavelength. Raindrops are such particles, with resulting rainbow phenomenon. But when there is no diffraction/dispersion of the light and there is no rainbow (when observer is at most angles) the clouds appear white with various intensities (dark when intensity is very low due to blocking of the sun-rays) but no blue. That's because light scattering by large particles such as water droplets (or even pollution smog, still much larger than O2 N2) is the same for all wavelengths (red or blue). The dependency of scattering on wavelength shown nicely on this picture:
https://en.wikipedia.org/wiki/Diffuse_sky_radiation#/media/File:Rayleigh_sunlight_scattering.png
is for O2 N2 particles only.

Thanks, Chris. I confess that when I wrote the post I knew I was cutting some corners, but was simply too lazy to look after a lot of things I have actually forgot. Now I have glanced a bit on some of the topics thanks to your post.

Anyway, just as some sort of half (actually more a quarter of) justification, when you go from "refraction" to "scattering" there is a difference in approach, that is, from geometric optics to physical optics. Ultimately, light passing through any kind of material medium is always scattering. In other words, scattering will always explain light refraction. I kept myself on the geometric level partly for clarity, but as I said above also partly from ignorance and a hell lot of "rust".

As for your remark "I don't quite understand the precise mechanism (Rayleigh did for what he received Nobel Price) but apparently no quantum energy excitement is involved as in infrared radiation absorption by greenhouse gasses.", yes, no quantum considerations needed, you can fully explain Rayleigh's scattering with classical electromagnetic theory, by use of the short wavelength approximation to Maxwell's equations

I guess I found a decent summary here.

 

[Horsa]

To tell you the truth when Britbox mentioned about the sky being bluer somewhere not only did I think the same as you but on a simpler level but I also got a song that I've heard a few times & like running through my head. This is the song that ran through my head. I haven't been able to stop singing it since.

 

[Horsa]

What does P.U.F.A. stand for, please?

What does L.C.P.U.F.A. stand for?

 

[Chris Koziarz]

Why is the scientific name for the puffin fratercula arctica when they live in Britain which is nowhere near the Arctic?

No clear idea in my head about this one as this is the first time I've heard of fratercula genus of birds. But very interesting species of birds. Perhaps, from what I've just read, fratercula arctica nests in the very nothern regions of Atlantic (Iceland, Norway, Greenland, Newfoundland), as opposed to other species of genus fratercula which may be breeding further south...

 

[Chris Koziarz]

Thanks, Chris. I confess that when I wrote the post I knew I was cutting some corners, but was simply too lazy to look after a lot of things I have actually forgot. Now I have glanced a bit on some of the topics thanks to your post.

Anyway, just as some sort of half (actually more a quarter of) justification, when you go from "refraction" to "scattering" there is a difference in approach, that is, from geometric optics to physical optics. Ultimately, light passing through any kind of material medium is always scattering. In other words, scattering will always explain light refraction. I kept myself on the geometric level partly for clarity, but as I said above also partly from ignorance and a hell lot of "rust".

As for your remark "I don't quite understand the precise mechanism (Rayleigh did for what he received Nobel Price) but apparently no quantum energy excitement is involved as in infrared radiation absorption by greenhouse gasses.", yes, no quantum considerations needed, you can fully explain Rayleigh's scattering with classical electromagnetic theory, by use of the short wavelength approximation to Maxwell's equations

I guess I found a decent summary here.

I didn't have time to digest that summary, mrzz but it's a nice one, thanks.

 

[Horsa]

No clear idea in my head about this one as this is the first time I've heard of fratercula genus of birds. But very interesting species of birds. Perhaps, from what I've just read, fratercula arctica nests in the very nothern regions of Atlantic (Iceland, Norway, Greenland, Newfoundland), as opposed to other species of genus fratercula which may be breeding further south...

Thank you very much for the information but we have puffins breeding in Britain & according to the information available to me fratercula arctica is the scientific name for a puffin. Puffins are very beautiful sea birds.

Edit: I got the answer, Chris. Although I've seen puffins breed & nest in Britain, I've double-checked & official records don't register this & they only have puffins down as breeding & nesting in the Arctic.

 

[Chris Koziarz]

What does P.U.F.A. stand for, please?

What does L.C.P.U.F.A. stand for?

Google brings this answer:
https://en.wikipedia.org/wiki/Polyunsaturated_fatty_acid
Would it be correct? If so, I have no deep knowledge in that area, and my organic chemistry basics are quite rusted in the back of my head. Would not mind refreshing them on this occasion...

 

[Horsa]

Google brings this answer:
https://en.wikipedia.org/wiki/Polyunsaturated_fatty_acid
Would it be correct? If so, I have no deep knowledge in that area, and my organic chemistry basics are quite rusted in the back of my head. Would not mind refreshing them on this occasion...

That would make sense in the context of what I read which only gave the abbreviations & mentioned fatty acids. Thank you very much. I guess the L.C.P.U.F.A. would stand for low calorie polyunsaturated fatty acids then.

 

[Horsa]

What do you think of the recent news about scientists taking pictures of a black hole?



I thought it was good & such a beautiful sight. It paves the way for them to find out more about the universe.

I also read this which I found fascinating. I hope you enjoy.

https://arxiv.org/ftp/arxiv/papers/1902/1902.11196.pdf

I also watched these videos which I found interesting.





I think Earth sounds like birdsong on a morning. What do you think?

 

[Horsa]

What is Aristotelian metaphysics, please?