Dalo

Why is the sky blue on Earth

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koti    140

The atmoshpere on earth acts as a prism refracting sunlight in such a way that we see a blue color sky. Mars has a different atmosphere (almost none compared to earth) so it refracts the sunlight differently giving a muddy brownish orange color like in your Mars photo. Sunlight is „white” color but it contains many other colors (all of them in fact) when light goes through an object - earths and mars’s atmosphere in this instance, it will refract into different colors.

https://spaceplace.nasa.gov/blue-sky/en/

 

Edited by koti

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Tub    10
18 minutes ago, koti said:

The atmoshpere on earth acts as a prism refracting sunlight in such a way that we see a blue color sky. Mars has a different atmosphere (almost none compared to earth) so it refracts the sunlight differently giving a muddy brownish orange color like in your Mars photo. Sunlight is „white” color but it contains many other colors (all of them in fact) when light goes through an object - earths and mars’s atmosphere in this instance, it will refract into different colors.

https://spaceplace.nasa.gov/blue-sky/en/

 

Something similar:  if i stand in my garden on a sunny day, the Sun's rays feel warm on my face, but if astronauts were to venture outside the ISS, without protective clothing, they would quickly freeze to death; so why do they not feel the same Sun's rays as heat?

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Dalo    8
27 minutes ago, koti said:

The atmoshpere on earth acts as a prism refracting sunlight in such a way that we see a blue color sky. Mars has a different atmosphere (almost none compared to earth) so it refracts the sunlight differently giving a muddy brownish orange color like in your Mars photo. Sunlight is „white” color but it contains many other colors (all of them in fact) when light goes through an object - earths and mars’s atmosphere in this instance, it will refract into different colors.

https://spaceplace.nasa.gov/blue-sky/en/

 

Yes, that is the explanation usually given. 

I wonder: is it scientifically, empirically, proven that colors do not come from the matter particles themselves? Why assume that something is being stopped or let through, and not assume that different matter reacts differently to em waves?

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swansont    6251
36 minutes ago, koti said:

The atmoshpere on earth acts as a prism refracting sunlight in such a way that we see a blue color sky. Mars has a different atmosphere (almost none compared to earth) so it refracts the sunlight differently giving a muddy brownish orange color like in your Mars photo. Sunlight is „white” color but it contains many other colors (all of them in fact) when light goes through an object - earths and mars’s atmosphere in this instance, it will refract into different colors.

https://spaceplace.nasa.gov/blue-sky/en/

 

No, it's not refraction. While the site mentions refraction, it later explains that the blue sky is the result of scattering (specifically Rayleigh scattering), which is wavelength dependent. Blue light is more strongly scattered, so we see blue sky as a result of light that was headed past us, being scattered toward us. For sunrises/sunsets the sun has a reddish hue because the blue light is more strongly scattered out of the path to us. It's more pronounced at these times because it has to travel through more atmosphere than when the sun is higher in the sky.

9 minutes ago, Dalo said:

Yes, that is the explanation usually given. 

I wonder: is it scientifically, empirically, proven that colors do not come from the matter particles themselves? Why assume that something is being stopped or let through, and not assume that different matter reacts differently to em waves?

If it were from absorption and emission of light, we would see specific wavelengths rather than a spectrum, since the transitions in the atoms/molecules are quantized. We do see this effect from the sun and in other instances (you can see emission and absorption lines, depending on the circumstances). The absorption lines in the solar spectrum are known as Fraunhofer lines. The continuous background is the blackbody spectrum.

https://en.wikipedia.org/wiki/Fraunhofer_lines

 

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Dalo    8
8 minutes ago, swansont said:

If it were from absorption and emission of light, we would see specific wavelengths rather than a spectrum, since the transitions in the atoms/molecules are quantized

I will accept the scientific explanations given on the subject. What I am interested in is whether the assumption, if it is one, that colors come from the em waves themselves, and not from what they encounter, is a necessary assumption.

I know it was Newton who was convinced that the spectrum could not come from the prism itself, and I wonder if this conviction is empirically provable.

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koti    140
16 minutes ago, swansont said:

No, it's not refraction. While the site mentions refraction, it later explains that the blue sky is the result of scattering (specifically Rayleigh scattering), which is wavelength dependent. Blue light is more strongly scattered, so we see blue sky as a result of light that was headed past us, being scattered toward us. For sunrises/sunsets the sun has a reddish hue because the blue light is more strongly scattered out of the path to us. It's more pronounced at these times because it has to travel through more atmosphere than when the sun is higher in the sky.

If it were from absorption and emission of light, we would see specific wavelengths rather than a spectrum, since the transitions in the atoms/molecules are quantized. We do see this effect from the sun and in other instances (you can see emission and absorption lines, depending on the circumstances). The absorption lines in the solar spectrum are known as Fraunhofer lines. The continuous background is the blackbody spectrum.

https://en.wikipedia.org/wiki/Fraunhofer_lines

 

Youre right, thanks for correcting me. Dalo listen to that ^^ and forget about the idea that the spectrum is coming from a prism.

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Dalo    8
1 minute ago, koti said:

forget about the idea that the spectrum is coming from a prism.

I would have no problem with that. It is certainly not a matter of life and death. But could someone answer my question?

Is is empirically provable that the spectrum does not come from the prism?

Or is it a (necessary) theoretical assumption?

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Janus    706
1 hour ago, Dalo said:

and yellow brown on Mars?

Mars_sky_at_noon_PIA01546.jpg

As already mentioned, the blue sky on the Earth is due to Rayleigh scattering.  Mars' atmosphere is so thin that this is normally not a significant factor.  The yellow brown of the sky is most likely caused by small dust particles in the atmosphere.  On Mars you only see blue in the sky when the sun is near the horizon, and then only as a halo surrounding the sun.  Just like on Earth, when the sun is near the horizon we see a greater amount of Rayleigh scattering making the sunset or sunrise orange or red, on Mars it can increase it enough to produce a region of blue sky.

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swansont    6251
28 minutes ago, Dalo said:

I will accept the scientific explanations given on the subject. What I am interested in is whether the assumption, if it is one, that colors come from the em waves themselves, and not from what they encounter, is a necessary assumption.

I know it was Newton who was convinced that the spectrum could not come from the prism itself, and I wonder if this conviction is empirically provable.

If it came from the targets, the spectrum would look different. As I said. We can identify elements from the spectrum of light they emit.

As far as the prism goes, turn off the source, and the light goes away. The prism is not the source of the light. 
We also have a handle on Snell's law of refraction, which explains how prisms and lenses work.

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koti    140
1 hour ago, Tub said:

Something similar:  if i stand in my garden on a sunny day, the Sun's rays feel warm on my face, but if astronauts were to venture outside the ISS, without protective clothing, they would quickly freeze to death; so why do they not feel the same Sun's rays as heat?

Because there is no atmosphere in space. The atmosphere keeps the temperatures on earth at comfortable levels for us. The temperature in space at earths orbit (and mostly in all other space in the Universe) is just a little above absolute zero. Also...if you would look into the sun from earths orbit without protective gear you would surely damage your eyes instantly if not go blind completely (because no atmosphere)

Edited by koti
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Dalo    8
6 minutes ago, swansont said:

As far as the prism goes, turn off the source, and the light goes away.

That is not much of an argument. Take all matter away and the light goes away too.

11 minutes ago, swansont said:

We also have a handle on Snell's law of refraction, which explains how prisms and lenses work.

I do not think Snell's Law would lose its validity if we assumed that the spectrum came from the prism.

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koti    140
3 minutes ago, Dalo said:

That is not much of an argument. Take all matter away and the light goes away too.

Light is electromagnetic radiation just like radio waves or X rays. Saying that a prism contains some source is like saying that your radio receiver produces radio waves, it jus doesn't make sense.

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Tub    10
2 minutes ago, koti said:

Because there is no atmosphere in space. The atmosphere keeps the temperatures on earth at comfortable levels for us. There is no atmosphere in space and the temperature in space at earths orbit (and mostly in all other space in the Universe) is just a little above absolute zero. Also...if you would look into the sun from earths orbit without protective gear you would surely damage your eyes instantly if not go blind completely (because no atmosphere)

Thanks, koti.

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swansont    6251
1 hour ago, Dalo said:

That is not much of an argument. Take all matter away and the light goes away too.

That tells you the light doesn't come from the prism.

Are you asking of the prism generates the spectrum, but white light doesn't actually have a spectrum? 

— We can do the same thing with diffraction gratings. We get the same spectrum.

— We can put a material in front of the the prism (or grating) which absorbs specific colors, and observe the absorption lines. This shows those colors were removed before encountering the prism, so the prism can't be the source. (e.g. the previously-mentioned Fraunhofer lines)

— We can combine colored light to make white light, without using a prism.

Individually they contradict the notion that the prism is the source of the spectrum. Collectively they should leave no doubt.

 

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Dalo    8

I found this old clip of the renowned scientist Bragg (junior) which sums up the dilemma quite nicely.

 

It is interesting to note that the change of color can be seen on the screen, but not in the water. (around the 7th minute)

I will leave the explanation to people more knowledgeable than me.

 

10 minutes ago, swansont said:

Are you asking of the prism generates the spectrum, but white light doesn't actually have a spectrum?

 

Certainly not. I am just wondering whether light in general could not be a local phenomenon created by the collision of em waves and matter. Which would mean that light itself is not an em wave.

This is purely speculative and I have no way of proving it I am afraid.

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pzkpfw    172

Having seen "rainbows" from the beveled edges of the glass doors at my office, I'd point out that Prisms can be made of simple glass.

That is, it's not the material that separates the colours, it's the shape (allowing the different refraction of different frequencies to produce the effect).

Add to that, that since passing a single pure colour into a prism will get just that same colour back out, I'm not seeing any validity to the idea that the Prisim itself is what creates the colours.

 

Nice pictures here: http://www.schoolphysics.co.uk/age11-14/Light/text/Prisms_and_colour/index.html

Edited by pzkpfw

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Dalo    8
Just now, pzkpfw said:

Having seen "rainbows" from the beveled edges of the glass doors at my office, I'd point out that Prisms can be made of simple glass.

That is, it's not the material that separates the colours, it's the shape (allowing the different refraction of different frequencies to produce the effect).

And since passing a single pure colour into a prism will get just that same colour back out, I'm not seeing any validity to the idea that the Prisim itself is what creates the colours.

 

Nice pictures here: http://www.schoolphysics.co.uk/age11-14/Light/text/Prisms_and_colour/index.html

You may be right of course. I am not claiming any specific way through which light is created. The only general idea I would like to discuss is the possibility that light is a local phenomenon, an effect of em waves, but not an em wave itself.

That does not mean I have any inkling as to how different substances react to em waves. Even assuming I am right, that would still be empirical issues that only scientific experiments and not pure speculation can address.

 

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koti    140
53 minutes ago, Dalo said:

 

...local phenomenon created by the collision of em waves and matter.

Thats not what light is. I think this might be the key for you to understanding swansont’s and Janus’s answers to your question.

 

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Dalo    8
1 minute ago, koti said:

Thats not what light is. I think this might be the key for you to understanding swansont’s and Janus’s answers to your question.

Maxwell's Treatise on electricity never mentions light as anything else but an electrical effect. I think the assumption that light is not an em wave would not change anything to his equations.

 

But please, remember, it is pure speculation. It is therefore more science fiction than science. I just wonder whether the assumption that light itself is an em wave is really necessary, and what it would mean for Physics if we assumed it is not. 

I think that it would make at least a very interesting intellectual exercise. Certainly not worth a Nobel Prize, but even if proven wrong, it would at least show which position this assumption takes in Physics and what the consequences are of denying its validity.

I purposefully did not place this thread in a scientific sub-forum to avoid the accusation of propagating pseudo-science.

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koti    140
6 minutes ago, Dalo said:

Maxwell's Treatise on electricity never mentions light as anything else but an electrical effect. I think the assumption that light is not an em wave would not change anything to his equations.

 

But please, remember, it is pure speculation. It is therefore more science fiction than science. I just wonder whether the assumption that light itself is an em wave is really necessary, and what it would mean for Physics if we assumed it is not. 

I think that it would make at least a very interesting intellectual exercise. Certainly not worth a Nobel Prize, but even if proven wrong, it would at least show which position this assumption takes in Physics and what the consequences are of denying its validity.

I purposefully did not place this thread in a scientific sub-forum to avoid the accusation of propagating pseudo-science.

I agree that this might be an interesting subject to discuss. Whether you approach light from particle or wave standpoint it won't change the fact that light is not a "collision of em waves and matter" Light propagation has nothing to do with collision with matter, light does not require collision with matter.

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Dalo    8
2 minutes ago, koti said:

light does not require collision with matter

I am not a physicist, but I find this very surprising. Even "invisible light" like X, gamma IR or UV rays are only known to us because we somehow succeed in making their effects visible.

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John Cuthber    3237
4 hours ago, koti said:

The atmoshpere on earth acts as a prism refracting sunlight in such a way that we see a blue color sky. Mars has a different atmosphere (almost none compared to earth) so it refracts the sunlight differently giving a muddy brownish orange color like in your Mars photo. Sunlight is „white” color but it contains many other colors (all of them in fact) when light goes through an object - earths and mars’s atmosphere in this instance, it will refract into different colors.

https://spaceplace.nasa.gov/blue-sky/en/

 

Wow!

You  cited a web page written by NASA for kids.

And you managed to fail to understand it.

 

The colour of the sky is not due to refraction.

It is due to scattering.

In the case of Mars, the particles of dust in the air are big enough to act as reflectors (rather than scatterers) and their colour shows up.

Essentially brown martian dust makes the martian atmosphere brown.

 

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koti    140
14 minutes ago, John Cuthber said:

Wow!

You  cited a web page written by NASA for kids.

And you managed to fail to understand it.

 

The colour of the sky is not due to refraction.

It is due to scattering.

In the case of Mars, the particles of dust in the air are big enough to act as reflectors (rather than scatterers) and their colour shows up.

Essentially brown martian dust makes the martian atmosphere brown.

 

Yes, this was corrected by swansont above. Is it a site for kids? My bad...Im sure it doesn’t make it wrong though.

18 minutes ago, Dalo said:

I am not a physicist, but I find this very surprising. Even "invisible light" like X, gamma IR or UV rays are only known to us because we somehow succeed in making their effects visible.

Ok, so you mean that in order to detect light or make it visible it is required for light to act with matter? That is correct but a very different issue from „I am just wondering whether light in general could not be a local phenomenon created by the collision of em waves and matter” which is nonsense.

 

Edited by koti

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John Cuthber    3237
18 minutes ago, koti said:

Yes, this was corrected by swansont above. Is it a site for kids? My bad...Im sure it doesn’t make it wrong though.

It's right- NASA are good at that sort of thing.

What is remarkable is your failure to understand it.

 

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