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  #1  
Old 10-28-2009, 01:13 PM
galexander galexander is offline
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Question Problem's Explaining the Sky's Blue Colour.


Recently I become aware of the fact that the accepted explanation of what causes the sky’s blue colour is entirely inadequate and that some other physical process must lie behind it. The explanation we have been given is that it is caused by the Rayleigh ‘molecular’ scattering of light. In this process the shorter wavelengths (i.e. the blue) are scattered the most. The equation covering the phenomena is as follows:

I = Io 8 πexp4 N α² (1 + cos²θ) / λexp4 R²

where α = polarizability
N = the no. of scatterers
R = the distance from scatterer
and θ = the angle of scatter.

The equation can also be summarized as I ≈ 1 / λexp4


This would imply that the shorter wavelengths scatter the most causing the blue colour.

Although apparently straight forward enough at first glance there are one or two problems with this explanation upon closer scrutiny :

1. The scattering of the blue light should occur just as much along the horizontal line of sight as it does along the vertical. This is as a result of the fact that the scattering phenomena is not altitude specific. However it does not appear to. Consider the following example; if the Earth’s atmosphere was of an equal density from bottom to top it would form a layer around 5 miles thick. Therefore if one were to look horizontally for a distance of 5 miles one would expect to see just as much blue coloured glow as one sees when looking directly upward. Moreover on an exceptionally clear day (that is when the atmospheric water vapour content is low) and looking from an appropriate mountain top, one could perhaps see for some twenty miles towards the horizon. In this case one would expect the air in this direction to glow with four times the brightness as it does directly over-head but it quite clearly does not.

2. The first, more fuller equation, would imply (as a result of the (1 + cos²θ) term) that the scattering at right angles is exactly half of that in a forward direction. But this would not tally with the observation that the sky’s brightness is even throughout. The equation would predict that the sky should glow more brightly in the blue around the Sun (where the scatter angle is low) and get progressively dimmer the further away from the Sun you get. It would appear therefore that the physical process which causes the sky’s colour to be even throughout is not related to scattering at all.

3. Finally it is stated that the shorter wavelengths scatter the most but what about the violet part of the spectrum which lies beyond the blue and is far more extensive than the latter? Since violet light is of a shorter wavelength than blue why is the sky therefore not violet in colour? An explanation for this is unfortunately lacking in the descriptions we have been given of Rayleigh scattering but what are we to make of this obvious omission?

Each of these three points when considered in isolation, strongly suggest that Rayleigh scattering is not at all adequate in explaining the sky’s blue colour. However when considered together three ‘hits’ must surely leave little doubt that an entirely different process lies behind the phenomena.

Since we apparently can’t adequately explain the sky’s blue colour, this must surely have consequences as far as our understanding of the immediate environment of space is concerned. For example is the Earth’s upper atmosphere not fluorescing in the Sun’s short-wave radiation? It has already been admitted that at least in part it does but is this also the cause of the blue colour we are so familiar with? If this is the case then the physics and chemistry supporting this would possibly be quite complex and beyond the scope of the present article.

In conclusion therefore it would appear that there is a possible problem with the exact physics describing what goes on in the Earth’s upper atmosphere.

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  #2  
Old 10-29-2009, 04:46 AM
albie albie is offline
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Default Re: Problem's Explaining the Sky's Blue Colour.

I would like to know why a sunset feels different to a sunrise. They should be the same process. I reckon a poet fashioned this matrix. He loved insects too! But changed his mind about dinosaurs. Good show.
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  #3  
Old 11-05-2009, 12:26 PM
galexander galexander is offline
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Default Re: Problem's Explaining the Sky's Blue Colour.

Quote:
Originally Posted by albie View Post
I would like to know why a sunset feels different to a sunrise. They should be the same process. I reckon a poet fashioned this matrix. He loved insects too! But changed his mind about dinosaurs. Good show.
While we are on the subject of poetry, imagine waking up one morning just after sunrise and the sky is already a fully violet colour.

You admire the distant view of interlocking tree covered hills covered in a violet mist.

Directly overhead the sky is much darker because the sun is still low on the horizon.

Suddenly the thought comes into your mind, wasn't it Rayleigh who first observed all of this?
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Old 11-05-2009, 12:44 PM
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EireEngineer EireEngineer is offline
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Default Re: Problem's Explaining the Sky's Blue Colour.

On an unrelated note, how do you get superscript characters to show up in this forum?
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Old 11-06-2009, 03:41 AM
Out of the Box Out of the Box is offline
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Default Re: Problem's Explaining the Sky's Blue Colour.

Quote:
Originally Posted by EireEngineer View Post
On an unrelated note, how do you get superscript characters to show up in this forum?
I'm not sure this forum allows superscript.

¹ ² and ³ are included in standard ASCII code at positions 185, 178 and 179 respectively. You can also find ¼, ½ and ¾ at positions 188, 189 and 190 respectively. ² and ³ can be found on your keyboard (they exist on mine) while the others are more easily obtained by copy-pasting it from a Wiki-page on ASCII.
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Old 11-06-2009, 01:06 PM
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EireEngineer EireEngineer is offline
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Default Re: Problem's Explaining the Sky's Blue Colour.

Quote:
Originally Posted by Out of the Box View Post
I'm not sure this forum allows superscript.

¹ ² and ³ are included in standard ASCII code at positions 185, 178 and 179 respectively. You can also find ¼, ½ and ¾ at positions 188, 189 and 190 respectively. ² and ³ can be found on your keyboard (they exist on mine) while the others are more easily obtained by copy-pasting it from a Wiki-page on ASCII.
You must have a nicer keyboard than mine. thanks you so much, it has been driving me nuts not having it easily accessible, like on every other forum on the planet. It never dawned on me to look in the ascii table....duh. Thanks a bunch.
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Old 11-07-2009, 04:24 AM
I cannot be fooled I cannot be fooled is offline
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Default Re: Problem's Explaining the Sky's Blue Colour.

Actually the reason why the sy is blue is very easy to explain.

Light from the sun reflect off the ocean and into space, the darkness of space amplifies this colour and the light entering our eye reflects it as blue.

light is the part of the electromagnetic spectrum that our eyes can see. Light from the sun or a light bulb may look white, but it is actually a combination of many colours filtered by our eyes, for example looking through a telescope reduces 3/4 of all light visible by the eye, the same as our atmosphere does, so our eyes filter the colour and blue is the easiest colour our eyes can see so the sky is perceived as blue when in fact it is many colours
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Old 01-10-2010, 03:42 PM
JazzRoc JazzRoc is offline
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Default Re: Problem's Explaining the Sky's Blue Colour.

The scattering of the blue light should occur just as much along the horizontal line of sight as it does along the vertical.
And it does. If you ever were a landscape painter you would KNOW to paint increasingly distant objects bluer.

In this case one would expect the air in this direction to glow with four times the brightness as it does directly overhead but it quite clearly does not.
Of course it doesn't. There is much scattering and absorption of light. Less light always arrives from objects which are further away by the inverse square law, for starters.

But this would not tally with the observation that the sky’s brightness is even throughout.
But it isn't. Any daytime panoramic picture will show you this. Where do you get such an idea?

The equation would predict that the sky should glow more brightly in the blue around the Sun (where the scatter angle is low) and get progressively dimmer the further away from the Sun you get.
And it does.

Why is the sky therefore not violet in colour? An explanation for this is unfortunately lacking in the descriptions we have been given of Rayleigh scattering but what are we to make of this obvious omission?
It is only an omission in your knowledge and understanding. The stratosphere absorbs the violet and ultraviolet, warming in the process, which is why it is warmer the higher you go in it. That process is entirely separate from Rayleigh Scattering.

Three ‘hits’ must surely leave little doubt that an entirely different process lies behind the phenomena.
Three 'misses' should tell you it's time to go back to school.

Is the Earth’s upper atmosphere not fluorescing in the Sun’s short-wave radiation?
Yes it is, kind of. It's fluorescing INFRA-RED radiation. It heats up.

It would appear that there is a possible problem with the exact physics describing what goes on in the Earth’s upper atmosphere.
That's the way it looks when you lack understanding.
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Old 02-13-2010, 09:22 AM
galexander galexander is offline
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Default Re: Problem's Explaining the Sky's Blue Colour.

Quote:
Originally Posted by JazzRoc View Post
The scattering of the blue light should occur just as much along the horizontal line of sight as it does along the vertical.
And it does. If you ever were a landscape painter you would KNOW to paint increasingly distant objects bluer.

In this case one would expect the air in this direction to glow with four times the brightness as it does directly overhead but it quite clearly does not.
Of course it doesn't. There is much scattering and absorption of light. Less light always arrives from objects which are further away by the inverse square law, for starters.

But this would not tally with the observation that the sky’s brightness is even throughout.
But it isn't. Any daytime panoramic picture will show you this. Where do you get such an idea?

The equation would predict that the sky should glow more brightly in the blue around the Sun (where the scatter angle is low) and get progressively dimmer the further away from the Sun you get.
And it does.

Why is the sky therefore not violet in colour? An explanation for this is unfortunately lacking in the descriptions we have been given of Rayleigh scattering but what are we to make of this obvious omission?
It is only an omission in your knowledge and understanding. The stratosphere absorbs the violet and ultraviolet, warming in the process, which is why it is warmer the higher you go in it. That process is entirely separate from Rayleigh Scattering.

Three ‘hits’ must surely leave little doubt that an entirely different process lies behind the phenomena.
Three 'misses' should tell you it's time to go back to school.

Is the Earth’s upper atmosphere not fluorescing in the Sun’s short-wave radiation?
Yes it is, kind of. It's fluorescing INFRA-RED radiation. It heats up.

It would appear that there is a possible problem with the exact physics describing what goes on in the Earth’s upper atmosphere.
That's the way it looks when you lack understanding.
Nice try JazzRoc.

The only problem is you were making it all up as you went along!

Quote:
If you ever were a landscape painter you would KNOW to paint increasingly distant objects bluer.
Do they really paint distant objects bluer? Well I also take quite an interest in art and I can tell you that you are categorically wrong.

And what about the perpetual blue fog visible on the ground which is predicted by Rayleigh's equation?

As for the rest of what you have said it is not worth commenting upon.
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  #10  
Old 02-13-2010, 10:13 AM
JazzRoc JazzRoc is offline
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Default Re: Problem's Explaining the Sky's Blue Colour.

The only problem is you were making it all up as you went along!

Of course I was. You win.

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