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Old 10-28-2009, 02:13 PM
galexander galexander is offline
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Join Date: Jun 2009
Location: Bucks, UK
Posts: 405
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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