> -----Original Message-----
> From: joe dees [mailto:email@example.com]
> Sent: Friday, April 16, 1999 9:04 PM
> To: firstname.lastname@example.org
> Subject: Re: virus: Kiddie Brodies (was: only two levels)
> At Fri, 16 Apr 1999 02:22:29 -0700, you wrote:
> >Joe Dees wrote:
> >>>Where do the laws of physics come from? It's the sort of question
> >>>only children and geniuses ask--certainly most physicists are far
> >>>too busy putting the laws to work.
> >>This is the paragraph to which you pointed me, in its
> entirety. Besides
> >>being rhetorical, it is simply plain wrong. Children are
> not considering
> >>such a question because they have not even gotten to the stage where
> >>they can, like adults, apply such laws, for they do not yet
> know them..
> >"Why is the sky blue?"
> >-Prof. Tim
> Asking why the sky and sea are blue,
Of course, modern physical theory, specifically quantum electrodynamics (QED), tells us that the vacuum can no longer be considered a void. This is due to the fact that, even in the absence of matter, the vacuum is neither truly particle nor field free, but is the seat of virtual particle-pair (e.g. electron-positron) creation and annihilation processes, as well as zero-point-fluctuation (ZPF) of such fields as the vacuum electromagnetic field. But for the sake of your questions we will put that aside.
White light, from the sun, is composed of all the colors of the rainbow. The
refractive index, is given by
n = (speed of light in vacuum)/(speed of light in medium).
Since the speed of light in medium is different for different colors (i.e. frequencies), the refractive index is different for different colors. So, a mixture of different frequencies (like the white light) can be separated (technical term is dispersed) using a medium. And that is how we get the rainbow - white light separated into its components by the water droplets.
Molecules of air in Earth's atmosphere are just the right size to send sunlight flying in all directions as the light strikes the air. This process is called "scattering" -- and it happens that due to the effect above, the blue part of sunlight is scattered more strongly than the red part because of the refractive index of the nitrogen in air. For most of the day, only the blue light is affected - it is scattered through the air, and reaches us from every direction, so the sky appears blue, while the Sun's colour is less blue, and more yellow, than it would be otherwise. So sunlight causes the sky to look blue. If you go more than about a dozen kilometers above Earth's surface, the sky begins turning dark because, up that high, there are fewer molecules of air to scatter sunlight. Volcanic events cause additional dust in the atmosphere and this dust absorbse the shorter wavelengths (blue light) making the sky look more red/orange.
Sea water looks blue for three reasons. It reflects the sky, and as the sky is filtered in colour, so the light available to reflect off the water or refract from objects in the water tends to reflect sky filtering. Secondly particles suspended in the water behave similarly to the sky to perform a filtering action.
Thirdly microorganisms in the water (plankton and zooophytes mainly) can through reflection or absorbtion radically alter perceived "water colour".
The mixture of colors in sunlight also explains why clouds look white. If you looked at a single water droplet from a cloud, it'd be clear. But clouds look white because their water droplets are big enough to scatter light of all colors -- and all colors combined make white. Clouds can also look dark, when they're in the shadow of other clouds -- or when the top of a cloud casts a shadow on its own base.
grass is green
Because the chlorophyl in the grass absorbs the longer wavelengths (reds and oranges) and shorter wavelengths (violet and blue) from sunlight, leaving yellow and green to reflect.
blood is red
The FeO2 in blood (haemoglobin) is bright red (absorbs all shorter wavelengths).
the sun is yellow
This is actually nothing to do with the Sun - it's all caused by the changing thickness of the air we are seeing it through. When we look up at the Sun for most of the day, we are looking almost straight out of the Earth's atmosphere, but at sunset (and sunrise), we are looking at the Sun through a much greater thickness of atmosphere.
At sunset, the effect which makes the sky look blue becomes much stronger. The extra thickness of the atmosphere lets it scatter not just the blue, but also the green and yellow parts of the light, leaving the Sun orangey-red in colour, and creating the series of bands from yellow through green to blue that you see left behind after sunset.
and the moon is white are not in the
The moon reflects mainly violet, blues and green light, all the longer wavelength colours are absorbed. THe atmosphere just happens to scatter enough of the blue light and reduces the dominance of the shorter wavelengths enough for the colour balance to strike us as being white again.
I would say that they are in a very similar category, but that the above are effects of the laws of Optics (Snell's law of dispersion or refraction), while Newtons laws of motion and the laws of thermodynamics are called laws because of their fundamental usefulness and many years of surviving assaults against their hypothesis.
The speed of light in a vacuum (299,792,458 m.s^-1, usually rounded to 3E9 m.s^-1 ) is a universal constant rather than a law. Gravitational effects on light are the subject of multiple laws.
Your last one is tricky. I am not sure I have a real answer myself. While http://members.icanect.net/~zardoz/light.htm has some clever theories, knowing that the current post-Everett QM model postulates gravity as a characteristic of fundamental space-time, my thinking is tending toward the idea that we have a failure of language, the light is simply following the constraints of local space time (I know about the double slit and pendulum experiment) but the model I tend to has the "gravity pendulum" modifing local spacetime which solves the problem, and we have seperated gravity from the equation which has very interesting implications in the quest for a TOE.
> same category as asking why objects in motion in a vacuum
> tend to stay in motion, why the law of entropy works and why
> the speed of light in a vacuum is 186286.3 mps or
> thereabouts, or why gravity will bend it.
> Joe E. Dees
> Poet, Pagan, Philosopher
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Regards TheHermit (BTW I am really enjoing your Kosovo threads. Just too damn busy to really join in :-( )