Re: The Value of Science: Explanation vs Prediction

Robert Carnegie wrote:
David Ewan Kahana wrote:
Thanks for the correction, and apologies
to Robert Carnegie, who had the right idea.

But I have no knowledge of these other people whom you mention,
Rayleigh and so forth ;-)


It should be Lord John Rayleigh to the likes of you and me ;->

(born John Willliam Strutt.)

...something about why the sky is blue? - which Genesis gets confused
over... daylight is created by God separately from the sun several days
later ;-)

Yup, you're remembering correctly, it's the same man.

Lord R. was a man of many talents ... he also worked out the
first steps toward the explanation of the blue color of the
sky.

He put the colour down to the (eponymous) phenomenon of
`Rayleigh scattering' of light by small particles, dust,
water droplets and so on, that were supposedly in the
atmosphere. He showed that small particles would
preferentially scatter light of short wavelength (blues and
violets), and worked out that the dependence on wavelength
was very strong (inverse fourth power of wavelength).

Since blue light has wavelength about twice as short as red,
it would be scattered more by the atmospheric particles than
red light, and thus the sky would tend to appear blue in directions
away from the sun.

It turned out that Rayleigh's idea wasn't quite correct as it
stood, though, because in general there isn't enough particulate
matter in the atmosphere to account for the colour.

However, Einstein showed in 1911 that basically the same
phenomenon occurs when scattering visible light off of the
gas molecules that make up the air, (oxygen, nitrogen), as
long as the molecules are high up in the atmosphere and
separated by relatively large distances.

Lower down in the atmosphere where densities are higher,
Einstein showed that scattering collectively off of many
molecules can be important, and one has to take account
of thermal fluctuations occurring in the density. In the
right limit his formulae reduce to Rayleigh scattering.

So Einstein's name has become associated with
the question as well.

So it appears that what happens is that scattering off
of molecules in the upper atmosphere already affects
blue light more, and scatters the blue coming from
the sun downward.

People still call the effect that makes the sky blue
Rayleigh scattering, though, which seems fair since
Einstein actually just refined the explanation.

For larger sized particles there is a transition to
so-called Mie scattering which is much more weakly
dependent on wavelength, and is forward peaked, unlike
Rayleigh scattering. Mie scattering produces the white
glare that you can see in the direction of the sun, when
there is a lot of particulate matter present, and also tends
to make fogs and mists appear whitish in the same direction.

Atmospheric optics is a complex business!

David

.

Relevant Pages

  • Re: The Value of Science: Explanation vs Prediction
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