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Hale_Bopp
2001-Dec-05, 02:44 AM
Found this on the National Science Teacher's Association web site.

"On December 4 in 1893, physicist John Tyndall, who explained why the sky is blue, dies at 73 in Hindhead, England. His research centered on the movement of heat and light through gases. He is known for the Tyndall effect - a light beam cannot be seen from the side when it moves through a clear liquid or gas, but when dust and other particles are in the medium, the light beam can be seen. The scattering of light by atmospheric dust, which is especially intense for blue wavelengths, accounts for the overall blue appearance of the sky. [from The Illustrated Almanac of Science, Technology, and Invention]"

I knew the effect, but never knew the name of it. A forgotten man in science history.

Rob

The Bad Astronomer
2001-Dec-05, 04:18 AM
"The scattering of light by atmospheric dust, which is especially intense for blue wavelengths, accounts for the overall blue appearance of the sky."


This is wrong. It's not dust, but nitrogen molecules that do the scattering.

GrapesOfWrath
2001-Dec-05, 09:13 AM
I found this question discussed in the sci.physics FAQ (http://www.math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html). It says "Tyndall and Rayleigh thought that the blue colour of the sky must be due to small particles of dust and droplets of water vapour in the atmosphere. Even today people sometimes incorrectly say that this is the case." But it does say that "The first steps towards correctly explaining the colour of the sky were taken by John Tyndall in 1859."

I tried to get to the http://www.nsta.org/ site, but I kept getting "Can't open file: 'access_log.MYD'. (errno: 145)", from both Netscape and IE.

Hale_Bopp
2001-Dec-05, 01:26 PM
True, but dust does add to the effect. Remember the spectacular red sunsets after the eruption of Moutn Pinatubo due to the dust scattering more light than usual. There are many things in out atmosphere that scatter light.

Rob

GrapesOfWrath
2001-Dec-05, 01:36 PM
I can now access the NSTA site, but couldn't find the Tyndall info, even with their site search. Where was it? Is it still there?

Hale_Bopp
2001-Dec-05, 02:59 PM
The quote was on the first page. It was a "This day in history" piece, so it might not be there today.

Rob

Hat Monster
2001-Dec-09, 07:32 PM
Nitrogen molecules should have a negligible effect if my MO theory knowledge holds true. There's little resonance or emission much below 550nm in a N<sub>2</sub> molecule, thanks to the triple bond. Argon, OTOH, should have an effect at around 490nm. Argon comprises 1% of the atmosphere by mass.

What I've always seen as being the primary cause of a blue sky is just plain crap in the atmosphere, such as dust, water droplets and particulates. The same things that make sun beams visible. You're actually viewing millions of tiny, non-gaseous, particles that are illuminated.

If a molecule or atom was to be the cause then the atmosphere would peak at a single wavelength, but anybody knows that there are different shades of blue as well as red and pink during a sunset.

GrapesOfWrath
2001-Dec-09, 09:15 PM
On 2001-12-09 14:32, Hat Monster wrote:
What I've always seen as being the primary cause of a blue sky is just plain crap in the atmosphere, such as dust, water droplets and particulates. The same things that make sun beams visible. You're actually viewing millions of tiny, non-gaseous, particles that are illuminated.
That sci.physics FAQ (http://www.math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html) has a pretty good discussion about why that is not so.