'(The Blog) With No Name', perhaps best described as a stream of notes and thoughts - 'remembered, recovered and (sometimes) invented'.

Saturday, April 30, 2005

A Pale View of BLUE Hills

Light (and its interplay with matter) is the flavor of these days and here is a bit more...

Hills, more so those with substantial green cover, appear blue to a distant observer. I have never quite figured out why. An answer I have heard often is 'for the same reason that the sky is blue'. I could not quite see the connection. My understanding is, thus, rather 'pale'.

Here is a page from the cluster of expository material maintained by John Baez which explains how the real reason behind the sky appearing blue was not really known until Einstein settled some key issues in the matter. It also has some stuff about Blue Hazes due to scattering of light by organic compounds called terpenes emitted by vegetation and the rare Blue Moon phenomenon.

One could theorize as follows: the hills emit light of two colors - overwhelming green from all the greenery and the much less of scattered blue light from the steady terpene emissions from the same greenery. If we see the hills from close, the green is predominant and overpowers the terpene blue. However, from a considerable distance, there is a thick intervening layer of atmospheric air which scatters blue very preferentially - it is as if we see the hills thru a 'blue film'. Now, a film with a particular color blocks out all other colors. The green of the hills gets cancelled by the intervening air and the much weaker terpene blue persists and that is that. One could also guess that without the terpene factor, hills would have appeared simply dark rather than blue from a distance.

But I am not sure if a thick layer of air, though it scatters blue light preferentially, can be conceived as a film that allows only blue light to pass. A colored film essentially ABSORBS all colors than the color it lets thru; on the other hand air does not seem to absorb the other visible colors than blue - these colors simply pass thru unscattered for much longer distances than blue. Then how does the green color from hills fail to reach a distant observer?

One more point that puzzles me: Clouds are either white or a deep violet ( I dont think even the darkest rainclouds are really black). One sees no shades in between. Why only these two colors? That white clouds appear perfectly white with no tinge of blue probably refutes the 'blue film' idea given above. But I have no better idea.


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