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href=3D"http://www.hanstoom.com/StockPhotos/Visions3/0176.html"& This explanation for DW doesn`t seem to be correct as written, perhaps on account of it`s brevity. Because of the sun`s great distance from us, light from it arrives at the raindrops as elsewhere on earth, essentially collimated (all the conceptual incident rays are aligned parallel). The light coming back to us from the raindrop is some fraction of this light that landed directly on the individual droplet, was internally reflected once or twice and then refracted directly back to us, if we happen lie within the requisite viewing angle, as the Wikipedia citation below indicates. Given the collimated-light input geometry, this returning light cannot come back to us from some kind of parasitic interaction with the region between the primary and secondary rainbows, drawing light from there and so making that zone actually darker. The apparent primary reason why this Alexander`s Dark Band (ADB) zone looks darker is given in the Wikipedia citation -- the ADB zone actually is darker. Much of the refracted raibow light that fails to reach us along our direct line of sight is scattered indirectly off other particles but only INSIDE the primary rainbow (single internal reflection): a wide band adjacent to the rainbow but only inside it appears a bit brighter, by indirect back reflection reaching the viewer. Similarly but conversely, the refracted light that fails to reach us directly from the secondary rainbow (double internal reflection) is also scattered but this time OUTSIDE the rainbow itself, so that a larger zone outside the secondary arc too appears brighter. The central ADB zone with little or no back-scattering is the same as the rest of the sky, and so appears a bit darker, by contrast with the local sky scene on either edge of this. A possible secondary reason not considered in that Wikipedia article is a powerful perceptual phenomenon that's well known in vision studies, a neural effect that generated a couple of Nobel prizes, `lateral inhibition`. This process happens within the visual system itself, enhancing existing local differences in lightness and creating the illusion of even greater darkness in an already relatively dark zone like ADB. Such neural enhancements and the resulting illusions are known collectively as Mach Band effects after 19-20thC European physicist-philosopher Ernst Mach, and are described briefly in Wikipedia also (type in `Mach Bands` and follow the links there for more info than you probably wanted to know). Steve Quoting Patrick Kelly <patrick.kelly@Dal.Ca>: > The region between the bows is called Alexander's Dark Band > (http://en.wikipedia.org/wiki/Alexander%27s_dark_band ) and is a > result of the rainbow itself. Briefly, the light that makes the > bows brighter has to come from somewhere, or as the saying goes, > you don't get something for nothing! > > Pat > > > On Jun 8, 2011, at 9:02 PM, David & Alison Webster wrote: > >> Hi Hans, June 8, 2011 >> Impressive photos in any case. >> >> In all three shots the sky below the lower arc is less blue and >> more white than above. I don't recall having noticed this in >> rainbows and the width of the band from red to violet is unusually >> narrow. I am wondering if this was just after a shower or just >> before fog rolled in. >> Yt, DW >> ----- Original Message ----- >> From: Hans Toom >> To: naturens@chebucto.ns.ca >> Sent: Wednesday, June 08, 2011 7:58 PM >> Subject: [NatureNS] 180 Degree Double Rainbow Photos >> >> Here are three photos from yesterdays 180 degree double rainbow >> from Portuguese Cove. I regret not rushing down to the ocean and >> snapping pics without the wires in the foreground and with the >> rainbows framing cargo ships swinging at anchor in the outer >> harbour. What was I thinking??? >> http://www.hanstoom.com/StockPhotos/Visions3/0176.html >> >> Hans Toom >> Portuguese Cove, Nova Scotia, Canada >> http://www.hanstoom.com/ >> No virus found in this message. >> Checked by AVG - www.avg.com >> Version: 10.0.1382 / Virus Database: 1511/3689 - Release Date: 06/08/11 >> > > > = = ======================================================================== > Patrick Kelly > Director of Computer Facilities > = = ======================================================================== > Faculty of Architecture and Planning > Dalhousie University > = = ======================================================================== > MAIL COURIER > 1459 Oxford Street 5410 Spring Garden Road > Halifax, Nova Scotia B3H 4R2 Halifax, Nova Scotia B3J 2X4 > Canada Canada > = = ======================================================================== > Phone:(902) 494-3294 FAX:(902) 423-6672 E-mail:patrick.kelly@dal.ca > = = ========================================================================
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