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>>> the rainbows fr This is a multi-part message in MIME format. --Boundary_(ID_FytV1Q8spjRSL5DCnKrkbw) Content-type: text/plain; charset=Windows-1252 Content-transfer-encoding: quoted-printable Hi Patrick & All, June 12, 2011 Actually, whether an area of sky will appear darker or lighter relative= to nearby areas depends not upon the amount of light reaching that area of= sky but upon the amount of light reaching the eye or camera lens from the = area in question. Getting back to the original photos the only contrast in brightness is = between the entire field below the primary rainbow and the sky above the pr= imary rainbow. For whatever reason, Alexander's Dark Band was not apparent. Yt, DW=20 =20=20=20=20 ----- Original Message -----=20 From: Patrick Kelly=20 To: naturens@chebucto.ns.ca=20 Sent: Sunday, June 12, 2011 4:54 PM Subject: Re: [NatureNS] 180 Degree Double Rainbow Photos Actually, the band is darker because there is less light reaching that ar= ea of the sky. Less light, darker sky The best explanation for rainbows is in the RASC's Observer's Handbook (h= ttp://www.rasc.ca/handbook/) In contains a section by Roy Bishop. retired e= meritus professor of physics at Acadia University. In his 11-page section, = Rainbows and Some Other Sky Phenomena, Over five pages are devoted to the r= ainbow, including the following: Alexander=92s Dark Band: With a raindrop-mandated minimum deviation of 13= 8=B0 for the light of the primary rainbow and 231=B0 in the opposite sense = for the light of the secondary rainbow, there is a 138=B0 + 231=B0 =96 360= =B0 =3D 9=B0 gap into which the light contributing to the two rainbows cann= ot enter. Thus the sky between the bows is darker than elsewhere, a feature= called Alexander=92s Dark Band after Alexander of Aphrodisias, a Greek phi= losopher who drew attention to it c. AD 200. The =93dark=94 band is only mo= derately darker than the region beneath the primary rainbow because of thre= e sources of light between the bows: skylight, external reflections off rai= ndrops (see Figure 1), and diffraction of light waves by the raindrops Roy has given several presentations on the rainbow, all the packed rooms.= Using a laser beam and a spherical flask filled with water (falling raindr= ops are spherical) he demonstrates quite clearly how the changing incident = angle of the light hitting the drop concentrates the refracted light into t= he bows, and why this gets leaves a darker area in between. A life-long naturalist. he also has the only known photograph of a double= rainbow over Woolsthorpe Manor, the birthplace of Sir Isaac Newton, the fi= rst human to properly explain the formation of a rainbow. http://www.cosmolearning.com/images/rainbow-over-isaac-newtons-birthplace= -in-woolsthorpe-manor-england-961/ A paper describing the circumstances of the photo was published in the No= tes and Records of the Royal Society of London. Pat On Jun 10, 2011, at 3:31 AM, Stephen R. Shaw wrote: 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 t= he raindrops as elsewhere on earth, essentially collimated (all the concept= ual 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 ind= ividual droplet, was internally reflected once or twice and then refracted = directly back to us, if we happen lie within the requisite viewing angle, a= s the Wikipedia citation below indicates. Given the collimated-light input= geometry, this returning light cannot come back to us from some kind of pa= rasitic interaction with the region between the primary and secondary rainb= ows, drawing light from there and so making that zone actually darker. The apparent primary reason why this Alexander`s Dark Band (ADB) zone l= ooks 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 ou= r direct line of sight is scattered indirectly off other particles but only= INSIDE the primary rainbow (single internal reflection): a wide band adjac= ent to the rainbow but only inside it appears a bit brighter, by indirect b= ack reflection reaching the viewer. Similarly but conversely, the refracte= d light that fails to reach us directly from the secondary rainbow (double = internal reflection) is also scattered but this time OUTSIDE the rainbow it= self, 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 s= ky 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 ne= ural effect that generated a couple of Nobel prizes, `lateral inhibition`. = This process happens within the visual system itself, enhancing existing lo= cal differences in lightness and creating the illusion of even greater dark= ness in an already relatively dark zone like ADB. Such neural enhancements= and the resulting illusions are known collectively as Mach Band effects af= ter 19-20thC European physicist-philosopher Ernst Mach, and are described b= riefly in Wikipedia also (type in `Mach Bands` and follow the links there f= or 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://e= n.wikipedia.org/wiki/Alexander%27s_dark_band ) and is a result of the rainb= ow itself. Briefly, the light that makes the bows brighter has to come fro= m 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 wonderi= ng 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 fro= m Portuguese Cove. I regret not rushing down to the ocean and snapping p= ics without the wires in the foreground and with the rainbows framing carg= o ships swinging at anchor in the outer harbour. What was I thinking??? http://www..hanstoom.com/StockPhotos/Visions3/0176.html Hans Toom