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Index of Subjects
Index of Subjects --Apple-Mail-259-1036277915 Content-Type: text/plain; charset=WINDOWS-1252; format=flowed; delsp=yes Content-Transfer-Encoding: quoted-printable Actually, the band is darker because there is less light reaching that =20= area of the sky. Less light, darker sky The best explanation for rainbows is in the RASC's Observer's Handbook =20= (http://www.rasc.ca/handbook/) In contains a section by Roy Bishop. =20 retired emeritus professor of physics at Acadia University. In his 11-=20= page section, Rainbows and Some Other Sky Phenomena, Over five pages =20 are devoted to the rainbow, including the following: Alexander=92s Dark Band: With a raindrop-mandated minimum deviation of =20= 138=B0 for the light of the primary rainbow and 231=B0 in the opposite =20= sense for the light of the secondary rainbow, there is a 138=B0 + 231=B0 = =96 =20 360=B0 =3D 9=B0 gap into which the light contributing to the two = rainbows =20 cannot enter. Thus the sky between the bows is darker than elsewhere, =20= a feature called Alexander=92s Dark Band after Alexander of Aphrodisias, = =20 a Greek philosopher who drew attention to it c. AD 200. The =93dark=94 =20= band is only moderately darker than the region beneath the primary =20 rainbow because of three sources of light between the bows: skylight, =20= external reflections off raindrops (see Figure 1), and diffraction of =20= light waves by the raindrops Roy has given several presentations on the rainbow, all the packed =20 rooms. Using a laser beam and a spherical flask filled with water =20 (falling raindrops are spherical) he demonstrates quite clearly how =20 the changing incident angle of the light hitting the drop concentrates =20= the refracted light into the bows, and why this gets leaves a darker =20 area in between. A life-long naturalist. he also has the only known photograph of a =20 double rainbow over Woolsthorpe Manor, the birthplace of Sir Isaac =20 Newton, the first 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 =20= Notes 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, =20 > perhaps on account of it`s brevity. > > Because of the sun`s great distance from us, light from it arrives =20 > at the raindrops as elsewhere on earth, essentially collimated (all =20= > the conceptual incident rays are aligned parallel). The light =20 > coming back to us from the raindrop is some fraction of this light =20 > that landed directly on the individual droplet, was internally =20 > reflected once or twice and then refracted directly back to us, if =20 > we happen lie within the requisite viewing angle, as the Wikipedia =20 > citation below indicates. Given the collimated-light input =20 > geometry, this returning light cannot come back to us from some kind =20= > of parasitic interaction with the region between the primary and =20 > secondary rainbows, drawing light from there and so making that zone =20= > actually darker. > > The apparent primary reason why this Alexander`s Dark Band (ADB) =20 > zone looks darker is given in the Wikipedia citation -- the ADB zone =20= > actually is darker. Much of the refracted raibow light that fails =20 > to reach us along our direct line of sight is scattered indirectly =20 > off other particles but only INSIDE the primary rainbow (single =20 > internal reflection): a wide band adjacent to the rainbow but only =20 > inside it appears a bit brighter, by indirect back reflection =20 > reaching the viewer. Similarly but conversely, the refracted light =20= > that fails to reach us directly from the secondary rainbow (double =20 > internal reflection) is also scattered but this time OUTSIDE the =20 > rainbow itself, so that a larger zone outside the secondary arc too =20= > appears brighter. The central ADB zone with little or no back-=20 > scattering is the same as the rest of the sky, and so appears a bit =20= > darker, by contrast with the local sky scene on either edge of this. > > A possible secondary reason not considered in that Wikipedia article =20= > is a powerful perceptual phenomenon that's well known in vision =20 > studies, a neural effect that generated a couple of Nobel prizes, =20 > `lateral inhibition`. This process happens within the visual system =20= > itself, enhancing existing local differences in lightness and =20 > creating the illusion of even greater darkness in an already =20 > relatively dark zone like ADB. Such neural enhancements and the =20 > resulting illusions are known collectively as Mach Band effects =20 > after 19-20thC European physicist-philosopher Ernst Mach, and are =20 > described briefly in Wikipedia also (type in `Mach Bands` and follow =20= > 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=20 >> ) and is a result of the rainbow itself. Briefly, the light that =20 >> makes the bows brighter has to come from somewhere, or as the =20 >> 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 =20= >>> more white than above. I don't recall having noticed this in =20 >>> rainbows and the width of the band from red to violet is =20 >>> unusually narrow. I am wondering if this was just after a shower =20= >>> 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 =20 >>> from Portuguese Cove. I regret not rushing down to the ocean =20 >>> and snapping pics without the wires in the foreground and with =20 >>> the rainbows fr