[NatureNS] 180 Degree Double Rainbow Photos

From: David & Alison Webster <dwebster@glinx.com>
To: naturens@chebucto.ns.ca
References: <662440C296464BD5B2DB2B509DE5008B@hans57ff9baf63>
Date: Sun, 12 Jun 2011 18:24:55 -0300
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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