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Hi Stephen & All, June 10, 2011
Thanks for the detailed explanation.
DW
----- Original Message -----
From: "Stephen R. Shaw" <srshaw@Dal.Ca>
To: <naturens@chebucto.ns.ca>
Sent: Friday, June 10, 2011 3:31 AM
Subject: Re: [NatureNS] 180 Degree Double Rainbow Photos
> 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/
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>>>
>>
>>
>> = =
>> ========================================================================
>> Patrick Kelly
>> Director of Computer Facilities
>> = =
>> ========================================================================
>> Faculty of Architecture and Planning
>> Dalhousie University
>> = =
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>> MAIL COURIER
>> 1459 Oxford Street 5410 Spring Garden Road
>> Halifax, Nova Scotia B3H 4R2 Halifax, Nova Scotia B3J 2X4
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>
>
>
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