[NatureNS] 180 Degree Double Rainbow Photos

Date: Fri, 10 Jun 2011 03:31:31 -0300
From: "Stephen R. Shaw" <srshaw@Dal.Ca>
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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
>>
>
>
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> Dalhousie University
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