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Hans Toom
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Hi Pat, DW, others
Regarding the source of relative darkness in ADB, Roy Bishop's first 3 =20=
lines of explanation make the same point that I was attempting to get =20=
at, only mercifully much more briefly, so I think we are actually all =20=
on the same page.
I don't get all of the last 3 lines of his qualification though, in the =20=
sense that 'skylight' would be the same all over at least locally =20
around the rainbow, and the same for generalized reflection from the =20
local raindrops' surfaces, while it's interesting that diffraction from =20=
the drops plays a role. He may mean only that generalized scatter in =20=
his three categories increases the local background glare which =20
inevitably worsens the contrast with the dark band, making the dark =20
band even harder to see.
This gets to DW's second point that it is very hard to see the band in =20=
the photo -- agreed. If I blur my eyes or stand back from the screen I =20=
think I can see a faint effect in the bottom half of the photo, but =20
wouldn't put money on it. The problem is that human contrast threshold =20=
lies at a few percent difference in lightness, so if the difference is =20=
less, say only 1%, you won't see it even if it is there. The way to =20=
resolve this would be to plug a good copy of the image into an analysis =20=
program like the excellent free NIH program ImageJ, do mouse transects =20=
across the region of interest and average many such scans. This would =20=
quantify the size of any average brightness dip on the inside of the =20
secondary bow, relative to the zone outside. A better remedy would be =20=
to search for a clearer image, as Alexander and his successors =20
presumably did make valid observations, with visual systems that had =20
just the same threshold limitations as ours.
Steve
On 12-Jun-11, at 6:24 PM, David & Alison Webster wrote:
> Hi Patrick & All,=A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 June 12, 2011
> =A0=A0=A0 Actually, whether an area of sky will appear darker or =
lighter =20
> relative to nearby areas depends not upon the amount of light reaching =
=20
> that area of sky but upon the amount of light reaching the eye or =20
> camera lens from the area in question.
> =A0
> =A0=A0=A0 Getting back to the original photos the only contrast in =20
> brightness is between the entire field below the primary rainbow and =20=
> the sky above the primary rainbow. For whatever reason, Alexander's =20=
> Dark Band was not apparent.
> =A0
> Yt, DW=A0
> =A0
> =A0=A0=A0
>> ----- Original Message -----
>> From: Patrick Kelly
>> To: naturens@chebucto.ns.ca
>> 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 =20
>> that area of the sky. Less light, darker sky
>>
>> The best explanation for rainbows is in the RASC's Observer's =20
>> Handbook (http://www.rasc.ca/handbook/) In contains a section by Roy =20=
>> Bishop. retired emeritus professor of physics at Acadia University. =20=
>> In his 11-page section, Rainbows and Some Other Sky Phenomena, Over =20=
>> five pages 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 =20
>> =96 360=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 =20
>> Aphrodisias, a Greek philosopher who drew attention to it c. AD 200. =20=
>> The =93dark=94 band is only moderately darker than the region beneath =
the =20
>> primary rainbow because of three sources of light between the bows: =20=
>> skylight, external reflections off raindrops (see Figure 1), and =20
>> diffraction of 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 =20
>> concentrates the refracted light into the bows, and why this gets =20
>> leaves a darker 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 =20
>> rainbow.
>>
>> http://www.cosmolearning.com/images/rainbow-over-isaac-newtons-=20
>> birthplace-in-woolsthorpe-manor-england-961/
>>
>> A paper describing the circumstances of the photo was published in =20=
>> the 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). =A0The 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. =A0Given 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. =A0Much 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. =A0Similarly 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. =A0The central ADB zone with little or no =20
>>> back-scattering is the same as the rest of the sky, and so appears a =
=20
>>> bit darker, by contrast with the local sky scene on either edge of =20=
>>> 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. =A0Such 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 =20
>>>> (http://en.wikipedia.org/wiki/Alexander%27s_dark_band ) and is a =20=
>>>> result of the rainbow itself. Briefly, the light that makes =A0the =20=
>>>> bows brighter has to come from somewhere, or as the saying goes, =20=
>>>> =A0you don't get something for nothing!
>>>> Pat
>>>> On Jun 8, 2011, at 9:02 PM, David & Alison Webster wrote:
>>>>> Hi Hans, =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0June 8, 2011
>>>>> =A0=A0Impressive photos in any case.
>>>>> =A0=A0In all three shots the sky below the lower arc is less blue =
and =20
>>>>> =A0more white than above. I don't recall having noticed this in =20=
>>>>> =A0rainbows and the width of the band from red to violet is =20
>>>>> unusually =A0narrow. I am wondering if this was just after a =
shower =20
>>>>> or just =A0before 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 =A0Portuguese Cove. =A0I regret not rushing down to the ocean =
and =20
>>>>> =A0snapping pics without the wires in the foreground and with the =20=
>>>>> =A0rainbows framing cargo ships swinging at anchor in the outer =20=
>>>>> =A0harbour. =A0What was I thinking???
>>>>> MailScanner has detected a possible fraud attempt from =20
>>>>> "www.hanstoom.com" claiming to be =20
>>>>> 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: =20
>>>>> =A006/08/11
>>>> =3D =3D =20
>>>> =
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=20
>>>> =3D=3D=3D=3D
>>>> Patrick Kelly
>>>> Director of Computer Facilities
>>>> =3D =3D =20
>>>> =
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=20
>>>> =3D=3D=3D=3D
>>>> Faculty of Architecture and Planning
>>>> Dalhousie University
>>>> =3D =3D =20
>>>> =
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=20
>>>> =3D=3D=3D=3D
>>>> MAIL =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=
=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0COURIER
>>>> 1459 Oxford Street =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=
=A0=A0=A05410 Spring Garden Road
>>>> Halifax, Nova Scotia B3H 4R2 =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0Halifax, =
Nova Scotia B3J 2X4
>>>> Canada =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=
=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0Canada
>>>> =3D =3D =20
>>>> =
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=20
>>>> =3D=3D=3D=3D
>>>> Phone:(902) 494-3294 =A0=A0=A0FAX:(902) 423-6672 =20
>>>> =A0=A0E-mail:patrick.kelly@dal.ca
>>>> =3D =3D =20
>>>> =
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
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=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=20
>>>> =3D=3D=3D=3D
>>>
>>>
>>>
>>
>>
>> =
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=20
>> =3D=3D=3D=3D
>> Patrick Kelly
>> Director of Computer Facilities
>> =
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=20
>> =3D=3D=3D=3D
>> Faculty of Architecture and Planning
>> Dalhousie University
>> =
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=20
>> =3D=3D=3D=3D
>> MAIL =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 COURIER
>> 1459 Oxford Street =A0 =A0=A0=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0=A05410 =
Spring Garden Road
>> Halifax, Nova Scotia B3H 4R2=A0=A0 =A0 =A0 =A0 =A0=A0Halifax, Nova =
Scotia B3J 2X4
>> Canada=A0=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0=A0Canada
>> =
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=20
>> =3D=3D=3D=3D
>> Phone:(902) 494-3294=A0 =A0=A0FAX:(902) =20
>> 423-6672=A0=A0=A0E-mail:patrick.kelly@dal.ca
>> =
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=20
>> =3D=3D=3D=3D
>>
>>
>>
>> No virus found in this message.
>> Checked by AVG - www.avg.com
>> Version: 10.0.1382 / Virus Database: 1511/3689 - Release Date: =20
>> 06/08/11
--Apple-Mail-1--988436484
Content-Transfer-Encoding: quoted-printable
Content-Type: text/enriched;
charset=WINDOWS-1252
Hi Pat, DW, others=20
Regarding the source of relative darkness in ADB, Roy Bishop's first 3
lines of explanation make the same point that I was attempting to get
at, only mercifully much more briefly, so I think we are actually all
on the same page.
=20
I don't get all of the last 3 lines of his qualification though, in
the sense that 'skylight' would be the same all over at least locally
around the rainbow, and the same for generalized reflection from the
local raindrops' surfaces, while it's interesting that diffraction
from the drops plays a role. He may mean only that generalized
scatter in his three categories increases the local background glare
which inevitably worsens the contrast with the dark band, making the
dark band even harder to see.
This gets to DW's second point that it is very hard to see the band in
the photo -- agreed. If I blur my eyes or stand back from the screen
I think I can see a faint effect in the bottom half of the photo, but
wouldn't put money on it. The problem is that human contrast
threshold lies at a few percent difference in lightness, so if the
difference is less, say only 1%, you won't see it even if it is there. =20=
The way to resolve this would be to plug a good copy of the image into
an analysis program like the excellent free NIH program ImageJ, do
mouse transects across the region of interest and average many such
scans. This would quantify the size of any average brightness dip on
the inside of the secondary bow, relative to the zone outside. A
better remedy would be to search for a clearer image, as Alexander and
his successors presumably did make valid observations, with visual
systems that had just the same threshold limitations as ours.=20
Steve
=20
On 12-Jun-11, at 6:24 PM, David & Alison Webster wrote:
<excerpt><smaller>Hi Patrick & All,=A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0=
June 12, 2011</smaller>
<smaller>=A0=A0=A0 Actually, whether an area of sky will appear darker =
or
lighter relative to nearby areas depends not upon the amount of light
<bold>reaching</bold> that area of sky but upon the amount of light
reaching the eye or camera lens <bold>from </bold>the area in =
question.</smaller>
=A0
<smaller>=A0=A0=A0 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 primary rainbow. For whatever reason, Alexander's
Dark Band was not apparent.</smaller>
=A0
<smaller>Yt, DW</smaller>=A0
=A0
<smaller>=A0=A0=A0</smaller>
<excerpt><fontfamily><param>Arial</param><x-tad-bigger>----- Original
Message -----</x-tad-bigger></fontfamily>
=
<bold><fontfamily><param>Arial</param><x-tad-bigger>From:</x-tad-bigger></=
fontfamily></bold><fontfamily><param>Arial</param><x-tad-bigger>
</x-tad-bigger><color><param>0000,0000,EEEE</param><x-tad-bigger>Patrick
Kelly</x-tad-bigger></color><x-tad-bigger> </x-tad-bigger></fontfamily>
=
<bold><fontfamily><param>Arial</param><x-tad-bigger>To:</x-tad-bigger></fo=
ntfamily></bold><fontfamily><param>Arial</param><x-tad-bigger>
=
</x-tad-bigger><color><param>0000,0000,EEEE</param><x-tad-bigger>naturens@=
chebucto.ns.ca</x-tad-bigger></color><x-tad-bigger> =
</x-tad-bigger></fontfamily>
=
<bold><fontfamily><param>Arial</param><x-tad-bigger>Sent:</x-tad-bigger></=
fontfamily></bold><fontfamily><param>Arial</param><x-tad-bigger>
Sunday, June 12, 2011 4:54 PM</x-tad-bigger></fontfamily>
=
<bold><fontfamily><param>Arial</param><x-tad-bigger>Subject:</x-tad-bigger=
></fontfamily></bold><fontfamily><param>Arial</param><x-tad-bigger>
Re: [NatureNS] 180 Degree Double Rainbow =
Photos</x-tad-bigger></fontfamily>
Actually, the band is darker because there is less light reaching that
area of the sky. Less light, darker sky
The best explanation for rainbows is in the RASC's <italic>Observer's
Handbook</italic>
=
(<color><param>0000,0000,EEEE</param>http://www.rasc.ca/handbook/</color>)=
In contains a section by Roy Bishop. retired emeritus professor of
physics at Acadia University. In his 11-page section, Rainbows and
Some Other Sky Phenomena, Over five pages are devoted to the rainbow,
including the following:
<bold>Alexander=92s Dark Band:</bold> With a raindrop-mandated minimum
deviation of 138=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 cannot enter. Thus the sky between the bows is darker than
elsewhere, a feature called Alexander=92s Dark Band after Alexander of
Aphrodisias, a Greek philosopher who drew attention to it c. AD 200.
The =93dark=94 band is only moderately darker than the region beneath =
the
primary rainbow because of three sources of light between the bows:
skylight, external reflections off raindrops (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 raindrops are spherical) he demonstrates quite clearly how
the changing incident angle of the light hitting the drop concentrates
the refracted light into the 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 first human to properly explain the formation of a rainbow.
=
<color><param>0000,0000,EEEE</param>http://www.cosmolearning.com/images/ra=
inbow-over-isaac-newtons-birthplace-in-woolsthorpe-manor-england-961/</col=
or>
A paper describing the circumstances of the photo was published in the
Notes and Records of the Royal Society of London.
Pat
On Jun 10, 2011, at 3:31 AM, Stephen R. Shaw wrote:
<excerpt>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). =A0The 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. =A0Given 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. =A0Much 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. =A0Similarly 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. =A0The 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. =A0Such 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
<<<color><param>0000,0000,EEEE</param>patrick.kelly@Dal.Ca</color>>:
<excerpt>The region between the bows is called Alexander's Dark Band
=
(<color><param>0000,0000,EEEE</param>http://en.wikipedia.org/wiki/Alexande=
r%27s_dark_band</color>
) and is a result of the rainbow itself. Briefly, the light that makes
=A0the bows brighter has to come from somewhere, or as the saying goes,
=A0you don't get something for nothing!
Pat
On Jun 8, 2011, at 9:02 PM, David & Alison Webster wrote:
<excerpt>Hi Hans, =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0June 8, 2011
=A0=A0Impressive photos in any case.
=A0=A0In all three shots the sky below the lower arc is less blue and
=A0more white than above. I don't recall having noticed this in
=A0rainbows and the width of the band from red to violet is unusually
=A0narrow. I am wondering if this was just after a shower or just
=A0before fog rolled in.
Yt, DW
----- Original Message -----
From: Hans Toom
To: <color><param>0000,0000,EEEE</param>naturens@chebucto.ns.ca</color>
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
=A0Portuguese Cove. =A0I regret not rushing down to the ocean and
=A0snapping pics without the wires in the foreground and with the
=A0rainbows framing cargo ships swinging at anchor in the outer
=A0harbour. =A0What was I thinking???
<bold><color><param>FFFF,0000,0000</param>MailScanner has detected a
possible fraud attempt from "www.hanstoom.com" claiming to
be</color></bold><color><param>0000,0000,EEEE</param> =
http://www..hanstoom.com/StockPhotos/Visions3/0176.html</color>
Hans Toom
Portuguese Cove, Nova Scotia, Canada
<color><param>0000,0000,EEEE</param>http://www.hanstoom.com/</color>
No virus found in this message.
Checked by AVG - <color><param>0000,0000,EEEE</param>www.avg.com</color>
Version: 10.0.1382 / Virus Database: 1511/3689 - Release Date:
=A006/08/11
</excerpt>=3D =3D
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
Patrick Kelly
Director of Computer Facilities
=3D =3D
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
Faculty of Architecture and Planning
Dalhousie University
=3D =3D
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
MAIL =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=
=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0COURIER
1459 Oxford Street =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=
=A05410 Spring Garden Road
Halifax, Nova Scotia B3H 4R2 =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0Halifax, Nova =
Scotia B3J 2X4
Canada =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=
=A0=A0=A0=A0=A0=A0=A0=A0=A0Canada
=3D =3D
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
Phone:(902) 494-3294 =A0=A0=A0FAX:(902) 423-6672
=
=A0=A0E-mail:<color><param>0000,0000,EEEE</param>patrick.kelly@dal.ca</col=
or>
=3D =3D
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
</excerpt>
</excerpt>
=
<fontfamily><param>Courier</param>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D</fontfamily>
<fontfamily><param>Courier</param>Patrick Kelly</fontfamily>
<fontfamily><param>Courier</param>Director of Computer =
Facilities</fontfamily>
=
<fontfamily><param>Courier</param>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D</fontfamily>
<fontfamily><param>Courier</param>Faculty of Architecture and =
Planning</fontfamily>
<fontfamily><param>Courier</param>Dalhousie University</fontfamily>
=
<fontfamily><param>Courier</param>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D</fontfamily>
<fontfamily><param>Courier</param>MAIL =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0=
=A0 =A0 =A0 =A0 =A0 =A0 =A0
=A0 COURIER</fontfamily>
<fontfamily><param>Courier</param>1459 Oxford Street =A0 =A0=A0=A0 =A0 =A0=
=A0 =A0 =A0 =A0
=A0=A05410 Spring Garden Road</fontfamily>
<fontfamily><param>Courier</param>Halifax, Nova Scotia B3H 4R2=A0=A0 =A0 =
=A0 =A0
=A0=A0Halifax, Nova Scotia B3J 2X4</fontfamily>
<fontfamily><param>Courier</param>Canada=A0=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0=
=A0 =A0 =A0 =A0 =A0 =A0 =A0
=A0=A0Canada</fontfamily>
=
<fontfamily><param>Courier</param>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D</fontfamily>
<fontfamily><param>Courier</param>Phone:(902) 494-3294=A0 =A0=A0FAX:(902)
=
423-6672=A0=A0=A0E-mail:<color><param>0000,0000,EEEE</param>patrick.kelly@=
dal.ca</color></fontfamily>
=
<fontfamily><param>Courier</param>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D</fontfamily>
No virus found in this message.
Checked by AVG - <color><param>0000,0000,EEEE</param>www.avg.com</color>
Version: 10.0.1382 / Virus Database: 1511/3689 - Release Date: 06/08/11
</excerpt></excerpt>
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