[NatureNS] 180 Degree Double Rainbow Photos

From: Patrick Kelly <patrick.kelly@dal.ca>
To: naturens@chebucto.ns.ca
Date: Sun, 12 Jun 2011 16:54:35 -0300
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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