🚀 go-pugleaf

In article <-DS-cA@engin.umich.edu>, ewald@math.lsa.umich.edu 
(Brian Ewald) writes...
>In article <2A6DEF17.1804@ics.uci.edu> bvickers@ics.uci.edu 
(Brett J. Vickers) writes:

>>peterk@public.sub.org (Peter Kittel) writes:
>>>Is it only me or does the sky beneath a rainbow also appear lighter
>>>to others than the sky above a rainbow (or wait, was it the other way
>>>around?)? But anyway, for my eye there is definitely a jump in
>>>brightness of the background sky (normally rain clouds) from beneath
>>>to above the rainbow. Any explanation for this?
>>
>>Optical illusion?  Two fields of the same color, separated by a line
>>of a brighter or darker color, give the optical illusion that one half
>>is lighter than the other.  The trick is to cover the line and see that
>>they are the same.  I've never tried this with a rainbow.
>>
> 
>It isn't an illusion.  The darker band of sky between the primary and
>secondary rainbows is called Alexander's Dark Band, after Alexander
>of Aphrodisias who noted it around the year 200AD.  (The secondary
>rainbow is the often seen rainbow above the primary, about twice as
>wide, on average one tenth as bright, and with the colors apparently
>reversed; it is caused by an extra reflection).  The bows themselves
>are caused by the light going through the angle of minimum refraction
>in the water droplets, although some, albeit considerably less, is
>refracted more.  With the primary rainbow, this is beneath the rainbow,
>with the secondary, it is above.  Therefore the sky between the two
>bows is, in fact, darker.
> 
>Brian D. Ewald
>ewald@math.lsa.umich.edu

The sky below the lower rainbow is also actually brighter than the
surrounding sky. I presume this is due to reflection from the front
surface of the drops. I was once on a hilltop in Calgary at sunset,
when a full double rainbow occurred. ( It being flat prairie, the
rainbow was as large as it could be for a ground observer).
The point on the sky exactly opposite the sun, on the horizon, in the 
centre of the semi-circles defined by the rainbows, was suffused by 
golden light, the colour of the sunset. This is also similar to the 
halo one sees around the head of one's shadow on dewy grass. 

===========================================================================
Coming soon! The Maarten vander Loeb              Pete Vincent
Memorial University Bomb Disposal
Correspondence School!