🚀 go-pugleaf

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.
>
>--
> ___            _    _     _  _        _
>(  _) ___ ___ _( )__( )_  ( )( ) o  __( ) _  ___  ___  ___
>(___)(_) (__=) (_)_ (_)_   (__) (_)(_((_)(_'(__=)(_) _(_)
>Brett Vickers (bvickers@ics.uci.edu)

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