Re: [asa] Noah's ark- rainbow question

From: David Bowman <>
Date: Tue Apr 14 2009 - 22:13:16 EDT

Regarding Dick F's optics:
> Hi Bernie:
> The rainbow effect comes whenever light is refracted off water
> droplets.
Rainbows fome from visible light refracting *and* reflecting off
water *drops* of rain--not the droplets of clouds.
> When I used to fly KC-135 tankers occasionally when we flew
> above the clouds and the sun was overhead we could see the
> shadow of our airplane on the clouds beneath with a circular
> rainbow around the shadow.
That effect is often called a *glory* rather than a rainbow (which
deals with precipitating *rain*). The optics of a glory are more
complicated than for a rainbow. In a glory the size of the
(mostly monodisperse) droplets in the cloud are close enough to
the size of the light's wavelengths that there are significant
diffration effects (besides reflection and refraction effects)
that combined have an overall effect that causes the redirected
light to be nearly backscattered back toward the sun with a small
angle of deviation. Hence it appears as a small circle about the
shadow on the cloud of the observer's head (classic glory) or of
the airplane containing the observer (the more common situation),
as the case may be, in a direction opposite of the Sun.
But in a rainbow the drops involved are macroscopic in size and
the diffraction effects are completely negligible. Rainbows can
be explained by the spherical geometry of the drops, the
dispersion of the index of refraction of the water, and the
mismatch of that index with that of the surrounding air. The arc
of primary rainbows is concentrated near an angle of about 42
degrees w.r.t. the direction opposite to the Sun due to a
two-to-one non-monotonic caustic in the intensity of the light
that undergoes a refraction upon entering the drop, an internal
reflection off of the back surface, and a second refraction when
leaving the drop. A secondary rainbow is due to light suffering
two internal reflections inside the drops before leaving them.
David Bowman
> BTW, someone else thinks he has found the Garden of Eden. Too
> early by my calculatios and in the wrong place.
> <>
> Dick Fischer, GPA president
> Genesis Proclaimed Association
> "Finding Harmony in Bible, Science and History"
> <>

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Received on Tue Apr 14 22:14:11 2009

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