[Tfug] Another OT Optics Question

[Bexley Hall]

Hi, Hu,

--- On Sun, 8/3/08, Hubert M Bath <bathhm at laposadagv.net> wrote:

> > Inceident ray comes back ~40 degrees refracted (and
> reflected).
> > So, can I think of rainbow as a "conic
> section" in a cone with
> > a 42 degree apex?
> 
> The 42 degrees applies to red light, and 40 degrees to
> violet, the 
> inside of the "primary" rainbow. These angles
> apply no matter the height 

Yes.  A function of the water droplets...

> of the sun.  Only if the sun is at the horizon, and
> sun's rays therefore 
> parallel to the earth's surface does the rainbow appear
> as a semicircle 
> --with its center at the horizon.  If the sun is higher,
> the rainbow 
> center is below the horizon and the rainbow less than a
> full semicircle. 

But, the shape is still that of a "circular arc", right?

>     If the sun is not so low, the rainbow moves down, so
> that its center is below the horizon, rather than at it.

Ah, OK.  That fits my memories of observed rainbows...

And, presumably, if the water vapor does not extend far
enough to each "side", that portion of the rainbow (arc)
disappears...

> > And, the apparent "diameter" of the rainbow
> is a function
> > of how far the water is from the observer?  E.g., a
> rainbow
> > thrown from the mist of a gargen hose would appear to
> have a much
> > smaller diameter than one from water vabor many miles
> distant.
> 
> I think that the *angular* diameter remains the same, tho,
> obviously, 
> the actual refraction-reflection-refraction is occurring
> much closer 
> than if the drops were from a distant rainstorm.  And,

To be clear... the rainbow *does* form a circular arc (?)
as a slice of that 40 degree cone  (?)  So, the farther
the observer iws from the phenomenon, the larger the
apparent radius becomes?

(sorry, this isn't my field so I need to take baby steps)

> because the sun 
> is presumably 'way above the horizon, the the arc of
> the spraybow would 
> be less than a semicircle.  In fact, if the sun is more
> than 42 degrees 
> above the horizon, a rainbow is impossible (because its
> "center" is more 
> than 42 degrees below the horizon.)

Oooh! This is an interesting observation!
But, are you sure of that?  I grew up in New England
pretty close to that 42 degree N line and I *know* I
saw rainbows as a kid (I am trying to remember if
I ever saw them in New Hampshire...)

Thx,
--don