[Tfug] Another OT Optics Question

[Jeremy D Rogers]

[snip]
> But, the shape is still that of a "circular arc", right?

Yep, always circular.

[snip]
> 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?

You're right, but I'd not say it's a slice of a cone in general. It
really is just a circle or part of a circle. Perhaps a better way to
think of it is that the light from visible wavelengths scatters from
raindrops (refracts on the way in, reflects off the backside, refracts
on the way out) at 30 degrees from the angle of incidence. Now that
happens in a cone pattern for each and every raindrop. But, you don't
see that cone, you only see one 'ray' from that cone because your eye
is only at one point on that cone. However, there are lots of
raindrops, so you can see any ray that reaches you eye from any
raindrop. Magically (actually geometrically), all those rays form
another 40deg cone from your eye to the cloud.

> (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...)

Ah, Hu did not mean latitude. You could be at high latitudes or low
latitudes, but if the angle straight from the sun down to the horizon
is 40 degrees, the location of the center of the rainbow would be 40
degrees below the horizon which means the top of the rainbow would be
right at the horizon and you would not see it. Imagine it's noon at
the equinox in Chicago (42deg), the sun is due south and 90-42=48 deg
above the southern horizon. There is a rainstorm to the north, but the
rainbow would be formed below the northern horizon with the center 48
degrees below the northern horizon, and the top forming just 8 degrees
below. If there are no raindrops between me and the ground (the storm
is a ways off), I don't get to see the pretty colors. However, if I
went up in a tall tower so that I look down at the rain cloud, I would
be able to see it. Although it would be easier to just spray the hose
in front of me while facing north.

Hope that's enlightening (hehe),
JDR

>
> Thx,
> --don
>
>
>
>
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