Author Topic: Twin Double Rainbows  (Read 995 times)

samdavo

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Twin Double Rainbows
« on: October 24, 2015, 08:30:54 PM »
Here's a good chance to use DCAD to illustrate a fairly complex situation in 3D. 
Two sets of double rainbows for two light sources - the sun, and the reflected sun. :)
I'll give it a crack "later" - unless someone beats me to it.
Cheers, sam

https://www.washingtonpost.com/news/capital-weather-gang/wp/2015/04/21/is-it-legit-rare-quadruple-rainbow-photo-goes-viral/
« Last Edit: October 24, 2015, 08:32:49 PM by samdavo »

samdavo

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Re: Twin Double Rainbows
« Reply #1 on: October 25, 2015, 03:04:39 PM »
OK,
quick first attempt ...
O = observer
OA = dirn of sun's rays (drawn as 10 degrees slope down)
OB = dirn of reflection of sun's rays (drawn as 10 degrees slope up)

R1 = first rainbow, which is a curve 41 degrees from OA
R2 = second rainbow, which is a curve 51 degrees from OA
R3 = first reflected rainbow, which is a curve 41 degrees from OB
R4 = second reflected rainbow, which is a curve 51 degrees from OB

The fact that R2 and R3 are tangents (dead ahead, top of arc) is a coincidence that follows from choosing 10 degrees for the sunray slope, given that primary and secondary rainbows are also separated by 10 degrees)

PS As per that Washington Post article, the arcs do indeed have common springing points on the horizon.

  PS I suspect an error - in that the arcs should be truncated at some  vertical plane -  where that plane represents the plane of water droplets.  See next post.
 
« Last Edit: October 25, 2015, 03:50:20 PM by samdavo »

samdavo

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Re: Twin Double Rainbows
« Reply #2 on: October 25, 2015, 03:44:26 PM »
ok, and this is what happens if you make those arcs into cones, and truncate them at some  vertical plane. (2 cents)
(PS Colours are random - nothing to do with rainbow colours)
« Last Edit: October 26, 2015, 12:14:05 AM by samdavo »

samdavo

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Re: Twin Double Rainbows
« Reply #3 on: October 25, 2015, 03:56:30 PM »
PS JUST FOR INTEREST
Only two rainbows this one (primary and secondary) - and already posted elsewhere - but this is a photo or two taken in Sydney a few months back, and pretty impressive in my opinion - late in the afternoon obviously (almost semicircular arcs).

Also illustrates how primary rainbows have red on top, and secondary are reversed with blue/violet on top (2 cents).

For 4 rainbows, it seems you need a large body of water BEHIND you to reflect the sun's rays.
« Last Edit: October 25, 2015, 04:07:31 PM by samdavo »

samdavo

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Re: Twin Double Rainbows
« Reply #4 on: October 25, 2015, 07:10:40 PM »
Final question,
If there's a pot of gold at the end of rainbow, how many pots of gold for two double rainbows? :)

PS Here's the same set of cones after exporting to Sketchup. (2 cents)
« Last Edit: October 25, 2015, 09:25:13 PM by samdavo »

samdavo

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Re: Twin Double Rainbows
« Reply #5 on: October 26, 2015, 02:55:50 PM »
Just "chewing the fat" here, but if I could somehow set the eye at the tip of all those conical shapes, and look in the opposite direction to the jpeg above, then I believe I would be able to see four circular arcs. 

BY THAT I MEAN that you would (hypothetically) be looking outwards along each of the radiating surfaces, so that they would appear as curves only, without the surfaces.

I may be wrong, but I suspect that would be easier to do in Sketchup than in DCAD, but to be honest, I am unable to do it in either.

The project continues (or am I chasing rainbows, lol ?)

samdavo

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Re: Twin Double Rainbows
« Reply #6 on: October 26, 2015, 02:58:56 PM »
Herewith my attempt (in DCAD) to set the camera at the apex of those cones, and look along the X axis.   The right hand view window shows the result.    Clearly, since X appears larger than Y or Z, then perspective suggests the view is looking the opposite direction to what I attempted. 

  (edit) PS,  DOH - I should have set points 1 and 2 in the opposite order  (i.e. 1 = target,  2 = eye / viewer)  - experiments continue. ...
(update) - still not getting tha arcs - maybe 41 degrees is a bit too steep for "limits of view"?  - might leave it for a while, maybe an idea will drop from the blue :) - cheers
« Last Edit: October 27, 2015, 01:00:11 PM by samdavo »

samdavo

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Re: Twin Double Rainbows
« Reply #7 on: October 26, 2015, 03:01:19 PM »
And herewith my attempt in Sketchup, again setting the Camera position at the apex.    This image is taken AFTER I have set the Camera there.   And yet again, the apparent "eye" does not appear to be at that point. ( 2 cents)
« Last Edit: October 26, 2015, 03:35:54 PM by samdavo »