# Intriguing optical illusion
**URL:** https://boards.straightdope.com/t/intriguing-optical-illusion/954782
**Category:** Miscellaneous and Personal Stuff I Must Share
**Created:** [November 22, 2021, 8:20pm UTC](https://boards.straightdope.com/t/intriguing-optical-illusion/954782 "2021-11-22T20:20:02Z")
**Posts on this page:** 6
**Page:** 2
### Author:  [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)
#### Post date: [November 25, 2021, 9:33pm UTC](https://boards.straightdope.com/t/intriguing-optical-illusion/954782/21 "2021-11-25T21:33:04Z")
> [@Lumpy](#):
>
> Sounds similar to the principle of a [Reflector sight - Wikipedia](https://en.wikipedia.org/wiki/Reflector_sight)
Well, there are some similarities in parts. The beamsplitter with lens and reticle design is the one most commonly used with radiuscopes. The reflection from a curved surface in there is basically a substitute for the lens, and effectively places the image at infinity – not really the way the radiuscopes work.
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### Author:  [@BigT](https://boards.straightdope.com/u/BigT)
#### Post date: [November 25, 2021, 9:48pm UTC](https://boards.straightdope.com/t/intriguing-optical-illusion/954782/22 "2021-11-25T21:48:13Z")
There was a Burger King playground (their version of the McDonald’s one) where you crawled around in a sort of jungle-gym maze type thing, and the windows on it were translucent hemispheres, pointing outward. They were my first exposure to these types of holograms, and part of what made the place special.
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### Author:  [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)
#### Post date: [November 25, 2021, 11:55pm UTC](https://boards.straightdope.com/t/intriguing-optical-illusion/954782/23 "2021-11-25T23:55:08Z")
> [@BigT](#):
>
> They were my first exposure to these types of holograms, and part of what made the place special.
Well, they’re not really “holograms”, but they do reproduce the wavefront from the object with great clarity, so they give the impression of one. There aren’t that many arrangements that will do so good a job. Besides the use of a parabolic mirror for imaging (either in Tanagra Theater mode or in the Museum of Science mode) and the paired parabolic mirrors placed so their vertices coincide with the other’s focal point, I don’t know of any other that gives so perfect an illusion. I’ve doodled out another one that I think could do it – another set of paired parabolas – but I haven’t been able to try it out yet.
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### Author:  [@commasense](https://boards.straightdope.com/u/commasense)
#### Post date: [November 26, 2021, 3:31am UTC](https://boards.straightdope.com/t/intriguing-optical-illusion/954782/24 "2021-11-26T03:31:01Z")
@CalMeacham: would you please describe, or point us to an illustration of, how a Tanagra Theater is set up, the positions of the mirrors, object, etc? The link you posted said it was similar to Pepper’s Ghost, which I have seen many times and understand pretty well, but didn’t provide any details of the Tanagra Theater setup or the differences. Thanks.
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### Author:  [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)
#### Post date: [November 26, 2021, 3:57am UTC](https://boards.straightdope.com/t/intriguing-optical-illusion/954782/25 "2021-11-26T03:57:14Z")
Unfortunately, no – the links I had to websites with diagrams all appear to be dead now.
There used to be a really good illustration at a German site that showed the Tanagra Theater with reduction, but the site’s gone now.
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### Author:  [@beowulff](https://boards.straightdope.com/u/beowulff)
#### Post date: [December 30, 2022, 11:40pm UTC](https://boards.straightdope.com/t/intriguing-optical-illusion/954782/26 "2022-12-30T23:40:19Z")
I saw this today.
Although it’s unrelated to the OP’s illusion, I thought it was interesting.
Here’s the photo as published from [Weather.com](http://Weather.com). It’s an image of flooding in Yellowstone.
> **[Top Weather Photos Of 2022 | Weather.com](https://weather.com/photos/news/2022-12-07-best-photos-2022)**
>
> Here are the top photos from the top weather stories of the year. - Articles from The Weather Channel | weather.com
I thought the image looked very strange. The flooding appeared to be solid, and on top of the road.
Then I realized this this was an example of the famous “crater / hill” illusion, and the image needed to be rotated 180°:
[[Album] imgur.com  ](https://imgur.com/a/Etakho5)
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