Got old lenses? Build a Cloaking Device! at
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Old September 25th, 2014, 10:33 AM   #1
Inner Circle
Join Date: May 2006
Location: Camas, WA, USA
Posts: 5,513
Got old lenses? Build a Cloaking Device!

Here's the trick. Get two pairs of single element lenses, possibly by dismantling old photo lenses, with focal lengths A and B. Arrange the lenses in line in this order A, B, B, A. Separate the pairs on the ends by the distance A + B. (Let's say you have a 200 mm and 75 mm lenses. The distance would be 275 mm.) The distance between the two "B" lenses is 2B * (A+B) / (A-B). (That's 330 mm in the 200 & 75 example.)

The result is a very narrow light path between the pairs of A&B lenses, but the end-to-end view is neutral. As long as the "cloaked" object doesn't block the center node, it becomes invisible.

The video from the University of Rochester:

The article from NBC News:
Scientists Show You How to Make an Invisibility Cloak (Sort Of) - NBC News

So, how does it work? The first 200mm lens makes a focal point 200mm from that lens. (You can see that point clearly in the video's thumbnail image above. Put the cloaked object here.) The first 75mm lens, which is 75mm from this point, straightens the rays. Unfortunately, the view is now upside down. That's where the second pair comes in. It inverts the effect and puts things right-side-up again. This works no matter the distance between the two pairs as the rays between the pairs are straight. So why the specific distance between the pairs? Because at this distance, you get the proper 3D effect as the viewer moves off center. At other distances, the background would move too much or too little as the eye moves off center.

Fun with optics!
Jon Fairhurst
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