Using the technology of tiny optical glass fibers it is possible to allow visible light and shadows to be seen through a concrete wall. I saw this on shown on a Nation Geographic Documentary and was intrigued because it has been around a while and I had not heard of it before.
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Using technology a company called Litracon in Hungary has mastered what some may think is a magic trick.
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These commercially available prefabricated transparent concrete blocks are manufactured using a combination of optical fibers and fine concrete.
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Translucent concrete was first invented in 2001. In 2004 Litracon's product (Litracon Classic®) was among the TIME magazine's most important inventions of the year. There are now other manufactures of translucent concrete throughout Europe.
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When incorporated into a buildings architecture you and get some spectacular effects.
Sources:
National Geographic Documentary Magastructures - Science of Concrete 86 minute video