PROTIP: Press the ← and → keys to navigate the gallery, 'g' to view the gallery, or 'r' to view a random image.
May 15, 2013 at 03:53PM EDT
May 15, 2013 at 04:30PM EDT
I’ve seen this in science museums. There are special filters that appear clear by themselves, but if two of them are put together, they can be anywhere from totally transparent to totally opaque, depending on their relative orientation. I wish I could remember the name…
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