Not really an optical illusion, but its optical and it's cool. Drag and drop in an image!
What: Ok, this one is clearly not an optical illusion, but it’s a great optical effect that translates really well into the digital era, that said, if you get your hands on a physical one the tactile response is very fun!
It’s inclusion is important because the kaleidoscope is one of the very first optical toys, the name coined by it’s inventor David Brewster, in 1815  … he was later knighted for his work, in 1831, so thats Sir David Brewster.
How To: In this version you can drag and drop an image from your desktop and it’ll be there! You can also alter the amount of shards. I think in a perfect world I’d have multiple hexagons mirroring, but I couldn’t quite get the math right (I’m no David Brewster). I’ll update this if I get it right!
Explain it: Carefully & perfectly positioned mirrors.
I've researched these illusions in my spare time but am clearly not any kind of expert and my explainations are pretty smooth brained, if you find something mis-cited, earlier examples, or general mistakes please new let me know via firstname.lastname@example.org, be kind!