I assume posted because of the latest 2swap video about it: https://www.youtube.com/watch?v=kL9BTbIGxLg

They are a fun way to demonstrate LCM and GCD too

https://en.wikipedia.org/wiki/Arithmetic_billiards

With curved reflecting boundaries and parallel rays you get the beauty of catacaustics. A familiar site in a cup of coffee

https://en.wikipedia.org/wiki/Caustic_(mathematics)

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