I like this one on wave propagation. https://landgreen.github.io/physics/notes/waves/waves/index.html https://landgreen.github.io/physics/notes/waves/waves/in

I like this one on wave propagation. https://landgreen.github.io/physics/notes/waves/waves/index.html https://landgreen.github.io/physics/notes/waves/waves/index.... (it's at the very bottom)