Huygens' principle
Huygens’ principle is not an easy physical concept to grasp and I've often tried to avoid it (if possible). But at last making this animation made it a little clearer (to me).
You can control:
v1 and v2 (light speeds in the media 1 and 2)
=distance between adjacent light rays
nr: number of rays
:angle of incidence (with respect to the horizontal line)
You'll probably need to adjust the font size and layout to match your screen resolution.
Gif in twitter:
https://twitter.com/sfera314/status/1495725283751649280?s=20&t=TB13K_u9BlM_upvy0uvSPQ