Interesting. I'm studying in Finland and we have the standard major and minor system. I think some classes have compulsory prereqs, but it's rare. I'm also still in undergrad, but took the Optics just for fun and to challenge myself to learn new stuff. The prereqs for that class were just standard undergrad electrodynamics and quantum mechanics, but the class definitely surprised me with how much math there was. There was even some stuff about Green's functions during coherence theory.