No one’s written a book about how spells interact so there isn’t a source, but simply logic. If you can’t understand that a spell isn’t about to bounce off of a metal shield then I don’t know what to tell you champ.
An icicle… traveling at a high speed and colliding against a shield… hmm… what would happen? Oh yeah, ice is brittle so it would bounce off and head another direction of course. That makes sense…