In Superman II Lois jumps into the Niagara River to test her theory that Clark is actually Superman. Clark uses heat vision to break a branch off of a tree in hopes that Lois can use it as a life saving flotation device. Did Clark's thick, thick lenses provide assistance to his heat vision? If not, how did his glasses not get melted or destroyed? They were not a part of his original uniform.
It's just a plot hole and there's no real explanation for it, other than one someone could just make up (ie super-focusing micro-heat vision). Something similar happened in the first episode of Lois and Clark. It's best to just ignore it.
And the beauty is, you can if you watch Superman II: The Richard Donner Cut. It's a 2006 recut of Superman II by the original director, who was replaced during filming of the second movie. In this version there is now a similar themed scene in the Daily Planet building at the start of the movie, which covers much of the same ground, so much of the Niagara Falls stuff has been chopped out. It's worth a look, if you haven't seen it already.