You can use logic to communicate a coherent theory of radiation.
At that point it becomes a question of Uniform-Standards-of-Evidence.
But more specifically to your question, "What qualifies as REAL-TRUE-FACTS?"...
Just like a court of law, both the prosecution and defense MUST agree on the facts (common-ground).
If you and your opponent disagree about a FACT, you must immediately stop the debate, and negotiate the point of disputed FACT.
In your specific (excellent) example, you would probably start with common-ground by discussing the visible spectrum and how a rainbow effect is produced by a prism, and then explain that the rainbow effect extends beyond the red and the blue on both sides, and then try and find some real-world examples of how they could test this and why you believe it is "important" in practical times.
You basically have to build a compelling case.
You can't just say, "it's a fact it's a fact it's a fact and you're a #?@!!o if you don't agree".
REAL-TRUE-FACTS are not always, necessarily "self-evident".
RE: OPINION =/= FACT