The difference between blue ray and green ray.

[anagogy]

I'm amazed that you went from asking as an OP, to now revealing this knowledge. Did you know it before, or did you learn something along the way when you recently responded?

I ask questions to gather new bits of knowledge from other people, not necessarily because I have no answers of my own. I'm always in the process of refining my knowledge, to see how my understanding matches or diverges from others understanding. We are all mirrors for one another.