Rainbows, How Do They Work?

So, a discussion Jennifer and I like to have is if everything in the Bible is to be taken as written, then light must have worked differently before the Flood.  I base this on rainbows being a new thing as a symbol of God's promise.  So in the pre-Flood world light could not be split?  But then how were there any colors?  Or if water wasn't able to split light, then how was it different?

It's a fun thing to talk about, at least for us.  It's no different than putting yourself in any other fantasy world and arguing the differences in physics.

Which brings me to XKCD's spin-off site "What If?" and more specifically pondering about rainbows in Tatooine.

Now beyond the concept of the what the system looks like, I have to ask, "When does it ever rain on Tatooine to even create a rainbow?"

But then back to my original biblical thought experiment, here's this XKCD strip: