What are rainbows?

Rashi:
A sign that God is mad at the world, but has elected not to deliver the destruction it deserves. He puts up a rainbow to convey this message; when he's not mad at the world rainbows do not appear. Indeed, there were several generations when no rainbows ever appeared. [This is a (a) untenable and (b) a modification of the Midrash which states only that two generations went without rainbows]

Ramban:
We ignore science at out own peril. When light hits water rainbows appear. This is built into nature. The rainbow serves as a sign in the same way that Lavan's pillar serves as a sign. There's no special message in it. It's not something external to nature, rather it is something that exists in nature that serves as a reminder of the deal God made with the creation following the deluge.

Additionally.
The Pslams speak (metaphorically?) about an angry God shooting lightning arrows at the earth. When ancient warriors wished to indicate hostilities had ended they'd hold up their bows either to show they were empty, or tyo show they weren't being used. (we hold up our hands now). The rainbow, which points its business end at the sky, is God's way of putting down his weapon, of saying hostilities have ended. (Ramban says this, too, (not the bit about the lightning arrows) but rejects it, because he won't ignore the fact that rainbows, pace Rashi, are a natural phenomenon that appear whenever light is refracted through water.)

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