Rainbows-Why are they here?

Why do rainbows exist? Science says its due to the sun shining after the rain falls, the cycle of photosynthesis. The Bible however, says something entirely different.

Everybodys heard of the story of Noahs Ark,God told Noah to build the ark and bring two of every animal on board along with his family as God was going to destroy all the evil things by making it rain for forty days and forty nights. Well after he did that, God made a promise to Noah that he would never flood the Earth to destroy flesh like that as long as the Earth is here, so to show his promise and remember the promise he made, God created a rainbow that would come out of the clouds after every rain. It makes me wonder if God made any other promises that resulted in the beauty of nature? I'm sure I'll find out,but I'm already learning and remembering things from the bible like man is not allowed to eat raw meat. Pretty cool. If you ask me though, I think God just wanted a rainbow to make the sky even more beautiful. Bible challenge Day 2.