This question came up during an email debate with a muslim. He was trying to show me that the bible has errors. This is one of the errors that he proposed (His words are in red):
What does the Bible say regarding Hydrology? Bible says in Genesis, Ch. No.9, Verse No.13 to 17, that After God, at the time of Noah submerged the world by flood, and after the flood subsided, He said I put up a rainbow in the sky as a promise to the humankind never to submerge the world again, by water.
To the unscientific person it may be quite good Oh rainbow is a sign of Almighty God, never to submerge the world by flood again. But today we know very well, that rainbow is due to the refraction of sunlight, with rain or mist. Surely there must have been thousands of rainbows before the time of Noah.
To say rainbow was not there before Noahs time you have to assume that the law of refraction did not exist – which is unscientific.
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No, you do not have to assume that the law of refraction did not exist before the flood. The world before the flood was very different from the world today. Basically, the bible hints that there was no such thing as rain before the flood, and therefore, no rainbow in the clouds. There is a detailed answer here