That's a good way to silence them.
That is a good example of what is referred in 1 Tim 4:2
I would simply point out that they were convinced at once time that the Bible was true and then came to believe that it was lie, then how sure they were they weren't being lied too again? But their response was always the same, they knew what the Bible said.
Like the bow that the LORD placed in the cloud, they would argue that it was written that it was rainbow . Of course in the KJV it reads that "a bow in the cloud" while there are other translations which do in fact use "rainbow" in lieu of bow in the cloud. So that proved them right in the opinion regarding what the scriptures said and naturally their position was that the rainbow was due to light refraction not some supernatural being.
Without going into the entirety of points, while it is true that the multicolored band of visible white light is the result of light refraction, if not for the bow in the cloud that the LORD covered the earth with like a garment then the the multicolored band of refracted light would not curve to since refracted light only travels straight, just at a different line of travel, or angle if you will.
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It is the bow in the cloud, or rather the curvature of the earth's atmosphere which gives the refracted light the appearance of being a multi-colored bow in the sky. Not to mention that the dry land appeared after the firmament was made from the waters that covered the earth.
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Then of course there is the verse which reads that the LORD said, And the bow shall be in the cloud; and I will look upon it, that I may remember the everlasting covenant between God and every living creature of all flesh that is upon the earth. Gen 9:16
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But like they say, if you can't dazzle them with brillance, baffled them with BS.