Rainbows
According to Genesis GOD created the first rainbow just after the flood when Noah sacrificed a load of animals. This random slaughter apparently pleased GOD because he decided to make a covenant with man never to send a flood to kill all flesh again. He seals this covenant with the creation of the rainbow.
Genesis 9:12-17: The rainbow that I have put in the sky will be my sign to you and to every living creature on earth. It will remind you that I will keep this promise forever. When I send clouds over the earth, and a rainbow appears in the sky, I will remember my promise to you and to all other living creatures. Never again will I let floodwaters destroy all life. When I see the rainbow in the sky, I will always remember the promise that I have made to every living creature. The rainbow will be the sign of that solemn promise.
To the ancient Hebrews who were ignorant of the true nature of the rainbow this idea would have seemed plausible enough. Under scientific examination however the idea that there was a time without rainbows is ridiculous. Consider the three possible conditions where rainbows would not form.
This could not have been the case. A lack of sunlight would have quickly driven all but a few special species to extinction. Genesis also mentions sunlight in the creation account.
The bible tells us there was atmospheric moisture. At the beginning of the flood epic it rains for 40 days and nights.
This scenario does not work either. The refraction of light is what makes sight possible, without refraction light would not be focused on the retina.
There is nothing in the Genesis text which says this was the first rainbow. Reading the classical Hebrew and Christian commentaries over the centuries (and certainly before Newton’s work on the prism), we find plenty who believed that this was not the first rainbow. Take the Jewish commentator Saadia Gaon for example (882-942 AD), who held the view that this was not the first rainbow – he could hardly have been asserting this on scientific grounds, as Newton might have.
But far earlier than Saadia is the Babylonian Talmud (compiled 6th century AD from earlier rabbinical sources), in which we find the rabbis commenting that the rainbow has been around since the beginning of creation (Babyloian Talmud, Seder Mo’ed, Pesahim 54b).
God sets his rainbow in the clouds. If there were already rainbows why would god have to set them in the clouds? The rainbow is also a sort of gift to man that will remind god to keep his promise, why would god give the Hebrews a gift they already had?
Your first question assumes that this was the first rainbow. There is nothing in the text to say this. You seem to forget that rainbows do not appear after every occurrence of rain, so for God to use a rainbow as a sign on this occasion does not necessarily mean there were no rainbows previously.
Your second question is irrelevant because the Bible says nothing about the rainbow being ‘a sort of gift to man’.
I’ve already demonstrated that the earliest Jewish commentaries didn’t interpret this as the first rainbow, so it’s clear the reading of the text you’re choosing isn’t exactly intuitive. It seems more like a motivated reading in a deliberate attempt to find problems with the text.