The Rainbow

According to the dictionary; its a bow of prismatic (brilliant) colors appearing in the heavens opposite the sun.
According to the Law of Physics; its caused by the refraction (change of direction) and reflection (return of) of the sun's rays in drops of rain.
There are even Legends regarding rainbows; its the tongue of the sun, the bridge between the living and the dead, if you pass beneath a rainbow you will be transformed woman into man and man into woman, the most popular being a leprechauns pot of gold can be found at the end of one.
Even a proverb (not biblical) "Rainbow at night shepherds delight. Rainbow in morning shepherds take warning."
But the best explanation for rainbows is found in the Book of Genesis. That it is a token of the covenant which the Lord made for perpetual (continuing or enduring forever) generations.