Hastings' Dictionary of the Bible (1898)
RAIN'BOW, a seven-colored semicircle produced by the reflection of the sun's rays from the drops of falling water, and appearing in its greatest brilliancy when the spectator is placed between the shining sun on the one side and a raining cloud on the other. It may be formed in waterfalls, fountains, etc., but when formed in the atmosphere it always shows that the rain has passed away. The same laws by which this effect is produced were probably in operation before the Deluge.
The rainbow, which had hitherto been a beautiful object in the heavens, was appointed as a sign of the covenant that the earth should not be again destroyed by a flood. " Gen 9:12-17.