"I do set my bow in the cloud, and it shall be for a token of a covenant between me and the earth." (Gen.9:13)
Besides the rainbow, you will find several other tokens God employs as reminders or commemorative signs between Himself and man, including: the ordinance of circumcision for Israel (Gen.17:11); the blood on the door lintels in Egypt (Exo.12:13); and Aaron's rod that budded (Num.17:10). In the case of the blood and the rainbow, the token was a promise of protection—a safeguard against the death angel, in the first instance; and in the second, assurance against the possibility of another flood of water on the earth. Obviously, God doesn't need reminders, but perhaps He chose to use this terminology to reassure us, His forgetful and fearful children.
Webster says that a rainbow is caused by "the refraction (change of direction) and reflection of the sun's rays in drops of water." That is only the secondary cause, however. The primary cause is the reflection of the Son's rays of love in drops of water. The prism of color is merely a picture of His multi-faceted grace. It reminds us, His beloved ones, that He always keeps His promises. The world will never again be overflowed by water, and you and I need never fear that we will drown in the sorrows or trials of this life. For He has promised, "When thou passest through the waters, I will be with thee; and through the rivers, they shall not overflow thee" (Isa.43:2a).
When there's a rainbow in the sky,
The clouds that frowned go smiling by.
'Tis a promise written there;
Of our Father's love and care;
When there's a rainbow in the sky.
---John W. Peterson