A Rainbow is a magnificent natural phenomenon that occurs when rays from a distant light source pass through drops of water. The coloring of a rainbow is dependent upon the refraction and reflection of the light passing through the raindrops, producing a distinct band of visible, colored light. According to Genesis 9, the rainbow was a covenant sign created by God in conjunction with his promise to never again flood the entire earth.
"And God said, "This is the sign of the covenant I am making between me and you and every living creature with you, a covenant for all generations to come: I have set my rainbow in the clouds, and it will be the sign of the covenant between me and the earth. Whenever I bring clouds over the earth and the rainbow appears in the clouds, I will remember my covenant between me and you and all living creatures of every kind. Never again will the waters become a flood to destroy all life. Whenever the rainbow appears in the clouds, I will see it and remember the everlasting covenant between God and all living creatures of every kind on the earth" - Genesis 9:12-16
Not only was a rainbow mentioned in Genesis as a mark of the covenant, but it was also mentioned several times elsewhere in the scriptures as a testament to the glory of God.
Biblical references of a rainbow