The Physics and Metaphysics of Rainbows

How Rainbows Work

What causes a rainbow to appear in the sky? It usually shows up when part of the sky is sunny and part of it is still dark with rain clouds. The sun shines from behind the person and onto the rain droplets in the air. The beam of light gets refracted on the way in to each droplet, reflected on the back, and refracted again on the way out.

Each color of light has a different wavelength and gets diffracted a different amount. Each color that a person sees in a rainbow comes from a different location and a different angle with the sun. One cannot come to close to a rainbow since it only appears when viewed from afar at the correct angle (generally 42°). The rainbow isn’t actually located anywhere in space and each person sees their own version of the rainbow.

The Meaning of Rainbows

This week’s parsha describes God’s covenant with Noah (Genesis 9):

And God said to Noah and to his sons with him,
I will maintain My covenant with you: never again shall all flesh be cut off by the waters of a flood, and never again shall there be a flood to destroy the earth.”
God further said, “This is the sign that I set for the covenant between Me and you, and every living creature with you, for all ages to come.
I have set My bow in the clouds, and it shall serve as a sign of the covenant between Me and the earth.
When I bring clouds over the earth, and the bow appears in the clouds,
I will remember My covenant between Me and you and every living creature among all flesh, so that the waters shall never again become a flood to destroy all flesh…

God tells Noah that the rainbow is a sign that the world won’t be destroyed. What is the rainbow’s significance and how is it a sign for anything? On a simple level — when it’s raining, people don’t know if the rain will stop or perhaps just keep going and destroy the world, like in the times of Noah. Before the clouds and moisture have disappeared, the rainbow is a clear sign that the sun is starting to shine again.

Yet it seems there’s more to the meaning of a rainbow. The prophet Yechezkel sees a rainbow in his vision of the divine throne (Ezekiel 1):

Like the appearance of the bow which shines in the clouds on a day of rain, such was the appearance of the surrounding radiance. That was the appearance of the semblance of the Presence of the LORD…

There’s something about the rainbow that hints to the Divine presence guarding the stormy world. The sign isn’t something tangible, like a stone or tree — it’s more faint and ethereal. It can be seen only briefly and you might miss it if you’re not looking for it. No one can get to close to it, and each person sees it a little differently. It’s faint but it hints to a light beyond the darkness, even if all one can see is its reflection.

Yechezkel was dealing with a traumatic period for the Jewish people, when they were going into exile in Babylonia. His vision is filled with frightening forces — a stormy wind, a flashing fire — people don’t know what will happen. But at the end of his vision, he sees a rainbow — a sign of the Divine, something beyond that will guide his people. In difficult times, from floods to exile, sometimes all that’s needed is this faint sign.