You've run to the window when the sun came back in the middle of the rain. There it is: a huge arc, colorful, stretching from one end of the sky to the other. It fades in minutes. Nobody paints it, nobody programs it. It just happens.
The rainbow has two stories at the same time. One is physics: light, water, and angles you can measure and even reproduce with a hose in the backyard. The other is faith: in the Bible, this same arc is the sign God chose to remember a promise. The two stories don't fight. They fit together.
In this article you'll understand exactly how the raindrop breaks up sunlight, why the arc is always on the side opposite the sun, what that second, fainter arc above it is, and what the Bible says about all of this. At the end, an inexpensive activity to do with your Unit.
Why does a raindrop turn into seven colors?
Sunlight looks white, but it isn't. It's a mix of all the colors together. While they travel stacked up, your eye sees white. You just have to separate them to see the rainbow hidden inside.
The raindrop is what does that separating. When a ray of sun enters a drop, it passes from air into water. Water is denser than air, so the light changes direction as it enters — this is called refraction, it's light "bending." Inside the drop, the ray hits the back, reflects, and comes back. As it leaves, it bends again. In once, reflects once, out once.
The brilliant detail is that not every color bends the same. Red light bends a little less; violet bends a little more. Each color leaves the drop at a slightly different angle. That fanning out is dispersion. A single drop sends you just a tiny ray of one color. But the sky has millions of drops, each at its own height, and together they build the whole arc: red, orange, yellow, green, blue, indigo, and violet.
Why is the arc always on the side opposite the sun?
Notice something next time: to see the rainbow, the sun is always behind you. Always. It's no coincidence.
There's an imaginary point in the sky called the antisolar point: it's the direction exactly opposite the sun, on the other side of your head. The light entering the drops comes back concentrated in a ring around that point. That's why the arc is behind your back relative to the sun — you look at the rain with the sun shining from behind.
That ring has a well-defined radius: about 42 degrees from the antisolar point. That's why the arc is always more or less the same size in the sky. And it's also why, the lower the sun (early morning, late afternoon), the bigger and higher the arc appears. At noon, with the sun high up, the arc hides below the horizon and you can barely see it. The best hours to hunt for rainbows are early morning and late afternoon, right after a burst of rain.
A curious detail: each person sees "their own" rainbow. Since it depends on the angle between you, the drops, and the sun, the Pathfinder beside you is seeing an arc built by other drops. No one can reach the "foot" of the rainbow — it moves along with whoever is looking.
The double rainbow: why is the second one reversed?
Sometimes, when the rain is heavy and the sun is clear, a second arc appears above the first, fainter and wider. It's the double rainbow, and it has a beautiful explanation.
In the main arc, the light reflects once inside the drop. In the second arc, the light reflects twice before leaving. That extra reflection does two things. First, it spends a bit of light with each bounce, so the second arc is always dimmer. Second, it reverses the order of the colors.
That's why, in the lower arc (the main one), red is on the outside and violet on the inside. In the upper arc (the secondary one), it's the opposite: red inside, violet outside. The two reds face each other. Between the two arcs there's a strip of noticeably darker sky, called Alexander's band. The next time you see a double arc, look for that dark strip in the middle — it's a lovely detail to point out to the Unit.
Read alsoThe seven days of CreationThe arc in the covenant: Genesis 9 and the flood
Now the other layer. In the Bible, the rainbow's first great appearance comes right after the flood. When Noah and his family leave the ark, God makes a promise and chooses a visible sign to seal it.
The text is direct. In Genesis 9:13, God says: "I have set my rainbow in the clouds, and it will be the sign of the covenant between me and the earth" (NIV). A covenant, in the Bible, is a serious agreement, a commitment. And the promise is clear: never again would a flood destroy all life on earth (Genesis 9:11).
Notice that the Bible calls it a "bow" — the same word used for an archer's bow. It's an image of war hung in the sky, but with no string and no arrow, pointing upward. A bow at peace. In the following verses, God repeats that, when the bow appears in the clouds, He will "remember" the covenant (Genesis 9:14-16). God does not forget, of course; the language is meant to teach us. The sign is for us to look up and remember the One who keeps His promises.
The arc around the throne: Ezekiel and Revelation
The rainbow does not disappear after Genesis. It returns in two of the most striking moments in the Bible — and in both it surrounds the presence of God.
The prophet Ezekiel has a vision of the glory of the Lord and can't find a better word to describe that radiance. He writes: "Like the appearance of a rainbow in the clouds on a rainy day, so was the radiance around him. This was the appearance of the likeness of the glory of the LORD" (Ezekiel 1:28, NIV). The same rainbow of the rain becomes a measure to describe the indescribable.
In Revelation, the apostle John is taken to see the throne of heaven. And there it is again: "encircling the throne was a rainbow that shone like an emerald" (Revelation 4:3, NIV). The sign of the promise from Genesis now frames the very throne. The message that runs through the whole Bible is the same: the God who promised is the God who reigns. Faithfulness from beginning to end.
Unit activity: make your own rainbow
The best way to understand is to see it happen. You can reproduce the rainbow with materials almost every Club has. Pick a sunny late afternoon and do it in the yard, with the Unit gathered.
With a hose: stand with your back to the sun, with the sun low behind you. Adjust the nozzle to make a fine mist (not a jet). Spray forward and to the sides. Ask the Pathfinders to look at the mist with the sun behind them — the arc will appear floating in the water. Each person sees their own; ask them to switch places and confirm. It's the physics of the sky, in your own hands.
With a prism or CD: on a sunny day, use a glass prism at the window to throw the colors on the wall. No prism? An old CD works: tilt the shiny face into the light and see the fan of colors. Then use the moment to read Genesis 9:12-17 in a circle and talk it over: a phenomenon that repeats all the time, chosen by God precisely because it's common, so that no one needs a rare miracle to remember the promise. If your Unit enjoys watching the sky, you can follow up with a night of observing the night sky and turn it into a series about creation.