Why does a rainbow always curve, and could you ever reach its end?
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After you watchWhy does a rainbow always curve, and could you ever reach its end?
The short answer
Refraction and internal reflection concentrate primary-rainbow light near 40–42 degrees from the direction opposite the Sun. Drops at the relevant angles form a ring around each observer’s antisolar direction. Moving changes which drops send the bright colors to the observer.
The whole story
How it works
Refraction and one internal reflection concentrate primary-rainbow light near 40–42 degrees from the antisolar direction. The hidden application changes the observer position; the model recomputes the lit ring within a fixed field of drops.
What people get wrong
A rainbow is not a painted stripe attached to a fixed set of raindrops. Changing viewpoint changes which drops send bright rainbow light to the observer.
The catch
The bright rainbow depends on illumination, viewing direction and available drops. From the ground, the lower part of the circle is often hidden by the horizon or lacks suitably placed drops.
Questions kids ask
Why does a rainbow always curve, and could you ever reach its end?
Refraction and internal reflection concentrate primary-rainbow light near 40–42 degrees from the direction opposite the Sun. Drops at the relevant angles form a ring around each observer’s antisolar direction. Moving changes which drops send the bright colors to the observer.
What is different in the hidden experiment?
After exploring a single drop, predict whether the same or different drops form a rainbow after an observer steps sideways. Keep the field fixed and watch the selected lit drops change.
What limit should I keep in mind?
A rainbow is not a painted stripe attached to a fixed set of raindrops. Changing viewpoint changes which drops send bright rainbow light to the observer.
Talk about it
- Before running the hidden experiment, explain your prediction.
- Compare the final picture with your prediction. Which visible change supports your explanation?
- What did the experiment keep fixed?
For grown-ups
Refraction and one internal reflection concentrate primary-rainbow light near 40–42 degrees from the antisolar direction. The hidden application changes the observer position; the model recomputes the lit ring within a fixed field of drops. A rainbow is not a painted stripe attached to a fixed set of raindrops. Changing viewpoint changes which drops send bright rainbow light to the observer.
Keep going
What else makes you wonder?
- If the rainbow is centered on your shadow, what happens to it on a really tall mountain or from a plane window?
- Why does the secondary rainbow that sometimes hangs above the bright one have its colors flipped?
- Could there be rainbows on other worlds, where the rain is made of something other than water?
