A SMALL EXPERIMENT IN WONDER
Double rainbow simulator
Explore primary and secondary rainbows. Change the sun angle and compare the reversed colours of a double rainbow in this interactive simulator.
Drag the picture to place the rainbow. You can also focus the picture and use the arrow keys.
A playful overlay. Move the horizon to hide the effect below it; trees and buildings above the line aren’t detected.
A LITTLE SOMETHING TO NOTICE
Primary and secondary rainbows
The second rainbow comes from light reflecting twice inside a drop. That extra bounce reverses its colours—and leaves it a little fainter.
Try this rainbow experiment
A double rainbow contains two different paths through water droplets. Light forming the primary bow reflects once inside a drop. Light forming the secondary reflects twice before returning towards the observer. The extra reflection changes the direction of the outgoing light and produces a larger, fainter bow with its colours reversed.
This page starts with both bows enabled in the Summer shower preset. Look for red on the outside of the primary and on the inside of the secondary. Switch the second bow off and on to compare their positions. Both are centred on the antisolar point, the direction directly opposite the Sun, rather than on a fixed spot in the landscape.
Next, raise Sun elevation slowly. As the Sun rises behind you, the centre of the rainbow moves further below the horizon in front of you. Around 42 degrees, the primary disappears below this level ground horizon. The wider secondary can remain above it until roughly 51–53 degrees, depending on colour. Lower the Sun to bring the arches back.
Try changing droplet size while keeping the Sun low. The fine bands just inside the primary are supernumeraries, not additional secondary rainbows: they come from interference between light waves. This simulator approximates spherical-drop scattering and uses an illustrated landscape. An observer above the rain may see parts of a bow below their horizon, a viewpoint this ground-level scene does not model.
FOLLOW THE LIGHT
How rainbows form: experiments to try
Sunlight bends as it enters and leaves a water droplet, and reflects inside it. Different colours bend by different amounts. One internal reflection produces the primary rainbow; two produce the fainter secondary with reversed colours. Light waves also interfere, creating fine extra bands when conditions are right.
For teachers: suggested for grades 5-8, with a 10-minute activity. Predict what will happen when the Sun rises, then test it. Compare two droplet sizes while holding other settings fixed, and describe how the bands change. Finish by explaining the reversed colours of the second bow. Use Share this sky to save the settings or copy an embed for your lesson page.
Rainbow simulator questions
Why is a rainbow about 42 degrees from the antisolar point?
Refraction and one internal reflection concentrate sunlight near a particular outgoing angle. The primary bow lies roughly 40-42 degrees from the direction opposite the Sun, depending on colour. The angle describes the bow's radius, not the Sun?s height.
Why does the rainbow disappear when the Sun is high?
As the Sun rises, the antisolar point sinks below the horizon. Above about 42 degrees of solar elevation, the primary bow falls below a level ground horizon. The larger secondary may remain visible until roughly 51-53 degrees. From above the rain, an observer can see parts of a bow below their horizon.
Why are the colours of the second rainbow reversed?
The secondary rainbow comes from two internal reflections in each drop instead of one. This changes the outgoing directions, reversing the colour order: red lies on the inner edge of the secondary and the outer edge of the primary.
What makes a fogbow look white?
Tiny fog droplets spread the light over a wider range of angles. The colours overlap strongly, making a broad, pale bow with possible faint colour at its edges. Choose Morning fog to explore this approximate effect.
What are supernumerary rainbows?
Supernumeraries are fine extra bands caused by interference between light waves taking different paths through droplets. They are easiest to see when drops have similar sizes. Choose Extra bands, then increase Size variation to see the fringes soften.
Does raindrop size change a rainbow?
Yes. Smaller droplets broaden the bow and its wave features; larger spherical drops make narrower, more closely spaced features. A mixture of drop sizes smooths fine bands. This model approximates spherical drops, so very small fog droplets and large flattened raindrops have limits.