In recognition of Pride Month, which celebrates the LGBTQIA2S+ community, assistant professor Xiyue Zhang answered some questions about how rainbows form. Zhang studies clouds in polar regions.
How do rainbows form?
Two key ingredients to form rainbows are sunlight and suspended water droplets. Sunlight appears white and is composed of electromagnetic waves of different wavelengths. When sunlight enters a rain droplet, it gets refracted (bent) because of the density difference between air and water. Then it gets reflected at the back of the droplet, and finally refracted again as it exits the droplet. Light of different wavelengths bends at slightly different angles, separating sunlight into an array of colors as it leaves the water droplet and enters our eyes. This is why we only see rainbows when the sun is low and behind us, and most likely after rain when the air is extra moist (full of suspended water droplets but not clouds).
Why are some rainbows complete circles (in the sky), and others are semicircles (touching the ground)?
We almost always see rainbows as semi circles on the horizon because the ground prevents the other halves from forming. If you rise, say in a hot air balloon, the rainbow will grow fuller as there will be air carrying water droplets below you. You may have seen photographs of circular rainbows taken from aircraft when the viewer is much higher in the sky.
Can other liquids make rainbows?
As long as it’s transparent, liquids other than water can make rainbows as they can also refract and reflect light. The exact geometry of the sunlight, rainbow and the viewer will depend on the density of the liquid.
Can rainbows be seen from space?
Because 99% of water in the atmosphere is found in the troposphere, about thirteen kilometers or eight miles above the surface, outer space is very dry and lacks the key ingredient water droplets. Therefore, we aren’t able to see rainbows from space. If you’ve seen photos showing rainbow-like features taken from the International Space Station, they’re most likely showing glory instead of rainbows. Glory forms when sunlight is reflected but not refracted by water droplets.
Why are there double (and triple!) rainbows?
Sometimes, there is enough moisture higher up in the sky to allow sunlight to be reflected twice inside water droplets. This is why when we see a double rainbow, the secondary outer arc shows colors arranged upside down compared to the primary inner arc. More reflection also means longer paths for the light to reach our eyes, making the secondary rainbows much fainter than the primary rainbows. Theoretically, sunlight can undergo even more reflections inside water droplets to form triple rainbows. But if you are lucky enough to have seen a double rainbow, you might have noticed how dim the secondary rainbow appears. A third arc would be even dimmer and harder to witness.
About the professor
Xiyue Zhang is an assistant professor in the Department of Physics. Zhang studies climate dynamics and polar clouds, which have important radiative effects on the climate of Earth’s surface.
