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Why do Rainbows Form?

Mary McMahon
Updated May 21, 2024
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Rainbows are a striking and beautiful meteorological phenomenon caused by the refraction of light. In order for rainbows to form, the sun must be near the horizon, and there must be a heavy fog, mist, or rain opposite the sun. An observer standing between the sun and the rainbow would see a 180 degree arc of colors which run through the visible spectrum from red to violet. In some cases, if conditions are right, double rainbows will form, with a pale rainbow in reversed colors above the primary rainbow, and in other instances, a fully circular rainbow has been observed, usually from inside an aircraft flying over the Earth.

Although rainbows have been observed, written about, and depicted in art for centuries, their cause was not understood until physics began to explore the properties of light. Essentially, the cause of a rainbow is the reflection of sunlight through individual drops of water. Light enters the water and reflects from the opposite side of the water drop, bouncing back through the point of entry. Because the angle or refraction of the light changes as it bounces back, it filters the light into different colors, which arrange themselves with red at the top and violet at the bottom because red is the longest wave length, and violet is the shortest.

When double rainbows form, the light refracts twice, causing a fainter rainbow to mirror above the primary one. Potentially, a triple rainbow could form as well, although it is extraordinarily rare. As soon as the water dissipates enough or the sun moves, the rainbow will disappear, because conditions are no longer optimal for the refraction of the light through the drops. It is also possible for a moonbow to form, although because humans have difficulty distinguishing color in the dark, it usually appears in shades of white and gray.

People seeking rainbows should go outside when the sun is near the horizon and there has been a heavy rain, or there is an active storm. If the observer turns his or her back to the sun and looks in the direction which is immediately opposite the sun, he or she may see rainbows if the conditions are right, with the apex of the arc located directly opposite the sun. In some instances, light conditions are right for rainbows to form in close proximity to an observer: otherwise, the rainbow will continue to look as though it is moving away from the pursuer until it finally disappears altogether, because the observer has actually passed through it.

Frequently Asked Questions

What causes a rainbow to form?

Rainbows form due to the refraction, dispersion, and reflection of sunlight in water droplets. As sunlight enters a droplet, it bends (refraction) and spreads out into a spectrum of colors (dispersion). Some light reflects off the inside surface of the droplet and bends again as it exits, resulting in the colorful arc we see.

Why do rainbows have multiple colors?

The multiple colors in a rainbow are due to the dispersion of sunlight as it passes through water droplets. Sunlight is composed of various colors, each with a different wavelength. When sunlight enters a droplet, these wavelengths bend at slightly different angles, separating into a spectrum of colors ranging from red to violet.

Can rainbows form at night?

Yes, rainbows can form at night and are known as moonbows or lunar rainbows. They occur when moonlight, which is reflected sunlight, encounters moisture in the air. Moonbows are often fainter than solar rainbows because moonlight is much weaker, but they can be seen when the moon is bright and the sky is dark.

Is it possible to reach the end of a rainbow?

No, it is not possible to reach the end of a rainbow. Rainbows are optical phenomena that depend on the observer's perspective and the position of the sun. As you move, the rainbow will appear to move as well, always maintaining the same distance from the viewer, making it impossible to reach its end.

Why do rainbows form a semicircular shape?

Rainbows form a semicircular shape because of the way light is refracted and reflected inside water droplets. The angle at which the light is reflected back to the observer is approximately 42 degrees for red light and 40 degrees for violet light, creating a circular arc centered opposite the sun. The ground limits the visibility, so we typically see a semi-circle.

Can rainbows be full circles?

Yes, rainbows can be full circles, although they are rarely observed as such from the ground. When viewed from a high vantage point, like an airplane, with the right conditions of sunlight and moisture, a rainbow can appear as a complete circle. This is because the observer can see the full extent of the raindrop's light circle without the ground obstructing the view.

All Things Nature is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a All Things Nature researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments

By anon245979 — On Feb 07, 2012

@anon134672: You should make one with a mirror and water. Look it up!

By anon134872 — On Dec 16, 2010

i am doing a science project and my question is how rainbows form and for my final project i have to make a rainbow out of glass. what should i do?

By Kaylaflocka Gill — On Nov 30, 2010

I'm sorry but this was not helpful.

By anon25958 — On Feb 05, 2009

i love this site it is so so so so so so so so so so so helpful!!!!!!!!!!!! :):):)

Mary McMahon

Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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