A Double Rainbow

by Tammy
(Barrie, Ont.)

A double rainbow

A double rainbow

October 18th at 6:15 pm in Barrie

a beautiful double rainbow appeared and gone within 15 minutes.

Barry's Response - Thanks, Tammy. Rainbows come in a lot of different variations. Wikipedia has them listed.

You've captured two distinct rainbows, where you can see the full spectrum in each. They'll also notice that the colors appear in reverse order in the second rainbow. Why? The pattern flips because of two reflections inside the drop. Sometimes you can see a third arc. The sky between the rainbows is darker than the rest.

How does a rainbow form? The refraction effect. We get light directly from the sun, and it's mixed with all the colors we know to give it its yellow hue. Transparent liquids or solids let light through, but at a slower speed than air. In addition to slowing down (a little), it bends when it enters an object, like a prism or water droplet.

You'll get used to it. The different colours, wavelengths actually, bend at different angles and come apart to form the rainbow pattern, like on the old Pink Floyd album.

The light enters sphere-shaped water droplets, like those floating in the air after a rainstorm, and then reflects off the inner backside of the sphere. At one end of the spectrum, red is bent 42 degrees, and violet is bent 40 degrees.

Meteorology and optics. They make great partners.

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If we viewed a double rainbow with a keen analytical mind and a deep understanding of its underlying physical principles, what might we discover?


Maybe like a physicist who develops technology and was curious about everything. To explain how a double rainbow forms, one might explain how a single rainbow forms:

- When sunlight enters a raindrop, it slows down and changes direction because of the change in refractive index (from air to water).

- Raindrops reflect light off their inner surfaces.

- In the transition back to air, the light undergoes further refraction as it exits the raindrop. The varying indices of refraction for different wavelengths also cause the light to disperse into its component colors (red, orange, yellow, green, blue, indigo, and violet).

- When light rays strike the inside surface of a raindrop at a certain angle, total internal reflection occurs, reflecting the light back inside.

- Eventually, the light exits the raindrop, forming a circular rainbow with red on the outer edge and violet on the inner edge.

Here's how we explain the double rainbow formation:

- The secondary rainbow happens when the light reflects twice inside the raindrop before it leaves. Due to this double reflection, violet is on the outer edge and red is on the inner edge.

- A secondary rainbow appears at a larger angle (approximately 50-53 degrees) than the primary rainbow (about 42 degrees).

Double rainbows have angles and colors due to mathematical and optical intricacies. Due to his scientific background, an expert in this field could appreciate the phenomenon both theoretically and empirically, showcasing the beauty and elegance of nature.

Comments for A Double Rainbow

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by: Jeff Jo

Total Internal Reflection is impossible in raindrops. The angle of incidence at the back of the drop is equal to the angle of refraction as it entered, because the path taken inside the drop forms an isosceles triangle with the surface normals. This angle is, by definition, less than the critical angle.

Each color is reflected everywhere within a 40° to 42° wide cone that originates on your shadow, like the beam of a wide flashlight. But it is much brighter in the outer 0.5°. Since this bright band occurs at different angles for different colors, we see colored bands.

But they aren't pure colors. Each band, after red, contains less-bright light of all colors toward the start of ROYGBIV. Inside 40°, the rainbow continues to the horizon as white. Yes, white is in the rainbow.

The secondary is not reversed, it is upside-down. The "beam" is about 130° wide and centered on the sun. Since this is more than 90°, it wraps around the top of the sky and is seen upside down, 10° above the primary. The sky is darker between the primary and secondary because the "white" is below the primary, but above the secondary.

From Barry - Let's shine a little light on this, now. Scientists like Albert Einstein and other physicists have developed principles of optics and physics that explain these concepts better.

I'm not sure if everyone would agree with the explanation given in the comment about Total Internal Reflection in raindrops. Rainbows are formed partly by Total Internal Reflection in raindrops. Before light leaves a raindrop, it goes through multiple reflections and refractions. Rainbows form when light disperses.

Things to think about: When sunlight enters and exits raindrops, it disperses into its constituent colors. Different colors bend differently as they pass through the drop, causing the rainbow to separate.

Secondary Rainbow: The explanation for the secondary rainbow may also confuse some readers. Raindrops reflect and refract light internally, resulting in the secondary rainbow, but it's not centered on the sun and doesn't wrap around the sky. A secondary rainbow usually has a wider angular spread and is fainter than a primary rainbow.

There is no white color in a rainbow. A rainbow is a spectrum of colors caused by the dispersion of light, not a single color. Occasionally, there may be a gap or a fainter region between the primary and secondary rainbows, which makes them appear less colorful.

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