A couple of questions about rainbows....?

[cedorsmulas]

1) How and why are double-rainbows formed?

2) Why are the colours reversed on the second rainbow?
Why do secondary rainbows occur SOMETIMES, but not at other times?? Why does secondary refraction only occur occasionally? What conditions make a secondary rainbow more likely ?

[kumorifox]

The secondary rainbow forms when light is reflected inside the raindrops. The primary rainbow forms from refraction and exiting the drop. The secondary rainbow forms when light is reflected back into the drop again before exiting. The second reflection inverts the colours, but not the direction of the arc.

[Southpaw]

A rainbow is formed when sunlight is refracted and reflected from raindrops. Since sunlight (white light) is composed of 7 different colors of different wavelengths, when the sunlight is refracted by the raindrop, the light bends but since each light has a different wavelength, each light bends by a different amount. As a result the 7 colors are seperated and we see a rainbow.

Primary rainbows involve the sunlight undergoing two refractions and one reflection while Double rainbows (Secondary Rainbows) involve the sunlight undergoing two refractions and two reflections. Because the light undergoes one more reflection than in a normal rainbow, the colors appear inverted. Also a Double rainbow is fainter than a primary rainbow because during each reflection, some light is lost each time it hits the edge of the drop.

[adaviel]

I believe secondary bows always form, but are usually too dim to see. You need really good conditions to see them, with a bright primary bow.

[Prometheus]

I googles "what is a double rainbow" and got this.

on twitter.
Find out how one of these rare rainbows occurs!
It is not uncommon to see a rainbow behind or ahead of a rain storm, but have you ever seen a double rainbow?

According to AccuWeather.com meteorologists, a ray of sunlight passes through a raindrop, reflecting off the back of the drop at varying angles.

Along with this reflection is refraction of light that causes of a spectrum of colors-- red, orange, yellow, green, blue, indigo and violet.

Certain angles and "bending" reflect light better for refraction to occur, and the amount of light refraction corresponds to wavelength and color.

For example, blue light is always refracted at a deeper angle than red light. This is the reason blue is found at the inside of the bow and red on the very outside.



Here you can see three rainbows, the double in the sky and the primary reflected on the AccuWeather.com Head Quarters in State College, Pa., in January 2008. Image courtesy of AccuWeather.com Meteorologist Jesse Ferrell.

Nature's natural color spectrum always elicits the same pattern (red, orange, yellow, green, blue, indigo, violet) when light is refracted, commonly known from the Roy G. Biv mnemonic.

While a primary rainbow is visible when light is reflected once off the back of a raindrop, a secondary and usually dimmer rainbow is spotted when light is reflected twice in a more complicated pattern.

The colors of the second rainbow are inverted, with blue on the outside and red moved to the inside. The second bow appears dimmer or cloudier because much more light is released from two reflections, and both bows cover a larger portion of the sky.

It is rare and unlikely, but three or even four rainbows can be seen on occasion, but only if they are reflected off of the earthly objects.

The best time to see a rainbow is in the early morning or late afternoon, when the sun is lower in the sky. When the sun is in a lower position, a higher bow can be seen.

Rainbows are actually a full circle, but we only see the top bow because the bottom half is below the ground.

Many rain droplets of all different sizes, not just one, are responsible for this phenomena. Perhaps billions of water droplets and sunlight reflections make a rainbow visible to the human eye.



A double rainbow in Lorain, Ohio, on May 8, 2010. Image courtesy of AccuWeather.com User Hanna1.

Content contributed by AccuWeather.com Meteorologists Henry Margusity and Jim Andrews.

Related to the Story:

What is the Best Time of Day For a Rainbow?

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