ASK TOM WHY: What causes double rainbows?

Dear Tom,

 

My grandson, Donovan, noticed double rainbows at about 5 pm on October 3 and we were wondering what causes this phenomenon.

 

RoseMary Jandes and Donovan, Downers Grove

 

Dear RoseMary and Donovan,

 

The beauty and awe-inspiring sight of double rainbows caught the attention of many readers that afternoon. The inner rainbow (the primary) is brighter and smaller. It forms when sunlight passes through raindrops and, after refraction and a single reflection within a raindrop, leaves the drop and returns to our eyes separated into the colors of the spectrum: red on top, followed by orange, yellow, green, blue, indigo and violet on the inside.

 

A wider, fainter rainbow (the secondary) forms outside the primary when some of the light is reflected twice within the raindrops before emerging, thereby reversing the sequence of colors.

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