How far is the rainbow?~ Images by Jonathon Stone.  ©Jonathon Stone.

"My sister and I were driving alongside a thunderstorm a few miles south of Knoxville, Tennessee. The storm had just passed the interstate and we could see a faint portion of a rainbow off in the distance.  Then something cool happened; a rainbow began to form in the mist of the vehicles in front of us.  It soon turned into a full spectrum appearing and disappearing in the mist.

What was really interesting though, when the trees cleared we could see that the two rainbows were connected around the same point and actually one rainbow!   Though one seemed just ahead of us and one miles away!"

Atmospheric
Optics

About - Submit Optics Picture of the Day Galleries Previous Next Today Subscribe to Features on RSS Feed

It is impossible to say how far away is a rainbow because it has no distance, no size or indeed real existence. It is purely a collection of rays from glinting water drops that happen to be intercepted by your eye.

It is valid, however, to ask how far away are the water drops forming it. But even then there are difficulties.

A rainbow formed by raindrops a few miles away is exactly the same (provided the water drops have the same size) as one made by nearby spray. The rays forming the rainbow's edge all lie along the same cone extending from your eye with an axis pointing towards the antisolar point.

To judge the drop distance we need other visual cues. Sometimes the rain or spray is in front of trees or buildings.

Spray usually has smaller droplets and these give a wider more diffuse bow. That behind the truck is broader.

But distant rain can have small droplets too - notice the supernumeraries in the sky at right. Small raindrops produced them.

And if the rainbow has no distance, no size, we can never find those pots of gold at its ends.