The Science of Rainbows

Some poets and artists of the Romantic era were not entirely happy with Isaac Newton’s scientific analysis of the rainbow. The poets Wordsworth and Keats believed that Newton “destroyed all the poetry of the rainbow by reducing it to prismatic colors." “Unweaving the rainbow,” as they put it, reduced its power and meaning. (See the last post on the meaning of the rainbow).

But Newton also had admirers among artists, especially Turner, Overbeck, Runge, Palmer, and Constable, who did this plein air sketch in 1812 of what he called “this most beautiful phenomenon of light”

From ancient times, people speculated on how many strands of colored light went into the rainbow. Ancients argued for two, three, or four. Newton reasoned that there are three primaries, but seven hues, namely red, orange, yellow, green, blue, indigo, and violet.

The rainbow forms when rays of sunlight bounce around inside raindrops that hang suspended in the air after a storm. These raindrops reflect and refract light back toward the viewer at a 42 degree angle. As the light bounces inside the spherical water droplets, the component colors of light bend in varying amounts, and exit the droplet at slightly different angles, creating the colors of the bow.

The main rainbow forms at 42 degrees from the “antisolar point,” the point in the sky below the horizon) that is 180 degrees away from the sun. As the sun gets lower in the afternoon sky, the antisolar point raises up closer to the horizon, and more and more of the full circle of the rainbow is visible.

It doesn’t matter whether the droplets reflecting the sunlight are close to the viewer or far away. What matters is the angle, as anyone who has seen rainbows in a sprinker’s mist will know.

In this photo by Andy Goldsworthy, a man is creating a rainbow by slapping the water with a stick and sending up a stream of droplets into the air.

Next week we’ll look at the secondary rainbow and the mysterious dark region between the two bows.
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Diagram of raindrop from Answers.com
Rainbows in Art, link.
Constable sketch from Victoria and Albert Museum, link.