Sunday, September 14, 2008

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.

8 comments:

Andrew Wales said...

Wordsworth was quite the killjoy. Of the growing trend of including illustrations in newspapers and magazines. The "intellectual elite" was disturbed by the growing propensity of illustrations in reading material and saw it as evidence of societal decay. Wordsworth wrote:

“Now prose and verse sunk into disrepute
Must lacquey a dumb Art that best can suit
The taste of this once-intellectual Land.
A backward movement surely have we here,
From manhood, -- back to childhood; for the age –
Avaunt this vile abuse of pictured page!
Must eyes be all in all, the tongue and ear?
Nothing? Heaven keep us from a lower stage!”
(Wordsworth, quoted in Heer & Worcester, 2004, page vii).

Murat Kayi said...

I haven't quite understood (or really grasped wether it was explained in your text at all, lol) why the rainbow is a bow, actually. I mean, I get the concept of breakin light up, but why isn't the rainbow a "rainbeam"?

Where does its form come from?

Does anyone know?

James Gurney said...

Andy, thanks for that quote. I love Wordsworth's poetry, but I had never heard of this opinion. What would he think of South Park?

Murat, the shape and location of the arc of the rainbow is defined in terms of the observer. All raindrops are sending light in all directions, but you only see the light from the ones that are 42 degrees from the antisolar point. Maybe someone else can explain it better.

Michael Chesley Johnson said...

I once saw, from a jet at 33,000 feet, a perfectly circular and perfectly complete rainbox below me. Awesome.

Andrew Wales said...

Hey, that picture of Jim Gurney with the rainbow reminds me of the song from Willie Wonka:

"Who can take a rainbow
Wrap it is a sigh
Soak it in the sun
and make a strawberry lemon pie?
The Candy Man can"

I think if Wordsworth were to see South Park he would promptly die of a coronary on the spot!

I used that quote in my master's thesis. I tried to trace the "anti-visual" sentiment that led to the "anti-comics" movement.

I would love to see a circular rainbow. As scientific as rainbows are, it is still something of a miracle to me when I see one.

Michelle Johnson said...

I too would love to see a circular rainbow. If I recall, all rainbows are technically circular, it's just that most are seen from the ground and the rest of the arc is blocked. It'd still be cool to see.

Andrew Wales said...

Many of the Bible prophets used the image of the rainbow to describe the appearance of God:

Ezekiel 1:28

Like the appearance of a rainbow in the clouds on a rainy day, so was the radiance around him. This was the appearance of the likeness of the glory of the LORD. When I saw it, I fell facedown, and I heard the voice of one speaking.

Revelation 4:3

And the one who sat there had the appearance of jasper and carnelian. A rainbow, resembling an emerald, encircled the throne.
Revelation 4:2-4

dimitris milionis non-blogs said...

The Rainbow Sphere seems to actually be round bubble a real all round ball of energy that center is below the ground and comes in several sizes one within the other, such violent spheres can cause burst of energy when in smaller sizes of 3-5 meters wide [this is very rare]. It seems we see it under such nice rainy conitions.


is it related to this, on a small human scale?

http://www.nasa.gov/home/hqnews/1998/98-075.txt