Chasing Rainbows

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Chasing Rainbows = PRICELESS

Rainbows have not only inspired poets through the centuries. They have also fostered legends and beliefs: that pots of gold can be found at the ends ... that rainbows at night mean good sailing, rainbows in morning mean dangerous conditions for sailing ... that a very greenish rainbow means continuation of rain. 

Despite the fallaciousness of such beliefs, everyone would surely agree with William Wordsworth, who wrote: My heart leaps up when I behold a rainbow in the sky.

Marvelous, yes, and mysterious, rainbows are truly one of nature's most splendorous creations. Elusive. Fleeting.  And surprisingly varied: sometimes double, sometimes partial, sometimes completely semicircular. 

But if you look carefully at rainbows, you'll discover some things that are invariable. For example: the color red always appears at the top of the primary arc, and blue at the bottom.   Why? Because rainbows result from splitting sunlight into its component colors when it passes through a water droplet. And the longer wavelengths, red and yellow, get bent less than the shorter wavelengths, blue and violet. 

From the ground, you see only the lower half of the reflected component colors. So you see only the effect of an arc, not the whole circle. You see the refracted light in millions of droplets, each at a different distance above the ground. The bow results from the cooperative effort of all the droplets, which refract for an instant, in a constantly shifting mix.  Since the sun's angle remains constant, the arc remains constant too, while it lasts. 

Where do you see rainbows? Where the sun is behind you, shining into droplets in clouds or mist in front of you. The sun's altitude determines how much of the arc is visible.  In early morning or late day, the rainbows appear as half circles. The radius of the primary arc will always be 42 degrees. If the sun is higher, you'll see less of the arc; at midday, you won't see any. 

Another curious thing about rainbows: they can have either one arc ... or more. If there's a single reflection inside the droplet, the result is a primary arc. If the reflection bounces back, you'll see a secondary rainbow-with colors in reverse order: red on bottom, blue on top. That's because the droplet's interior acts as a mirror, reversing the refraction and scattering. 

Several more points: the bigger the droplets, the more scattering of color and the wider the band. Very tiny droplets produce only thin, nearly white bows. There are fog bows ... dew bows ...and steam bows (thank you Charles for the input).

Look and admire.

Je sais les cieux crevant en éclairs, et les trombes

Et les ressacs et les courants :  je sais le soir,

L'Aube exaltée ainsi qu'un peuple de colombes,

Et j'ai vu quelque fois ce que l'homme a cru voir !

I know the lightning-opened skies, waterspouts,

Eddies and surfs; I know the night,

And dawn arisen like a colony of doves,

And sometimes I have seen what men have thought they saw!

-Arthur Rimbaud, Le Bateau Ivre