Rainbows: A Truly Spectacular Piece of Art

RainbowBy: Michael Watson, LLB

As we stepped out of the front door at 06.15 this morning, a few raindrops were starting to fall.  The entire sky was aglow with an orange-brown hue.  Turning to the west, we were startled by the dramatic sight of a bright, very colourful rainbow stretching over 100 degrees of sky.

We know that rainbows are caused by the reflection and refraction of light rays from the Sun through circular drops of water that are suspended in or falling through the atmosphere.  The required conditions are (i) early morning or late afternoon, so that the Sun is low in the sky – no more than about 30 degrees elevation above the horizon – (ii) an approaching or departing rainstorm, and (iii) a sharp demarcation between the storm and the sky conditions that precede or follow it, so that there is a clear path between the Sun and the water droplets in the air.

The photos show the invariable arrangement of the colours of the spectrum from green on the inside of the bow through red-yellow to violet on the outside.  The second close-up photo shows – unusually prominently – the fainter secondary bow, outside the primary bow.

Notice as well these things:

  1. The separation of the colours in the primary bow is quite sharp.  In many rainbows the colours are smudged together, with a gradual and indistinct gradation from green to violet.  This is caused by very small water droplets.  When the water droplets are large, the colours in the bow are much more sharply separated.  Because of the sharp colour division, we guessed that when the rainstorm hit on our drive to work, the raindrops would be quite large, and indeed they were – fat drops splattering onto the windshield.
  2. The sky is much brighter inside the bow than outside; this is always the case, but not usually so obviously.  In effect sunlight is “collected” from outside the bow and is concentrated into a band (the rainbow) that is exactly 42 degrees from the anti-solar point (the spot, below the horizon, that is 180 degrees away from the Sun).
The colours of the secondary bow are reversed from the primary bow, with violet on the inside and green on the outside.  This is because rays of light from the Sun undergo one additional reflection off the inside surface of water droplet before emerging into the observer’s eye.

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