Friday, 29 August 2008

Paint the whole world with a Rainbow

When you have lots of sunshine and showers around like we’ve had recently (well OK we’ve had more showers than sunshine) then it’s ideal conditions for rainbows. We see rainbows when the sun shines through the raindrops. More precisely the sunlight is refracted in the raindrop and is split into the different colours that make up the sunlight – you might remember doing something similar in the school science lesson when you shine a bright light through a prism (a glass pyramid). The refracted light is then reflected off the back of the raindrop at an angle of around 42o, which defines the angle in the sky that we see the rainbow. The blue light is a shorter wavelength and so is refracted at a bigger angle than the longer wavelength red light, which means that in the bow you see the red at the top and the blue near the bottom.

Sometimes you can see a secondary rainbow and I’ve included a picture above I took of a primary and secondary rainbow this year in Cornwall. The secondary rainbow occurs when the light undergoes a double reflection in the raindrop. Because this is a second reflection the colours occur upside down compared to the primary rainbow, and they are dimmer. We call the area in between the two bows Alexander’s band after the ancient Greek Alexander of Aphrodisias who wrote about it. It is possible on very rare occasions to see a third bow, but as by this stage the light is very dim and it appears in the direction of the Sun it is extremely difficult to spot.

There are a few different mnemonics that help you remember the seven colours of the rainbow, but my favourite is ‘Richard Of York Gave Battle In Vain’ – that’s Red, Orange, Yellow, Green, Blue, Indigo, Violet.

Rainbows have been talked about for many years. The ancient Greeks wrote about rainbows as a path made by Iris (the messenger of the Gods) between heaven and earth. Chinese mythology speaks of a slit in the sky sealed by the Goddess Nüwa using stones of five different colours. The Bible in the story of Noah talks about the rainbow of a sign from God that life would never again be destroyed by floods. But perhaps the most famous is that the Leprechauns keep their pot of gold at the end of the rainbow. Unfortunately you will never reach the end of the rainbow for two reasons. The first is that because it’s an optical effect then it moves as you move and so you can never reach the bottom. Secondly, and perhaps more importantly, a rainbow is really a circle, it’s just that we see half of it.

There are two things that stick in my mind most about rainbows. The first is the fantastic painting by Constable of Salisbury Cathedral from the Meadows. I first saw it in the National Gallery, and read about the painting of it in a great book by John Thornes (called John Counstable’s Skies, which I can definitely recommend to you). My second abiding memory is from the Wordsworth poetry I studied at school. His 1802 poem "My Heart Leaps Up When I Behold The Rainbow" begins:

My heart leaps up when I behold
A rainbow in the sky:
So was it when my life began;
So is it now I am a man;
So be it when I shall grow old,
Or let me die!…

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