As the sun slips below the horizon the top edge of it briefly 'flashes' green. You quickly look at your drink - you don't remember ordering absinthe - but rest assured, the chances are you have been lucky enough to see the elusive 'green flash'
What causes it?
As light passes from the vacuum of space into the atmosphere, which acts like a prism, it slows down by 0.03%. This causes the light to bend or refract towards the surface of the earth. The white from the sun is made up of many different colours of light, all of which have a different wavelength. The wavelength (or colour) of light affects how much it is refracted on entering the atmosphere, with red light refracted the most and blue least (as in rainbows).
Imagine the image of the sun as being made up of red, green and blue images. Light from the 'red image' will be refracted more than that from the green and blue. So, the 'red image' will appear lower than the green, which will similarly appear lower than the blue. At sunset, or sunrise, this effect is intensified as light travels through a slightly thicker atmosphere. As the sun disappears below the horizon, the 'red image' will disappear first and the blue last.
The atmosphere causes blue light to be scattered more than red or green - the reason why the sky appears blue - so light from the 'green image' - the 'green flash' - will normally be the last thing you see as the sun disappears below the horizon.
On very rare occasions, the atmosphere may be clear enough to allow some of the blue light to reach us and cause a 'blue flash' as the sun sets.
Why don't you see a green flash every time the sun sets?
The phenomenon lasts only a fraction of a second, so unless you know where to look and when, the chances of seeing one are very slim indeed. Viewing conditions need to be just right too.
Optimal viewing conditions
Watching the sun set over an ocean horizon on a clear evening will be a good start, as you will have an uninterrupted view through clear unpolluted air. Your line of sight should be almost parallel to the horizon and you need to really concentrate at the top edge of the sun as it is about 98% set. If you are lucky, you will see the top edge of the sun turn green for a brief moment, before disappearing below the horizon.
In one of its guides, the National Trust recommends looking for the green flash from Zennor Head in south west Cornwall, probably because atmospheric conditions are likely to be better here than in other areas in the UK.
"I have looked for it during quite a few sunsets but have only seen it once!"
Former BBC Broadcast Meteorologist Byron Chalcraft said "I have looked for it during quite a few sunsets but have only seen it once! The sun was setting over the sea on a nice clear evening in Cornwall and immediately after the top of the sun's disc went below the horizon there was a brief, bright green flash." Colleague Peter Gibbs hasn't been so lucky "I've looked long and hard at many a sunset, but never caught a glimpse!".
Take Care!
Even with the sun low in the sky, concentrated observation with the naked eye can still cause damage to your eyesight, so it is important to take precautions and watch through smoked or filtered glass.
