green flashPhotos of green flash taken in Puerto Escondido.

Sunset’s green flash: fact or fiction?

For decades, Topic Number One among Puerto Escondido’s sunset aficionados has been the elusive Green Flash. Now cynic, that I am, my theory was that these alcohol and substance-infused individuals, after gazing intently at that brilliant red orb as it slipped gently into the ocean, were experiencing an optical illusion, as a negative image — the color green — was emblazoned onto the retina.

Not so, says my science consultant, the reclusive Dr. K. as he now explains: When the sun sets, sometimes the last bit of light from the top of the solar disc itself is an emerald green colour. This phenomenon is known as the “green flash” or “green ray.” It is not an optical illusion, nor does it fill the sky with green light. Usually, the effect is very subtle, but occasionally the result is intense.

The green flash is quite common and will be visible any time the sun is rising or setting on a clear, unobstructed and low horizon. From Puerto, that means the horizon must be exceptionally clear at sunset with no apparent clouds or haze. The flash typically lasts one or two seconds. It helps to look away until the last possible instant to avoid over-saturating your vision.

What makes the Green Flash The explanation for the green flash involves refraction, scattering, and absorption. First, the most important of these processes is refraction. Sunlight is bent by the earth’s atmosphere resulting in our image of the sun on the horizon appearing roughly a solar diameter *above* its actual position. That is, if there was no atmosphere, the solar disc would already have disappeared below the horizon.

Refracting white light, combining all the colours, through any clear substance such as water, glass, or air, causes the different light wavelengths to be refracted at different angles, similar to a spectrum from a prism or rainbow.

Blue light with shortest wavelengths is refracted the most. Red light with longest wavelengths is refracted, or bent, the least. Thus, the sun’s disc in red light would appear to be lower in the sky than the solar disc in blue light. The blue disc is actually the last to set after green.

The blue light is “Rayleigh-scattered” away. (Rayleigh was a British physicist.) The size of air’s gas atoms causes blue wavelength light to be scattered more than the other colours. This effect makes the sky blue. Air has a weak yellow optical absorption band. When the sun is overhead, this absorption isn’t significant, but once near the horizon, the solar light travels through approximately 38 times more air, so even a weak absorption becomes substantial. A setting sun loses most its bright yellow colour.

Summarising, a simple explanation for the green flash is: Refraction separates the visible solar images by colour (spectrum). At just the right instant, the red/orange image of the sun sets, The yellow image is absorbed and . . . The blue image is scattered away. We are left to view the remaining, and sometimes brilliant, green solar crescent.

—Warren Sharpe