At setting sun

When day is done,

While lanyards ye do lash.

Where sea meets sky

Keep watchful eye

For the Mystical Green Flash.

— Anonymous

Some say they have seen it, but most never have. Others say it does not exist, and yet others think they have seen it but really have not.

The green flash occurs in that fleeting, nearly subliminal instant just as the top of the sun dips below the horizon before the mind can really wrap around it.

It is real. Film and tape have captured it in various configurations. To see it the conditions must be right and the eye must be quick.

The green flash is a phenomenon caused by a combination of scattering and refraction of sunlight by Earth’s atmosphere. Refraction has the greater effect, but scattering enhances it by removing the shorter light waves to make the sun appear redder as it sets.

Sunlight is a combination of wavelengths of light in the color spectrum that we see in a rainbow. The grade-school mnemonic ROY G BIV (red, orange, yellow, green, blue, indigo, violet) are the colors of the rainbow according to our perception of color.

There are no boundaries between the colors. There is a continuous progression of wavelengths from 400 to 700 nanometers, the intensity of which is a bell curve with a maximum of 550 nm. This is in the center of the region that we perceive as green, to which our eye is most sensitive.

Sunlight is white until the atmosphere scatters it, removing the blue of the sky and leaving the yellow sun.

The atmosphere also refracts the light, bending it around Earth’s curvature such that we see the sun set when it is already below the horizon.

When the sun is at very low angles, the atmosphere acts like a prism. As with the glass in a prism, the atmosphere bends different colors by different amounts. It disperses the light, separating the colors and making a small green band above the upper limb of the sun and a red band below the lower limb.

As the sun sets, red sets first and green sets last as air bends the light of different colors over the horizon.

There is always a green flash of some duration and intensity as the top of the sun disappears under the horizon, leaving the green band for an instant. Under normal conditions, it is so small as to be barely noticeable, if at all.

A good green flash requires a mirage, the same thing that makes islands appear to float in the distance or oases to appear in the desert. A mirage, caused by a disturbance in the density structure of the atmosphere, spreads out the colors and makes the flash last longer.

Mirages often occur over water because the large heat capacity of water causes the temperature and density contrast with the overlying air that favors a mirage.

However, beware the “fool’s flash,” which is a greenish after-effect caused by a fatigued retina from looking too long at the bright-reddish setting sun.

Many seasoned observers have seen it only once or never, so do not be disappointed to be among them. Just keep watching.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.