Green Flash Phenomenon

Most of you have heard of the Green Flash Phenomenon via the movie 'Pirates of the Caribbean: At World's End' (2007).

Green Flash (and Green Ray) is the term given to rare meteorological optical phenomena that occur transiently around the moment of sunset or sunrise, when a green spot is visible for a short period of time above the sun, or a green ray shoots up from the sunset point. According to Pirate Lore, it was a flash that shoots up into the sky at the last glimpse of sunset, signaling the return of a soul from the world of the dead, or trying to get back from Davy Jones' Locker.

These phenomena usually are seen at an unobstructed horizon, such as over the ocean, but are possible over cloud tops and mountain tops as well. The green appearance usually lasts for no more than two seconds. Rarely, the green flash can resemble a green ray shooting up from the sunset or sunrise point.

Green flash occurs because the atmosphere causes the light from the Sun to separate, or refract, into different frequencies. Green flashes are enhanced by mirages, which increase refraction. A green flash is more likely to be seen in stable, clear air, when more of the light from the setting sun reaches the observer without being scattered. One might expect to see a blue flash, since blue light is refracted most of all and the blue component of the sun's light is therefore the last to disappear below the horizon, but the blue is preferentially scattered out of the line of sight, and the remaining light ends up appearing green.

In addition to atmospheric refraction and lensing, another primary cause of the Green Flash Phenomenon may be due to a phenomenon discovered by American researchers in 2014. They found that when two photons of infrared light collide with a human retina cell, they cause what the researchers call 'a double hit'. It creates the perception of bright neon green[1]. The researchers discovered this inadvertently while working with powerful infrared lasers - which should have been invisible to them - yet the researchers consistently kept seeing bright green flashes from the source of the laser. Because the Green Flash Phenomenon occurs exactly where we would expect the bulk of refracted infrared sunlight: just beyond red but not by much. The most likely hypothesis is that the Green Flash Phenomenon is refracted and magnified infrared light that 'double-hits' the retina or camera lens, creating the effect of bright green.

[1] Palczewska et al: Human infrared vision is triggered by two-photon chromophore isomerization in PNAS - 2014