Six Characters in Search of an Author has a familiar ring, but few, perhaps, recall that the play was written by the Italian dramatist and Nobel Laureate, Luigi Pirandello. It is a paradoxical exploration of the nature of illusion, a favourite theme of Pirandello's, and one on which he once had this to say: "We deceive ourselves continuously by the spontaneous creation of a reality which, from time to time, reveals itself to be vain and illusory." The "green flash", it seems, is such a case in point.
The green flash is a rarely observed momentary eruption of green light emanating from the upper edge of the setting sun. As rays of light pass through the atmosphere, they are bent or "refracted" a little, causing the image of the sun that we see near the horizon to be slightly higher than it ought to be. But the amount of refraction depends on the wavelength of light - so the "blue image" of the sun is displaced slightly higher in the sky than its "red image". In the absence of any other complication, therefore, the setting sun ought to have blue rim along its top.
But blue light is "filtered" very effectively by the molecules of air and dust, leaving, we always thought, the colour green as the shortest wavelength to survive the long trip through the atmosphere when the sun is near the horizon. Occasionally, a freak thermal structure of the atmosphere makes the air act like a giant magnifying glass, and this last image of the setting sun is momentarily enlarged to produce the spectacular green flash.
The flash came to popular attention in 1882 when Jules Verne's Le Rayon Vert or "The Green Ray", had the phenomenon as its central theme. It had come to scientific prominence two decades earlier when James Joule published an account of it in the Proceedings of the Man- chester Literary and Philosophi- cal Society. But now, nearly a century-and-a-half later, it seems that the green flash is not green at all, but merely looks like green: images from the proverbially unmendacious camera clearly show it to be yellow.
The explanation, apparently, lies in the fact that someone waiting to watch the flash will usually have been staring at the setting sun for several minutes. Since the setting sun is red, cells of the eye that detect the red are overloaded, and temporarily incapable of registering weak signals in this colour. And if you subtract the undetected red from a yellow flash, you are left with what everyone who has ever seen it thinks they see: a green flash.