For a long time, the green flash had the reputation of a sailor's tall tale, mostly because it asks for conditions most people never stand in: a distant, sharp horizon and unusually clean air. But the flash is textbook atmospheric optics, photographed thousands of times, and it lasts just long enough to distrust. As the last notch of sun slides below the horizon, its color can swing from orange to a startling emerald for one or two seconds—the famous flash—before the light vanishes. The explanation is a stack of three quiet effects: air bends sunlight like a lens, bends blue more than red, and finally the atmosphere's long low path deletes most of the blue on the way to your eye. What survives at the final moment, at the top rim of the sun, is green. It is not an afterimage from staring—properly, you never stare—and it is not the green of auroras, which come from oxygen atoms emitting their own light, the subject of why auroras lean on one oxygen line.
A prism made of sky
Refraction is the first ingredient. Air is slightly denser than empty space, and light moving through ever-denser layers near the horizon bends downward, which lifts the apparent position of the sun—sunsets are in a sense delayed by refraction, the disk appearing where it already isn't. The key extra is that the bending depends on wavelength: blue light refracts slightly more than red. So the atmosphere produces not one image of the sun but a faint stack of them, a red sun sitting lowest, a green-to-blue sun highest, overlapping in the middle. In everyday conditions the overlap hides everything; the rims are thin and the blue rim is missing entirely. But at the horizon, the stack gets vertically stretched—magnified, in effect—by the extreme refraction, and the rims separate enough to matter. The top edge of the last sliver is the green-to-blue rim. That is the raw material of the flash.
The horizon swallows the disks bottom-first; the topmost, most-bent image is the last light—and its top rim is blue-green.
Why green survives when blue cannot
Sharp readers will ask: if blue bends the most, why not a blue flash? Because the horizontal path through the atmosphere at sunset runs through hundreds of kilometers of air, and air molecules scatter short wavelengths overwhelmingly—this is the same Rayleigh scattering that makes the daytime sky blue and sunsets red, the family of effects that turns volcanic sunsets vivid. By the time light grazing the horizon reaches your eye, most violet and blue has been scattered out of the beam. The green band is the shortest wavelength that still arrives in force. The red end needs no scattering to vanish: it is bent least, sits lowest in the image stack, and slips below the horizon first. So the closing moments of a clean sunset run orange, yellow, pale green—and occasionally a last brilliant green.
Ozone adds a final editorial pass. Atmospheric-optics references note that ozone absorption removes a slice of orange light on long paths, cleaning up the transition and helping the green stand apart from the red rather than blending toward yellow. The green flash, in other words, is a group project: refraction separates, scattering deletes the blue, the horizon deletes the red, ozone deletes a bit of orange, and your eye receives a moment of almost pure green.
The mirage that turns a rim into a flash
Many green flashes need one more actor: a mirage. When the air near the sea surface is warmer or cooler than the air above it, light rays curve, and the sun's image distorts—flattening, doubling, or stacking into a shape like a leaning mushroom. An inferior mirage, over warm water, can compress the green top rim into a brief blister that detaches at the moment the sun disappears; this is the classic green flash of beach photographs. A superior mirage, over cold water, can hold a spectral green line above the horizon for a few seconds. The flash is short because everything involved is brief: the rims' separation, the mirage's geometry, and the sun's own apparent motion, half a degree of disk sinking in a couple of minutes.
Where the horizon matters: flashes demand a view whose edge is far away and sharp—the open sea, a great lake, a prairie, or from an aircraft over cloud decks. A hilly rooftop sunset ends with the sun touching land, not the true horizon, and the layered refraction never fully develops. Haze and pollution mute the green by scattering it too. The blue-green boundary at the heart of the rim—exactly the hue range where naming gets slippery—can be explored off-screen in Blue or Green?, where the label you assign wobbles long before the color itself changes.
How to actually see one
Conditions first: a clean, distant horizon; clear, haze-free air; and a stable atmosphere, ideally with a temperature layer near the surface—ocean coasts deliver this regularly. Timing second: the show is the final ten seconds, so watch the horizon, not the sun; glance, don't gaze, and never through binoculars or a telescope without proper solar filtration, because magnified sunlight damages the retina before pain can warn you. Some hunters let a camera with a safe solar filter do the staring and watch the screen. Purists chase flashes at sunrise too, where the geometry runs in reverse and the crowd is thinner.
Once you have seen one, the legend quality dissolves into something better: a small public demonstration that the sky is a lens, a filter, and occasionally a stage magician. The sun sets red-orange every evening; the atmosphere merely saves its one green trick for the viewers who stood in the right place with a horizon that goes all the way down.