At sunset, the Sun’s last edge can appear to light up green for a second or two. This “green flash” is not a new kind of light emitted by the star: it is a brief effect caused by the way the atmosphere bends and disperses its light, made visible by a clear horizon and favourable conditions.
Why does the Sun look different near the horizon?
White sunlight is made up of several colours, each associated with a different wavelength. As it passes through the atmosphere, it does not travel in a perfectly straight line: it is refracted, or bent. The amount of bending varies slightly according to wavelength. This is called dispersion.
When the Sun is low, its light passes through a greater thickness of atmosphere than when it is high in the sky. This geometry makes the effects of refraction and dispersion more noticeable near the horizon. It does not mean that the air becomes denser because the path is longer: the light travels a greater distance through the atmosphere.
The atmosphere then acts a little like a prism, but the comparison has its limits. A prism clearly separates colours by sending them in different directions. In the atmosphere, dispersion is weak: the coloured images of the Sun remain very close together. The effect is therefore seen at the edge of its disc and lasts only a very short time, rather than taking the form of a clearly separated rainbow.
Green is not a new kind of light from the Sun
As sunset approaches, refraction shifts the apparent image of the Sun slightly. Because the bending depends on colour, the edges of its disc do not line up perfectly. In certain conditions, the last visible segment can take on a green tint. The “green flash” thus describes an appearance produced by the path light takes through the atmosphere, not a particular green emission from the star.
The colour observed also depends on what happens to the light before it reaches the eye. Colours do not all pass through the atmosphere in the same way: blue components, in particular, are scattered more. What we see is therefore linked to refraction, dispersion and visibility conditions, not to a simple splitting of light into perfectly distinct bands.
The expression can refer to several visual forms: a coloured rim on the Sun’s upper edge or a brief, distinct glow. It does not always mean that an isolated ray appears to shoot up above the disc. The description depends on the form the phenomenon takes and on what the observer can distinguish at the horizon.
A real phenomenon, but one that depends on observation
A distant, unobstructed horizon makes the phenomenon easier to observe, as it allows the Sun to remain visible until its last apparent edge. Low clouds, fog or turbulent air can obscure or blur the effect. The sea often provides an open horizon, but the phenomenon is not confined to the coast: it can also be observed over flat land, if visibility and atmospheric conditions are favourable.
Air stability also matters. Layers of air at different temperatures can alter the path of light and sometimes produce distorted or overlapping images of the Sun. For example, an image published by NASA shows a sunset over San Francisco Bay with a double-image effect attributed to warm air near the Golden Gate Bridge. This observation illustrates how local conditions can complicate the apparent shape of the disc; it does not, on its own, describe all green flashes.
So do not expect the same display at every sunset. The colour may be subtle, the effect very brief, or nothing visible may happen. A photograph or video can make the phenomenon easier to examine, but it is no substitute for a physical explanation: the result also depends on how the image was captured and processed.
Observe without looking directly at the Sun
The brevity of the green flash is no reason to stare at the Sun: looking directly at it can damage your eyes, even at sunset. Ordinary sunglasses do not protect your eyes when viewing the Sun. Never look at the Sun through binoculars, a camera or a telescope without a suitable solar filter placed in front of the instrument.
The green flash is thus a small demonstration of the difference between an object and its image. The Sun does not suddenly change colour: it is the light that reaches us after passing through the atmosphere that can create this impression. The observation is brief, but the mechanism links the precise properties of light to the geometry of the horizon and the state of the air.
References
- For centuries, sailors have said they saw this strange phenomenon at sea: they were right! — Futura
- APOD: 2015 December 2 - Golden Gate Sunset: Green Flash — NASA Science
Photograph: Vlad Podvorny — Wikimedia Commons — CC BY 2.0.
