The green flash happens when the atmosphere bends different colors of sunlight by slightly different amounts near the horizon. As the Sun disappears, the upper edge can briefly appear green. It is real, but it is fleeting and easiest to see over a distant, clear horizon.
Air bends and separates sunlight
Earth’s atmosphere is denser near the surface, so it refracts light. Because the refractive effect varies by wavelength, colors from the Sun’s image are displaced slightly from one another. Near sunset, this can leave a green rim visible at the upper edge for a short time. [1]
Why it is rare
The Sun is bright and the color separation is small. Haze and clouds can hide the thin green edge, while a clear, distant horizon gives the light a longer path through the atmosphere. Some green flashes also involve mirage conditions, so the visible forms are not all identical.
Do not stare at the Sun
The flash occurs while the Sun is low, but direct viewing can still harm the eyes. Never use binoculars, a telescope or a camera viewfinder to look at the Sun without suitable solar filters.
Is the green flash an illusion?
No. It is a real atmospheric optical effect, although some reported flashes can be influenced by visual effects or mirages.
Sources and evidence
- San Diego State University — Explaining green flashes ↗
Explains atmospheric dispersion and different forms of green flash.
- UCSB ScienceLine — The green flash ↗
Describes refraction and dispersion as the common explanation.
Sources consulted: 11 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: Initial publication; explanation checked against linked institutional and scientific sources.