A green flash is a faint green glow or brief green spot sometimes visible as the Sun disappears below the horizon. It occurs because Earth’s atmosphere refracts sunlight by wavelength, separating the colors slightly as the light travels through a long path near the horizon.
The effect is easiest to see from an unobstructed horizon, especially over the ocean, cloud tops, or mountain tops. Clear, stable, layered air and mirage conditions can magnify the final green edge of the Sun. The visible flash usually lasts no more than one or two seconds, although rare atmospheric configurations can produce longer or repeated flashes.
A green flash is often confused with the broader “green ray,” in which a green beam appears to rise from the horizon. Blue light is refracted more strongly, but atmospheric scattering usually removes much of it from the observer’s view, leaving green as the most noticeable final color. Jules Verne’s 1882 novel The Green Ray helped popularize the phenomenon.