The Green Flash at Sunset

For a fraction of a second, just as the sun fully disappears below a clear, unobstructed horizon, observers occasionally report seeing a brief flash of green light at the very top edge of the setting sun. This isn't an optical illusion or a trick of the eye adjusting to darkness -- it's a real, physically explainable phenomenon rooted in how Earth's atmosphere bends light. As sunlight travels through the atmosphere at a low angle near the horizon, it passes through a far greater thickness of air than sunlight arriving from directly overhead, and that extra atmosphere acts much like a prism, bending different wavelengths of light by slightly different amounts. Because the atmosphere refracts blue and green light more strongly than red and orange light, the sun's disk effectively separates into a stack of overlapping colored images, with the blue-green image positioned slightly higher than the red image at any given moment. Ordinarily, this separation is far too subtle to notice, and the effect is often further weakened because shorter wavelengths like blue and violet get scattered away by the atmosphere before they ever reach the observer's eye, a related but separate phenomenon from the refraction responsible for the green flash itself. Green light, however, scatters less than blue and refracts more than red, leaving it as the dominant remaining color at the very instant the red and orange portions of the sun's disk have already dropped below the horizon. This precise, narrow window is why the flash lasts only a second or two and why it requires a perfectly clear, distant horizon with no clouds, haze, or obstructions to be visible at all.