A green flash can be observed at sunset or sunrise, although sunset sightings are most common. It's best seen from a place with an unobstructed view of the horizon. Green flashes result when the atmosphere reflects the sun's light, functioning as a prism and splitting the light into different colors. The short wavelengths of blue and violet light are scattered by various molecules in the atmosphere, leaving the green glow that appears for only a second or two. Learn more about the mysterious green flash with these videos.