What causes a green flash?

A green flash is a rare optical phenomenon that occurs just before sunrise or just after sunset. It happens when the sun is just below the horizon, causing the light to refract through the Earth's atmosphere, separating its colors. Due to the varying wavelengths of light, the shorter blue and green wavelengths can be briefly visible, creating a flash of green. This effect is most commonly observed in clear, unobstructed horizons over the ocean.