What causes the green flash that can appear at sunrise or sunset?

The green flash is a brief, colorful visual rarity caused by the atmospheric refraction of sunlight when the sun is very close to the horizon.

How does it work?

When the Sun is very low, its light must pass through a greater amount of Earth's atmosphere. The different wavelengths of light bend (refraction) in slightly different ways, acting like a small prism. Red, orange and yellow light bend upward more strongly and remain visible even after the solar disk disappears. Green light, with a somewhat shorter wavelength, bends just above the horizon, creating a separate, bright flash right after sunset or just before sunrise.

• Refraction bends light as it passes from a less dense medium into a denser one (the lower atmosphere).
<li The dispersion separates the colors, just as a raindrop does with rainbow light.
• Extinction (light scattering) removes the shorter wavelengths (blue, violet), leaving green as the last visible color.

Conditions needed to observe it

The green flash is usually visible under the following conditions:

• A clear, unobstructed sea or mountain horizon that is not clouded over.
• A uniform atmospheric temperature that creates a stable gradient (the mirage effect).
• The Sun's surface low enough that the main disk has disappeared but there is still enough light for the green to be perceptible.
• The observer's eye must be properly focused (without the diffraction effect of a solar lens) and the observer should look just above the horizon, preferably with a clear field of view.

Why it is often green

Sunlight contains a continuous spectrum of colors. At the edge of the solar disk, red light bends upward more strongly than green, which in turn bends more than blue. Since the shorter wavelengths (blue and violet) scatter quickly in the atmosphere, they rarely reach the observer. Therefore, the green component is the last color that remains distinct and bright, which explains why the flash is usually green, sometimes with a slight yellowish or reddish tinge.

On rare occasions, atmospheric variations can allow brief flashes of blue, purple or even orange, but green is the most common because it lies at the edge of the light that is not completely scattered.

Tips for observing it

• Look for an elevated, clear vantage point (cliffs, boats or elevated plains) with a clear horizon.
• Use binoculars or a telescope to stabilize the image and magnify the faint flash.
• Aim your gaze just above the horizon, preferably against a dark background (sea, lake or clear sky).
• Be patient: the effect lasts only 1 to 5 seconds and can repeat several times during the same sunrise or sunset.

In short, the green flash is the result of Earth's geometry, atmospheric scattering and the bending of sunlight, which turn the edge of the Sun into a brief flash of green light as the sun disappears below the horizon.