What Causes Green Flashes at Sunset? The Science Behind One of Nature’s Rarest Optical Phenomena

What Is a Green Flash?

A green flash is a brief green-colored burst or rim of light visible at the upper edge of the Sun just as it sets—or, more rarely, just as it rises.

It typically lasts:

• 1 to 2 seconds

• Occasionally slightly longer in ideal conditions

The green flash occurs during the final moment of sunset, when the last visible sliver of the Sun appears tinted green before disappearing completely.

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The Science Begins with Refraction

To understand green flashes, we must begin with atmospheric refraction.

When sunlight passes through Earth’s atmosphere, it bends. This bending happens because light travels at different speeds in air of varying densities.

Near the horizon, sunlight travels through a much thicker layer of atmosphere than when the Sun is overhead.

This thicker atmospheric path increases refraction.

Refraction causes the Sun’s image to appear slightly higher in the sky than its true geometric position.

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Dispersion: Splitting Sunlight into Colors

Sunlight appears white, but it is made of many wavelengths—each corresponding to a different color.

When light refracts, it also disperses. This means different wavelengths bend by slightly different amounts.

• Blue and violet bend more strongly.

• Red bends less.

• Green falls in between.

This separation of colors is similar to what happens when light passes through a prism.

At sunset, atmospheric dispersion slightly separates the colors of the Sun vertically.

Under normal conditions, the separation is too subtle to notice.

But at the final moment of sunset, geometry enhances the effect.

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Why Green Becomes Visible

As the Sun sinks below the horizon:

1. The lower portion disappears first.

2. The upper rim remains visible longest.

3. Refraction and dispersion separate colors slightly.

Blue and violet light scatter heavily due to Rayleigh scattering. This scattering removes much of the blue light from the direct path to your eyes.

Red and orange light dominate most sunsets.

However, at the very top edge of the Sun, green light can momentarily become the most visible remaining wavelength.

For a brief second, the upper rim flashes green.

Then it vanishes.

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Why Green and Not Blue?

You might wonder: if blue bends more strongly, why don’t we see a blue flash?

The answer lies in atmospheric scattering.

Shorter wavelengths—especially blue and violet—scatter in all directions when passing through the atmosphere. This scattering is why the daytime sky appears blue.

Because blue light is scattered away from the direct line of sight at sunset, it rarely remains concentrated enough to form a visible blue flash.

Green, being slightly longer in wavelength, survives scattering more effectively than blue but still bends enough to separate from red.

This makes green the most likely visible color in the flash.

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Types of Green Flashes

Green flashes are not all identical. Atmospheric conditions determine their type.

1. Inferior Mirage Flash

Occurs over ocean horizons when warm air sits above cooler water. Temperature differences bend light in complex ways, enhancing color separation.

2. Mock Mirage Flash

Occurs when atmospheric layers create multiple distorted images of the Sun’s upper edge.

3. Subduct Flash

Happens when the Sun appears compressed and flattened near the horizon.

All types rely on temperature gradients in the atmosphere.

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Why Green Flashes Are Rare

Green flashes require precise conditions:

• A clear, unobstructed horizon (often over ocean or desert)

• Minimal atmospheric turbulence

• Clean air with low pollution

• Stable temperature layers

Clouds, haze, or urban pollution often obscure the subtle color separation needed for a visible flash.

Additionally, the event is extremely brief. Many people simply blink and miss it.

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The Best Places to See a Green Flash

Green flashes are most commonly observed:

• Over large bodies of water

• In desert regions

• From high elevations

• In areas with low humidity and clean air

Sailors historically reported frequent sightings because the ocean horizon provides an uninterrupted view.

Elevated coastlines are particularly favorable.

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The Role of Temperature Inversions

A temperature inversion occurs when a layer of warm air sits above cooler air near the surface.

These inversions increase atmospheric refraction and can magnify dispersion effects.

This enhanced bending helps separate green light from red at the Sun’s edge.

Inversion layers are common over oceans at sunset, making coastal regions ideal for sightings.

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Sunrise Green Flashes

Green flashes can also occur at sunrise.

However, they are harder to see because:

• The observer must anticipate the exact moment of sunrise.

• Atmospheric conditions must be ideal before the Sun becomes too bright.

Sunrise flashes may last slightly longer because the Sun’s light intensifies gradually rather than fading rapidly.

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Historical Observations

Green flashes were once considered myth or exaggeration.

In the 19th century, scientific documentation increased as optical physics advanced.

The phenomenon gained popular attention through literature, including references by Jules Verne, who described it romantically in his novel Le Rayon Vert (“The Green Ray”).

Modern atmospheric science confirmed the physical explanation.

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The Sun’s Changing Shape at Sunset

As the Sun approaches the horizon, it often appears flattened.

This flattening occurs because refraction lifts the lower portion slightly more than the upper portion.

The distortion contributes to the brief visibility of the uppermost rim.

When the final sliver remains, dispersion isolates green light at the edge.

Then it disappears completely.

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Can Cameras Capture Green Flashes?

Yes—but timing is crucial.

High-resolution cameras with telephoto lenses increase the chance of capturing the flash.

However, never look directly at the Sun through optical equipment without proper solar filters.

Eye safety is essential.

Professional observatories and space agencies such as NASA monitor solar phenomena using specialized instruments, though green flashes are primarily an atmospheric event rather than a solar one.

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Green Flashes on Other Planets?

Theoretically, green flashes could occur on planets with atmospheres capable of refracting and dispersing light.

The exact color would depend on:

• Atmospheric composition

• Particle density

• Refractive index

On planets with different gases, the dominant flash color could vary.

On airless bodies like the Moon, no atmospheric refraction means no green flash.

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Why the Phenomenon Feels Magical

The rarity, brevity, and vibrant color of the green flash give it a mystical reputation.

Unlike sunsets—which unfold gradually—the green flash is sudden.

It feels like a secret reward for patient observation.

But its beauty comes from physics:

• Refraction

• Dispersion

• Scattering

• Atmospheric layering

It is geometry and light interacting precisely at Earth’s horizon.

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How to Increase Your Chances of Seeing One

If you want to see a green flash:

1. Find a location with a clear, flat horizon.

2. Choose a day with minimal haze or pollution.

3. Watch carefully during the final seconds before sunset.

4. Do not blink at the critical moment.

Patience and clear conditions are key.

You may need several attempts.

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Conclusion: A Flash of Physics at the Horizon

What causes green flashes at sunset?

They occur when atmospheric refraction and dispersion separate sunlight into its component colors as the Sun dips below the horizon. Blue light scatters away, red light fades, and green briefly becomes visible at the Sun’s upper rim.

The event lasts seconds.

It requires precise conditions.

It is entirely natural.

The green flash is not supernatural or mythical—it is a stunning demonstration of how Earth’s atmosphere bends and filters light.

At the edge of day, just before darkness settles, physics reveals itself in a brilliant emerald spark.

If you watch carefully, you may witness one of the rarest and most beautiful optical effects our planet offers.