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Green flash

The green flash is a rare atmospheric optical phenomenon that occurs briefly at sunset or sunrise, manifesting as a vivid green spot, rim, or ray at the upper edge of the Sun's disk just as it appears to vanish or emerge from the horizon.^[1] This effect arises from the refraction of sunlight through layers of Earth's atmosphere, which bends and disperses the light into its spectral colors similar to a prism, with shorter-wavelength green light (around 550 nm) being the last visible component before red and yellow hues are filtered out by greater atmospheric absorption and scattering.^[2]^[3] The phenomenon is most commonly observed over a clear, distant horizon such as an ocean or flat plain, where atmospheric conditions are stable with minimal turbulence or inversion layers; it typically lasts only 1–2 seconds and requires the observer's eye to be adapted to low light levels for optimal visibility.^[4] Variations include the inferior-mirage green flash, produced by a temperature inversion creating a mirage that lifts the green rim into view, and the superior-mirage green flash, seen over cooler air layers, though the classic form is the mock-mirage or direct green rim flash without significant distortion.^[1] Favorable viewing sites, like coastal areas or high altitudes with unobstructed western or eastern horizons, enhance chances of observation, particularly in subtropical latitudes where the Sun sets more vertically.^[5] Scientifically, the green flash exemplifies astronomical refraction's extreme variability near the horizon, where the atmosphere's density gradient causes differential bending: blue and green wavelengths refract more than red, but extinction (scattering) removes shorter blues and violets, isolating green as the prominent color.^[6] First documented in the 19th century, with early observations by Captain Back during an 1836–1837 expedition and a reliable scientific record by W. Swan in 1865, later explained by James Prescott Joule in 1869,^[7]^[8] it has been captured in photographs and simulations, confirming its physical basis rather than physiological illusion, though retinal adaptation can influence perception.^[1]

Observing the Phenomenon

Optimal Conditions

The green flash is most reliably observed under conditions providing a clear, unobstructed view of the horizon, such as over vast oceans, large lakes, or flat prairies, where minimal atmospheric turbulence allows the upper limb of the Sun to appear sharply defined against the skyline.^[9]^[10] Foreground obstacles like hills, buildings, or trees must be absent, and the observer should be positioned at a sufficient elevation—typically a few meters above sea level—to avoid local distortions while benefiting from distant horizons.^[4] Temporally, the phenomenon occurs precisely at sunset or sunrise, when the Sun's disk is extremely low on the horizon, usually within the final minute as the upper edge vanishes below it.^[6] Atmospheric stability is crucial, requiring low humidity to minimize scattering, clean air free of pollution, dust, or aerosols that could obscure fine details, and the presence of temperature inversions—where warmer air overlies cooler surface air—to enhance refraction without excessive bending.^[6]^[9] Geographically, the green flash appears more frequently in tropical or subtropical latitudes, where consistent clear weather and stable atmospheric layers over oceans facilitate repeated opportunities, compared to higher latitudes with more variable conditions.^[10] Under these ideal circumstances, the flash endures for 1 to 2 seconds on average, though it can extend slightly longer in the presence of mirages, and is visible to the unaided eye as a brief emerald burst atop the vanishing solar disk.^[9]^[10]

Viewing Techniques

To observe the green flash effectively, viewers should employ optical aids such as binoculars or telescopes that magnify the horizon without using filters that block short wavelengths, as these can obscure the green light; low-power binoculars (around 7x to 10x magnification) are particularly useful for scanning the upper limb of the setting sun without excessive distortion.^[11]^[9] However, direct viewing through such devices should be limited to the final moments when the sun is nearly below the horizon to prevent eye damage from intense sunlight.^[12] Positioning is crucial for extending the line of sight over the Earth's curvature and minimizing obstructions; elevating the viewpoint—such as from a hilltop, tall building, or boat at sea—allows for a clearer, more distant horizon, ideally over water where atmospheric layers are layered predictably.^[11]^[12] A height of just a few meters above sea level can suffice for certain flash types, but higher elevations help in avoiding local turbulence.^[11] For timing, consult astronomy apps or perform calculations to determine precise local sunset times, adjusting for observer elevation and refraction effects to anticipate the exact moment the sun's upper edge vanishes, typically lasting only 1-2 seconds.^[11]^[12] Focus intently on the horizon line without blinking during this critical interval to capture the fleeting event, while averting gaze earlier to avoid retinal bleaching from the bright sun. Green flashes at sunrise are more challenging because the sky is brighter and eyes are not dark-adapted; direct focus on the rising sun's upper limb is needed, with similar caution for optical aids.^[11] Common pitfalls include confusing the green flash with lens flares in optical aids, which appear below the true astronomical horizon and lack the precise green coloration, or mistaking non-green wavy horizon distortions from strong temperature inversions for the flash; the authentic green flash manifests as a sharp, brief green rim, spot, or ray precisely above the sun's position in a stable atmosphere. While some types of green flashes (such as inferior-mirage flashes) may show accompanying distortions, the key is the distinct green color and brevity at the moment of disappearance or appearance.^[11]^[12]

Scientific Explanation

Atmospheric Refraction

The primary mechanism behind the green flash is atmospheric refraction, which bends incoming sunlight due to the gradient in air density from the Earth's surface upward. Light rays from the Sun, traveling through layers of air with varying refractive indices, curve concave toward the denser lower atmosphere, elevating the apparent position of the Sun above its true geometric location at the horizon. This effect allows the upper portion of the solar disk to remain visible briefly after the bulk of the Sun has set geometrically below the horizon.^[13] The atmosphere's density decreases with altitude, creating a refractive index gradient where denser, lower air near the surface—resulting from higher pressure and cooler temperatures—bends light rays more strongly than higher, rarer air. Rays grazing the horizon pass through this steep gradient, experiencing maximum deflection, while rays from higher parts of the Sun traverse less dense layers and bend less. This differential bending is particularly pronounced during sunset or sunrise over a flat horizon, such as an ocean, where the air density profile is relatively uniform but inverted near the surface. In the presence of an inferior mirage, caused by a thin layer of warm air overlying cooler air near the ground or water, the refraction distorts the lower limb of the Sun, creating an inverted, mirrored image below the true disk. This mirage effect compresses and lifts the undistorted upper limb momentarily above the horizon line, isolating it as the rest of the solar image dips out of sight. The inferior mirage arises from the strong negative temperature gradient in the surface layer, enhancing the overall refractive bending and contributing to the brief visibility of the phenomenon. Mathematically, atmospheric refraction can be described using Snell's law applied to a stratified atmosphere, where for a ray passing through layers, n \sin z = \text{constant}, with n the refractive index and z the zenith angle. The refractive index of air approximates n \approx 1 + \frac{P}{RT}, where P is atmospheric pressure, R the gas constant for dry air, and T the temperature in Kelvin; this form reflects the direct proportionality of n - 1 to air density, which varies with height due to the pressure-temperature profile. Integrating Snell's law over the continuous gradient yields the curved ray path responsible for the observed elevation. This vertical stretching, or elongation, of the Sun's image results from the varying refraction across its angular diameter: lower rays bend more sharply through the dense boundary layer, while upper rays experience milder deflection, separating the top edge from the compressed base and enabling its isolated appearance at the critical moment.

Dispersion Effects

The refractive index of air varies with the wavelength of light, being higher for shorter wavelengths like those in the blue-green spectrum than for longer wavelengths such as red. This wavelength-dependent variation, known as dispersion, causes different colors of sunlight to bend by slightly different amounts when passing through the atmosphere.^[2]^[14] The atmosphere functions analogously to a weak prism in dispersing sunlight, with shorter wavelengths refracted more than longer ones, resulting in a vertical spreading of the solar image near the horizon. For instance, the refractive index for red light is approximately 1.000292, while for blue light it is 1.000295, leading to a total chromatic dispersion of about 0.006° (or 20 arcseconds) out of the overall atmospheric refraction of roughly 0.53°.^[2]^[15] As the lower limb of the setting sun dips below the horizon due to refraction, the differential bending isolates the green portion at the top of the image, which flashes briefly before vanishing—typically lasting around 1-2 seconds under ideal conditions. This occurs because the red and orange components, experiencing less refraction, set first, leaving the green segment momentarily visible against the sky.^[14]^[2] Green light, peaking at approximately 550 nm, persists after the yellow and orange hues (around 570-600 nm) disappear owing to this differential refraction, as the atmosphere's prismatic effect progressively hides longer wavelengths.^[2]^[15] The angular separation δθ arising from dispersion can be approximated as δθ ≈ (dn/dλ) * θ, where dn/dλ represents the dispersion coefficient (the rate of change of refractive index with wavelength) and θ is the angle of incidence. This relation, derived from the wavelength dependence of refraction, quantifies how the spectrum spreads vertically at low solar altitudes.^[16]

Green Ray

The green ray is a rare variant of the atmospheric optical phenomenon observed at sunset, manifesting as a brief, narrow beam or spike of emerald-green light extending upward from the upper limb of the sun just as it disappears below the horizon.^[17] This distinct ray-like appearance differentiates it from the more common green flash, which appears as a broader burst.^[1] The formation of the green ray results from the combined effects of atmospheric refraction and dispersion acting on the sun's rays passing through a thin layer of air near the horizon. Refraction bends the light path, lifting the green portion of the spectrum slightly higher than the surrounding colors due to the wavelength-dependent refractive index of air, while dispersion separates the sunlight into its spectral components, isolating the green segment as a detached, vertical extension.^[18] This process creates a transient spike lasting approximately one second before the green light is extinguished as the solar disk fully sets.^[1] Visibility of the green ray demands exceptionally stable atmospheric conditions, with minimal turbulence and temperature gradients that could induce mirages distorting the horizon. It is most reliably observed over calm bodies of water, such as oceans or lakes, where the air layer remains uniform and mirage effects are reduced, allowing the clear projection of the ray.^[17] The term "green ray" emerged in 19th-century scientific and literary accounts to describe this specific ray-like manifestation, distinguishing it from broader green flashes reported in earlier observations. Popularized in Jules Verne's 1882 novel Le Rayon Vert, the name drew from meteorological descriptions by figures like John Tyndall and earlier seafarers, emphasizing its elusive, beam-like nature.^[19] Photographic evidence of the green ray often requires long-exposure techniques to capture its fleeting duration, revealing the upward-shooting spike in images taken from elevated coastal vantage points. Notable examples include photographs by atmospheric optics researcher Les Cowley from the Canary Islands, showing the ray as a sharp green extension amid a reddening sunset sky, and similar long-exposure shots documented by Andrew T. Young from San Diego, illustrating the phenomenon's transient verticality under stable conditions.^[17]^[20]

Green Rim

The green rim is a subtle variant of the green flash, manifesting as a thin, uniform green layer encircling the upper edge of the setting or rising sun. This horizontal band appears just before the sun's full disappearance below the horizon, providing a brief glimpse of emerald light separated from the surrounding spectrum. Unlike more intense forms of the phenomenon, the green rim is not a sudden burst but a persistent edge, often serving as a precursor to the complete green flash.^[1] The cause lies in partial atmospheric dispersion, where shorter green wavelengths refract more strongly than longer red ones, allowing the green light to linger above the horizon while red dips below. This differential refraction exploits the prismatic effect of the atmosphere acting on sunlight near the horizon. Basic principles of atmospheric refraction amplify this color separation without requiring extreme mirage conditions.^[1] Optimal for observation in slightly turbulent air, the green rim contrasts with variants needing stable atmospheric layers, as mild turbulence can enhance visibility of the rim without distorting it entirely. It typically lasts up to 5-10 seconds, rendering it more accessible for capture with basic magnification tools compared to fleeting spikes. Due to its narrow profile, unaided eye detection remains challenging, though it establishes a foundational display of chromatic refraction.^[1]

Blue Flash

The blue flash is an exceedingly rare atmospheric optical phenomenon that manifests as a brief burst of blue light at the moment the upper limb of the setting or rising sun vanishes below the horizon, distinct from the more commonly observed green flash. It occurs under extreme conditions, such as high-altitude locations or polar regions, where the atmosphere is exceptionally clear and stable, often featuring a strong temperature inversion layer that enhances refraction effects.^[21]^[22] The mechanism mirrors the refraction processes involved in the green flash but isolates blue light at approximately 450 nm wavelength after the green segment has dissipated, due to the greater atmospheric dispersion of shorter wavelengths like blue and violet. This separation arises from the bending of sunlight through layers of varying air density, where an inferior mirage magnifies the color differential, allowing the blue portion to briefly dominate before absorption scatters it away.^[21]^[22]^[23] Reports of blue flashes include observations in mountainous regions, such as Southern California at elevated coastal sites. These sightings are fleeting, lasting mere seconds, and often captured in photographic sequences showing the transition from green to blue.^[21]^[22] Visibility is severely limited by the human eye's lower sensitivity to blue light compared to green, compounded by atmospheric absorption and Rayleigh scattering that preferentially diminish shorter wavelengths. Scientific validation comes from ray-tracing simulations that replicate the color isolation under specified inversion profiles, alongside rare photographs confirming the phenomenon as a natural extension of dispersion-driven optics.^[21]^[22]^[23]

Historical and Cultural Context

Discovery and Early Observations

The green flash, a brief optical phenomenon observed at the horizon during sunset or sunrise, has roots in early discussions of atmospheric optics, though definitive accounts emerged later. Possible references to related horizon effects appear in ancient Greek texts, such as Aristotle's Meteorologica (c. 350 BC), where he describes mirages and the bending of light rays near the Earth's surface due to varying air densities, potentially alluding to refractive illusions at the horizon.^[24] These early mentions framed such sightings as natural optical occurrences rather than supernatural events, laying groundwork for later scientific inquiry. An early documented account dates to 1837, when Captain George Back of HMS Terror described a green light on the Sun's upper limb at sunrise during an Arctic expedition.^[25] Systematic observations began in the 19th century amid growing interest in atmospheric refraction. In 1815, Stephen Lee, Secretary of the Royal Society, documented the dispersive effects of the atmosphere in his paper "On the Dispersive Power of the Atmosphere, and Its Effect on Astronomical Observations," noting red and green color fringes on the Moon and planets near the horizon caused by wavelength-dependent bending of light.^[26] The earliest undisputed scientific record of the green flash itself dates to 1865, when W. Swan observed a "dazzling emerald green" burst at sunset from the Isle of Wight, England, attributing it to atmospheric conditions rather than retinal aftereffects.^[8] These reports shifted perceptions from folklore to verifiable optics, though skepticism persisted regarding its physical reality. By the late 19th century, the phenomenon gained wider attention through expeditionary astronomy, with observers like French scientists during solar eclipse voyages in the 1870s noting similar horizon color separations, though often conflated with spectral lines in the solar corona.^[6] Initially dismissed by some as a physiological illusion—such as retinal fatigue from staring at the bright Sun—eyewitness accounts accumulated, evolving the green flash from maritime myth to a subject of optical study.^[27] In the 20th century, telescopic and photographic evidence solidified its acceptance as a genuine atmospheric effect. The first black-and-white photograph capturing the green flash was obtained by French astronomer Lucien Rudaux at sunset and published in 1925, demonstrating its objective nature beyond subjective vision.^[28] Further validation came in the 1950s through detailed spectroscopic analyses by astronomers, confirming the role of dispersion, followed by D.J.K. O'Connell's pioneering color photographs from the Vatican Observatory in 1960, which captured the phenomenon's vivid emerald hue under clear conditions.^[29] These advancements transformed the green flash into a cornerstone example of refraction in modern optics textbooks.

Depictions in Literature and Media

The green flash has captured the imagination of writers and artists, particularly in 19th-century literature, where it symbolizes elusive hope and romantic revelation. In Jules Verne's 1882 novel The Green Ray (Le Rayon vert), the protagonist Helena Campbell embarks on a quest across Scotland to witness the phenomenon, which Verne portrays as a rare optical event granting insight into true love and protection from deception, drawing on an invented Scottish legend that romanticizes it as an omen for emotional clarity and fulfillment.^[30] This depiction blends scientific curiosity with romantic idealism, positioning the green ray as a metaphor for fleeting beauty and the pursuit of authentic connection in French Romantic traditions.^[31] The phenomenon's symbolic role extends to 20th-century cinema, where it often evokes themes of longing and epiphany. Éric Rohmer's 1986 film The Green Ray adapts Verne's novel, following a young woman whose summer solitude culminates in witnessing the flash, symbolizing emotional rebirth amid personal melancholy and the end of summer.^[31] In popular media, the green flash appears in the 2007 film Pirates of the Caribbean: At World's End, where it serves as a mystical signal of a soul's return from the dead, reinforcing its lore as a harbinger of supernatural or transformative events among sailors.^[32] Documentaries have further highlighted the green flash as an optical wonder, blending science with visual spectacle. A 2023 Aeon Video short explores the phenomenon through a Dutch filmmaker's efforts to capture and explain it, emphasizing its rarity and atmospheric mechanics while evoking wonder at natural illusions.^[33] In the 2010s, the green flash gained traction in modern media through viral videos and photography, amplifying public fascination. Social media platforms like TikTok and Instagram have hosted countless user-captured clips of the event, often shared as breathtaking moments of natural magic, contributing to its status as an accessible yet elusive spectacle.^[34] Photography enthusiasts have featured green flash images in contests, such as those organized by outlets like My Modern Met, showcasing high-resolution captures that highlight its brief, emerald glow and boosting awareness of atmospheric optics.^[35] Despite its prominence in Western narratives, depictions of the green flash in non-Western cultures remain underexplored in mainstream sources, though specific accounts are sparsely documented.

References

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Explaining Green Flashes
Green flashes are by-products of the large variations in astronomical refraction near the horizon. Although there are several kinds of green flash, almost every ...
[2]
Red Sunset, Green Flash - HyperPhysics
As shown above, the green flash is a refraction phenomenon which occurs when the light enters the Earth's atmosphere at a precise low angle which brings the ...
[3]
THE GREEN FLASH
The green flash, or the greening of the top of the setting sun, or of rising Venus, is a good natural example of the effects of refraction and scattering in the ...
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Green Flash - Mount Wilson Observatory
Definition and description. The green flash is an atmospheric refractive phenomenon where the top edge of the Sun will momentarily turn green.
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https://isaac.exploratorium.edu/~pauld/physics/atmospheric_optics/green_flash.html
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The green flash and other low sun phenomena - NASA ADS
Many who have seen the green flash held that it is purely physiological in origin, fatigue of the retina causing the complementary colour to be seen after the ...
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Green flashes usually are brief: a duration of one or two seconds is typical at moderate latitudes. The simulations will give you some notion of the time scale ...
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green_flash
Nov 7, 1999 · It requires optical help from refraction by layers in the atmosphere to make it easily visible. These same layers make mirages. Mirages and ...
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Dec 16, 2024 · Green flashes are brief bursts of green light at sunrise/sunset. View from sea horizon, tall buildings, or hilltops. Use telephoto lens and ...
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What's a green flash and how can I see one? - EarthSky
Dec 7, 2023 · The green flash is an optical phenomenon that you can see shortly after sunset or before sunrise. It happens when the sun is almost entirely below the horizon.
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The textbooks present atmospheric refraction as a complicated subject, replete with arcane formulae, bizarre series expansions, and approximations that ...
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I lived a long time in the tropics and often heard of, but never saw ...
Oct 21, 1999 · "The green flash does indeed exist. It occurs when the sun is rising or setting. It is a consequence of the dispersion of light by air. Light ...
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This experimental imitation strongly supported the generally accepted view that the green flash is primarily due to atmospheric refraction, dispersion and ...
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The theory which attributes the green sensation to retinal fatigue, though still sometimes met with, is conclusively disposed of by the simple observation that ...
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All about the green flash and the different types of green flashes, how they form, their appearance, an image gallery and how to see them.
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Green light is refracted more and the 'green sun' is very slightly higher than the red one. But the effect is small and cannot be seen with the unaided eye.
[17]
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the green ray seen just as the last glimpse of the sun disappears below the sea horizon was originally introduced into meteorology by Tyndall as an evidence ...
[18]
Pictures of Green Flashes
This sunset shows two inversions: a surface-based inversion that produces the line just at the sea horizon, and a higher one that made the mock-mirage flash.<|control11|><|separator|>
[19]
What produces the "blue flash" during sunset? | Astronomy.com
thickest along the horizon — refracts the Sun's light.Missing: optics examples limitations
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Dec 16, 2024 · The blue flash is a type of green/blue flash that occurs just as the last sliver of the sun sinks below the horizon, creating a brilliant burst of color that ...
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May 15, 2011 · ... green flash. The transparent air and an inversion layer were ... A green rim of the setting or rising sun occurs due to differential ...
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The oldest account of mirages seems to be Aristotle's brief mention in the Meteorologica (c. 340 or 350 BC) at 373 b in Book III (p.253 of the Loeb Library ...
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Jun 24, 2023 · The green flash occurs when the sun's topmost part rises/sets in clear skies. The sun has a stack of colors, and the flash is capricious.
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Aug 22, 2019 · Retinal fatigue was commonly held to be the “cause” of the flash. After looking into a brightly colored light our eyes become fatigued, and upon ...
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This was a black-and-white photo taken at sunset to prove that the flash was a physical phenomenon, not just an afterimage.
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Oct 28, 2018 · The young woman starts weeping. Then, suddenly, at the very moment when the glowing orb finally goes under, it gives off a subtle green flash.
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Green Flash was the term given to rare optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible for a short period ...
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Jul 26, 2023 · A Dutch filmmaker's quest to document and explain the fleeting 'green flashes' known to illuminate his native horizons.
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May 29, 2025 · You may have heard of the green flash as a superhero or even as an ... Viral Video Critters Features. The Green Flash: Optical Illusion ...<|control11|><|separator|>
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Photographer Captures Rare “Green Flash” Coming From Venus
Feb 26, 2024 · A photographer in Sweden captured a rare green flash off Venus on a cold night. These occurrences are caused by properties of light.