Sunrise and Sunsets are always stunning to watch and some rare occurence during these events makes it more beauiful to watch. Yes, there is an interesting and a rare meteorological optical phenomenon called the green flash. It is a green spot or flash visible for only few seconds at the top edge of the sun during sunrise or sunset.
The green flash was much popularized after the french author Jules Verne’s novel “Le Rayon-Vert” (The Green Ray) published in 1882. Though there are myths around this, scientists have established that the green flash is indeed real, and not an optical illusion.
So, What is the reason behind the green flashes?
Two effects combine to yield a green flash. It is actually caused by the refraction or bending of light and then the effect is enhanced by a mirage to notice.
Light is made up of wavelengths and each wavelengths has color. The color ranges from 400 nm (violet end) to 700 nm (red end) in the visible spectrum. In 1672, Isaac Newton used a prism and identified the seven colors – Violet, Indigo, Blue, Green, Yellow, Orange and Red in the visible spectrum.
Red, Orange, and Yellow have longer wavelengths, and Violet, Indigo, Blue, Green colors have shorter wavelengths. When the white sunlight enters the Earth’s atmosphere, it refracts or bends the light according to its wavelengths. So the white sunlight gets separated into different colors. As Red, Orange, and Yellow have longer wavelengths, it refracts less, and it is absorbed by the particles in the atmosphere. As Violet, Indigo, Blue, Green colors have shorter wavelengths, it refracts more, bents further around the horizon and the colors stays visible longer.
You may wonder, if Violet – Blue colors stay longer, then why are these flashes green in color?
This is because blue and violet light are scattered by the particles in the atmosphere. Hence it leaves the green light the most visible during the few seconds when the sun sets below or rises above the horizon. Green is the color most often seen, but rarely it can be blue flashes, yellow or even violet.
There is a reason you don’t see a flash with every sunrise and sunset because this refraction is not enough. The effect is very minor to see the flash with our own eyeballs. A mirage is also necessary for the phenomenon to be seen. Mirage is a optical phenomenon in which light rays bend via refraction to produce a displaced multiple images of a distant objects.
The inverted bottom part of the ship is not a reflection and this image is not blurry or out of focus. This is a mirage. Mirage magnifies and enhances the effect and allows us to see the green flashes.
The best way to see a green flash is to view sunrise or sunset from a distance with a clearly visible horizon line especially looking down from a high location.
They frequently occur at the ocean but can be viewed at deserts, enormous flat grasslands, hillside as well as mountaintops and tall buildings. The green flashes are regularly seen by pilots from airplanes, particularly when flying westwards as the sunset is slowed.
On very rare occasions, when the sun sinks into a coastal fog or at distant cumulus clouds, it forms the Cloud-top flash. Sometimes a rare double green flash can be observed, and if the atmosphere is layered, we may see it as a series of green flashes.
Cloud-top flash 
Double green flash
The sun’s upper rim becomes green which is called the Green rim, and often a green rim changes to a green flash and back again during the same sunset. In 1929, Adm. Richard Evelyn Byrd and his crew, on his expedition to Antarctica, saw the green rim on and off for about 35 minutes.
Adm. Richard Evelyn Byrd 
Green Rim
There are four categories of green flashes.
The Inferior mirage flash / (I-Mir) – They are oval and flat, and they occur close to sea level when the surface is warmer than the air above it.
The mock mirage flash / (M-Mir) – They are thin, pointy strip from the upper rim of the Sun and they occur higher up in the sky when the surface are colder than the air above.
The Sub-duct flash – The sun form an hourglass shape due to a phenomenon atmospheric inversion and the upper part of the hourglass shape turns green for up to 15 seconds.
The Green Ray – It is very rare and it is a beam of green light that shoots up from the green flash or is seen immediately after the sun sets.
An extremely rare green flash of the moon was captured on photograph in 2011.
Moon 
Venus 
Jupiter
A green flash also may be observed in bright planets such as Venus and Jupiter.
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https://earthsky.org/earth/can-i-see-a-green-flash/
https://www.livescience.com/26376-green-flash.html
https://www.aaas.org/catching-elusive-green-flash
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https://en.wikipedia.org/wiki/Green_flash
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https://www.nationalgeographic.com/science/article/green-sunsets-are-100-real
https://www.skybrary.aero/index.php/Green_Flash
https://www.washingtonpost.com/weather/2021/04/02/green-flash-photo-virginia-sunrise/
https://www.tropicalbreezeresort.com/blog/what-is-the-green-flash/
https://www.usatoday.com/videos/tech/science/2019/03/12/what-causes-rare-green-flash-during-sunsets/3138260002/
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https://www.atoptics.co.uk/atoptics/gf1.htm
http://spiff.rit.edu/classes/phys445/lectures/atmos/atmos.html
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