As an unfortunate person who has never had the privilege of spotting the amazing green flash effect that occasionally appears at the end of the sunset, I was very intrigued by this APOD and hope to be able to spot one in the future now knowing its cause. The phenomenon of the green flash and the even rarer blue flash that occurs as the sun sets is pictured here and was taken in Spain. A viewer's visibility of the green flash requires that the land is flat and has a distant horizon, such as over an Ocean. It is a common misconception that the green flash is a result of a reaction in our eyes where the red cones are saturated therefore causing the green flash to occur as an after image. The true cause of the green flash can be explained by the basic properties of Earth's atmosphere. As light enters the Earth from the Sun it slows down and refracts, which at sunset or sundown means it bends downwards. The blue light is bent more than the red light in the atmosphere because it is nearer to the ultraviolet frequencies (shorter wavelengths) and is slowed down more. The flash appears green because the blue light is scattered more than the green light in the atmosphere and usually the blue is entirely removed, except in this rare case where both can be seen. Both the green and the blue flash were able to be seen in this photograph because the atmosphere was astoundingly clear and did not scatter the blue light as much as it normally would have.
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