Green flashes and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible, usually for no more than a second or two, above the sun, or a green ray shoots up from the sunset point。

　　The reason for a green flash lies in refraction of light (as in a prism) in the atmosphere: light moves more slowly in the lower, denser air than in the thinner air above, so sunlight rays follow paths that curve slightly, in the same direction as the curvature of the Earth. Higher frequency light (green/blue) curves more than lower frequency light (red/orange)， so green/blue rays from the upper limb of the setting sun remain visible after the red rays are obstructed by the curvature of the earth。

　　简单说来就是：在日出和日落时，当太阳只有一小部分在水平线之上，而空气又十分清新的话，绿太阳便会出现，维时仅仅数秒。我们知道，太阳光通过大气层时会被折射，正如三棱镜一样，把太阳光的白光折射为彩虹七色。波长较短的光线会被折射得较多，所以在日出或日落的一刻，我们最先或最后所看见的都应该是蓝光。但是，蓝色的光在空气中较易被散射，不易看见，而往往我们看见的，是比蓝光波长稍长的绿光，这便是绿太阳的由来。可是如果空气中有太多灰尘的话，就连绿光也会被散射开，我们就看不见绿太阳了。