Green flashes also known as emerald flashes are optical phenomena that are visible during the time of sunset or sunrise. The term ‘flash' means a momentary brightness and the green flash can be observed occasionally. Amid the huge expanse of scarlet and orange suddenly a green flicker is seen. This seems like a weather illusion, but in reality it is not an illusion but a natural phenomenon that is caused by atmospheric refraction.

The green flash is visible when a portion of the sun changes color during sunset and from orange or crimson, the color turns greenish or bluish. In fact, the green flash appears at that last moment when the sun is completely on the verge of setting. When the sun goes beyond the horizon, the last glimpse that one gets to see is the unusual tinge of bluish green.

The emerald or greenish color persists for a very short time. Green flashes are positioned either above the sun or just emerges from the sunset point. Normally a green dot appears first above the sunset point and then it is accompanied by green rays and green flashes. The green flashes rise at such position that they can be seen from any altitude.

However the green flashes can be best viewed best from the point that lies a few kilometers above the sea level. The green flashes when viewed from substantial heights look like a thin glowing line within the horizon. Thus, the green flashes are unusual occurrences amid the usual orange, red and crimson colors of sunset.