This rare optical phenomena is called green flash. it occurs sometime right after sunset or right before sunrise. It happens when the sun is almost entirely below the horizon. For a second or two the upper rim of the sun appears green in color. Sometimes the flash is more yellow very rarely it is eren blue. It is caused by atmospheric bending and scattering of sunlight. The atmosphere acts like a weak prism separating sunlight into different colors. The three colors of light rays then overlap. The green light of the upper rim of the sun remains slightly visible. This effect is magnified by the atmosphere itself. If the atmosphere is exceptionally clear blue flashes can visible as well. This phenomenon can occur over land but it is mostly seen over water.