The green flash. Time-lapse photograph of the green flash, an elusive effect visible at sunset for a few seconds when the last portion of the solar disc goes beneath the horizon. The green flash is caused by the atmospheric refraction of the sunlight. The refraction is larger for green and blue light than for red light and this means that the image of the Sun viewed in green light will appear to be slightly higher above the horizon than the image in red. When the upper part of the red image approaches the horizon the green flash may be seen as a sudden change in colour from yellow (top image) to green. Sometimes, as in this case, a final violet-blue flash is also seen.