Lingering daylight after the sun is below the horizon is another effect of refraction. Light travels at a slightly slower speed in Earth’s atmosphere than in space. As a result, sunlight is refracted by the atmosphere. In the morning, this refraction causes sunlight to reach us before the sun is actually above the horizon. In the evening, the sunlight is bent above the horizon after the sun has actually set. So daylight is extended in the morning and evening because of the refraction of light. Note: the picture greatly exaggerates this effect as well as the thickness of the atmosphere.
Sunlight after Sunset
