At sunset, the sun is near the horizon; hence, sunlight has to travel a longer distance through the atmosphere to reach us. During this journey, most of the blue colour present in sunlight, which has a shorter wavelength, gets scattered from our line of sight. Thus, the red colour present in sunlight, which has a longer wavelength and gets scattered the least, reaches our eye. As we see red colour everywhere overhead, the sun and the surrounding sky appear red at sunset.
