Why is the sunset red?
The position of the sun (from where we are) is different as time goes by from the afternoon to the evening. In the early afternoon the sun is right above us while in the evening it comes on the side of us. So the light from the sun goes a longer distance and reaches us in the evening than it does in the afternoon. In the evenong , because the sunlight travels a longer distance. The blue of shot wave length disperses while the red of long wave length keeps going and reaches where we are and disperses above us. This is why the sky is red at the sunset. The figure shows the position of the sun in the afternoon and in the evening.
To sum up, the red of long wave length does not so easily disperse (beacause of the air and the dust) and reaches farther. So the sunset is red.
Rainbow
2015 身の回りの光と色の現象