Because of the angle it makes with the earth, the sun must, at sunrise and sunset, travel longer in the atmosphere before reaching the earth. In other words, it travels a greater distance into the atmosphere. A long journey means that the shorter wavelengths (on the blue side) of the spectrum are more and more diffused, thus letting the wavelengths of red, orange or yellow reach us or reflect on the clouds. An almost pure atmosphere gives us an orange sunset. The different colors depend on the different particles in the atmosphere and clouds that bounce sunlight at different angles. The orange and red colors of sunsets are intensified by pollution, as well as by the ashes and smoke released by fires or volcanic eruptions.