For many in Australia clear blue skies are a common sight, but why is it that we see it this colour, and why is it different during sunrise and sunset. 

Earth's atmosphere contains air molecules and many types of particles, including ash, dust and ice crystals. As sunlight travels through the atmosphere it is scattered in all directions by these molecules and particles.   

Air molecules are smaller than the wavelengths of visible light and are therefore more effective at scattering shorter wavelengths than longer wavelengths. It is important to note here that the human eye sees shorter wavelengths as greens, violets and blues, with longer wavelengths seen as yellows, oranges and reds. Therefore, when we are looking up at the sky, away from direct sunlight, blue light hits our eyes from all directions, making the sky appear blue. 

At sunset and sunrise, when the sun is closer to the horizon, sunlight has to pass through more of the atmosphere before reaching our eyes. Scattering of light removes shorter wavelengths, leaving the longer wavelengths of yellow, orange and red to reach our eyes.