It is a beautiful summer night and you are standing outside waiting for the sun to set. The wind blows across your skin leaving ripples of little hairs sticking straight up. The smell of fresh cut grass wakens your spirit as you lie on a hill beneath a large oak tree. Suddenly the sky becomes a vivid neon orange and the clouds, brighter than the sky, look like mirrors. Hues of pink and red abruptly break into the orange making the sky a plethora of warm colours. The cumulus clouds change colour and become almost iridescent. You wonder, "Why does this happen? Why are sunsets so colourful and clouds so unique?"

The sky behaves this way because of the light rays from the sun and the earth’s atmosphere. Firstly, the sun is made up of pure white light. On earth we see the sun as yellow during the day and orange or red during sunset because of scattering, but if you were in space the sun would be white because there is no atmosphere to block it’s ray’s paths. Scattering occurs because the earth has an atmosphere.

Earth’s atmosphere is made up of seventy-eight percent nitrogen gas and twenty-one percent oxygen gas. Argon and water, either in the form of vapor, droplets, or ice crystals, make up the rest of the atmosphere. In addition, many small particles inhabit the atmosphere such as dust, ashes and soot, pollen, and salt. The earth’s atmosphere gets increasingly denser the closer one gets to earth. It is because of these pollutants and the density of the atmosphere that make the sky different colours.

Pure white light, which the sun is made of, consists of the colours: red, orange, yellow, green, blue, indigo, and violet. Light travels in wavelengths at a certain frequency. Wavelengths are the lengths between the crests of each the waves and frequencies are the numbers of waves per second. Red travels at the lowest frequency and has the longest wavelength, where as blue has the fastest frequency and the shortest wavelength.

Light, in space, travels in a straight line. When light from the sun hits the earth’s atmosphere it continues to go straight until it bumps into something, such as dust or water particles. All different colours are affected differently and all can be absorbed, but higher frequencies, such as blue, are absorbed more often than lower frequencies, such as red. This is called Rayleigh scattering, named after Lord John Rayleigh, an English physicist who first described it in the 1870’s.

The sky is blue during the daytime because of Rayleigh scattering. The blue, because of it’s shorter wavelength, bumps into gas molecules that are very tiny, and are then absorbed by them. The blue light is then radiated everywhere, and the sky is blue. Red and orange light is not absorbed because their wavelengths are long and tend not to bump into small gas particles. As one looks at the horizon during the day time, it looks white. That is because it takes a longer time for the blue light to hit your eye, thereby passing through more air and the light gets scattered in more directions.