- Written by Nadia O'Carroll
There are three factors that create our perception of the sky colours – light, our own perception and the Earth’s atmosphere
Light expressed simply, light consists of waves - travelling energy in the form of vibrating electric and magnetic fields. Sunlight appears to be white, but by passing sunlight through two prisms Isaac Newton demonstrated that white light is actually a mixture of colours called the visible spectrum. We can see this in rainbows, when water in the atmosphere splits white light into the colours of the visible spectrum – red, orange, yellow, green, blue, indigo and violet (in order of longer to shorter wavelengths). Ultraviolet light has even shorter wavelengths; it may be visible to other species but is not visible to us.
Perception we perceive light through 3 different types of receptors in the retina of our eyes. Our vision is most receptive to the red, green and blue wavelengths. We also have some automatic vision adjustments in our brain, which have evolved as a survival mechanism to help us separate natural colours clearly. It is strange to think that nothing actually has colour itself. All we really see is a combination of waves of light that are reflected and absorbed by surfaces. Eg green plants contain chlorophyll, which absorbs blue and red colours, but reflects green; therefore our eyes perceive the plants as green.
Atmosphere the white light emitted by the sun passes through the Earth’s atmosphere, and the molecules and particles in the atmosphere have various effects on the waves of light. The shorter the wavelength of light, the greater the chance of being scattered.
During the day, the molecules of oxygen and nitrogen present in the atmosphere scatter the higher energy, shorter wavelength waves, so we see this as blue light. Since violet light has the shortest wavelength, it would seem that the sky should be violet. However it is not, partly because some violet light is absorbed by the upper atmosphere, and partly because our eyes are more receptive to blue than violet. As a survival mechanism to distinguish colours, our eyes also adjust to see the sky as a pure and constant blue.
At night there is minimal scattering of light so the night sky appears a constant black. Clouds and haze appear white, because they contain such large molecules that all the different wavelengths of white light are scattered equally.
At sunrise and sunset the sun’s light has to pass through more atmosphere than when the sun is directly above, this scatters the longer wavelength colours such as reds, oranges and yellows, (and colour combinations). And these colours can be scattered even more by particles such as dust, soot and salt which further enhance the colours.
The processes that create colours in the sky may be complex and difficult to understand, but the beauty that is created can be appreciated by anyone who spares a few minutes to look around.