On a warm summer day you lie on the grass and gaze at the endless blue sky. Maybe you’re daydreaming, or simply resting when a thought comes to your mind. Why is it blue and not purple, or green, or any other colour of the visible spectrum? After all, how much more spectacular would the sky be, had it been painted in the colours of the rainbow?
Many attribute the monopoly of blue to water, or water droplets and vapours in the skies, but that is not the case.
Water would acts like a prism, scattering white light into its components, much like the rainbow after a storm, so we would see more than one shade.
If not water, what makes the sky blue?
The answer to that is a bit more complicated and involves some physics and chemistry, but I’ll try to keep it simple.
Air is made up of gases, and those gases consists of molecules, tiny, ever-moving particles. Light, on the other hand, consists of waves traveling at different frequencies, short and more energetic at the left end of the spectrum, and longer and slower on the right.
So it’s only natural that light with shorter, high frequency wavelengths will encounter and bounce off air molecules more often, making the colours on the left dominant in a clear sky.
If you look back at the spectrum, you will notice that purple is also visible and even shorter than blue, so then why isn’t the sky violet?
Truth is, it is, the sky is actually a mixture of blue and purple, but because our eyes are more sensitive to blue, the receptors filter out the violet shades leaving us with a pure sheen of blue.
But the sky is not always blue. At dusk and dawn we see a different view. This is because while the blues are scattered all around, the greens, reds and yellows travel on. Eventually, the short waves are all spent, leaving only the long. This is what happens when the sun is far and angled, as it is at sunset and dawn, making the right end more dominant and the horizon glowing in orange.
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