Why is the sky blue? A simple explanation on how we see light
A basic understanding of how light works will clear everything up
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Your support makes all the difference.Why is the sky blue? Or more accurately, why do we see the sky as blue? The answer lies in why we see rainbows when the sun shines during or after rainfall.
Whereas the colour of a rainbow is determined by how light passes through water droplets, the colour of the sky comes from how light passes through the air.
To understand why this is, a brief physics lesson is required – about how we see light.
When the sun’s light reaches the Earth’s atmosphere, it is scattered by the tiny nitrogen and oxygen molecules present in the air.
Sunlight is made up of a spectrum of different colours that look white when we see them all mixed together, so when it is scattered, or refracted, we see different colours.
Colours with shorter wavelengths, violet and blue, are scattered the most, so more of the blue light is scattered towards our eyes than other colours.
'So why isn’t the sky a violet colour?' you may ask. This is because our eyes are much more sensitive to the colour blue.
The reason we can’t see stars during the day is because the blue light that gives the sky its colour is much brighter.
The sky also tends to fades to pale as its nears the horizon, because the light from the horizon has had further to travel through the air and so has been scattered more.
So what about rainbows?
Just like with the gas particles in the air, rainbows form when the colours in sunlight are scattered by water droplets, whether it be from rain, or the spray from a fountain or waterfall.
Light travels slower through water than air, so the light is bent as it enters the raindrop and the light is split into the spectrum of colours.
Does the sea reflect the sky?
A common misconception is that the sea is blue because it reflects the blue sky. In actual fact, the sea is blue for similar reasons as the sky.
When sunlight hits the ocean, the colours with longer wavelengths, red, orange, yellow and green, ar absorbed so that blue is left over.
The amount of water also alters how much light is absorbed, which is shallower sea appears lighter and deep oceans darker.
What’s in the sea, for example how much mud or seaweed is present, will also affect how blue the water appears because it alters how the sunlight is absorbed.
This article has been updated. It was originally published in June 2018.
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