Why is the sky blue?

After days of rain, a patch of blue sky is a sight for sore eyes. But why is the sky blue?

Let’s start with the Sun. Light from the Sun appears white but it actually consists of many different colours. We can see these different colours of light in a rainbow or when white light passes through prism. As the white light from the Sun travels through the Earth’s atmosphere, it collides with particles of air.  The different colours, or wavelengths, of light are scattered by these collisions by different amounts.  Blue light (shorter wavelengh) is scattered more than red light (longer wavelength).

So, when the Sun is high in the sky, blue light is scattered in all directions as sunlight passes through the atmosphere and we see the sky as blue.

But it’s a different case when the Sun is close to the horizon at sunset or sunrise. Sunlight from the low Sun has to pass through much more atmosphere before it reaches your eyes meaning most of the blue light has already been scattered leaving just the red. The result is the beautiful colours we see at sunset and sunrise and, very occasionally, a flash of green light.

When we see a rainbow we can see a range of colours from red (longer wavelength) through yellows and greens to blues and eventually violet (shorter wavelengths). So if shorter wavelengths are more easily scattered why don't we see the sky as purple?

The answer lies with how our eyes react to light. Inside the eye there are two types of cells that react to light. 'Rods' are sensitive to brightness and three types of 'cones' are responsible for detecting colour. The three types of cones are sensitive to lights of certain wavelengths.

The 'blue' cones are more sensitive to blue than violet, so when you look up at the sky, the cones tell your brain you are seeing blue even though there is violet there.

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