Why is the sky blue?

Ever wonder why the sky is blue? Or why sunsets are red? The Ancient Egyptians believed that the sky was personified by the goddess Nut, who was thought to arch her body over the Earth, forming the physical sky. The Ancient Greeks believed in the primordial God of the sky, Ouranus (Uranus) who like Nut, was thought to form the literal sky. According to Greek mythology the sky was a “solid dome of bronze, decorated with stars, whose edges descended to rest upon the outermost limits of the flat earth”.

Nut
Nut, Goddess of the sky. Image: Bruna, Source

 To us a clear cloudless day-time sky is blue. And the reason for this is Rayleigh scattering. But before we get into the nitty-gritty, let’s define some of the basics.

The white light we see from the sun is actually made up of all the colours of the rainbow. As Isaac Newton showed in 1666, when he used a prism to separate out all the different colours to form a spectrum.

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As white light passes through a prism it separates out to form a spectrum. Illustration by Yasmin of “Weird and Wonderful Science”.

The light energy from the sun, much like the ocean, travels in waves. These waves can be short and “choppy” or long and “lazy” as you progress from one end of the spectrum to the other. The visible part of the spectrum ranges from red light with a wavelength of about 700 nanometres (nm), to blue with a wavelength of about 400 nm.

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The different wavelengths of light. Red has the longest wavelength of 700 nm and blue has the shortest of 400 nm. Illustration by Yasmin of “Weird and Wonderful Science”.

Light travels in a straight line, until it meets a particle or surface. Then one of three things can happen to the light ray:

  • It can be reflected (like a mirror).
  • It can be bent (like a prism).
  • It can be scattered (bouncing off in different directions).

Now we know the fundamentals, we can start to answer some of these questions.

Why is the sky blue?

Sunlight reaches Earth’s atmosphere and is scattered in all directions when it hits the gases and particles in the air. Just like when you’re walking down a busy corridor, you might get bounced around and jostled. Rayleigh scattering, discovered by Lord Rayleigh in 1871, is the scattering that occurs when the particles in the atmosphere are smaller than the wavelength of light. So, for the Rayleigh scattering of visible light, the particles must be smaller than 400 to 700 nm. The nitrogen and oxygen molecules found in our atmosphere fit perfectly with this condition.

According to Rayleigh’s scattering, upon collision with these small particles the different wavelengths of light will be scattered by different amounts. A much higher percentage of short wavelength light will be scattered than long wavelength light. So, when the sun’s light reaches the Earth’s atmosphere, the shorter blue wavelengths are scattered more strongly towards our eyes than the red wavelengths on the other side of the spectrum. This is why we see a blue sky.

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Blue light gets scattered by nitrogen and oxygen molecules as it enters our atmosphere. Illustration by Yasmin of “Weird and Wonderful Science”.

You may notice that closer to the horizon, the sky fades to a lighter blue or white. This is because the light from the horizon has had further to travel through the air before it reaches your eyes, and so has been scattered and re-scattered many times, and in many directions. This increased scattering means that less blue light reaches us, which is why the sky fades to a lighter blue or white.

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The sky appears to fade to a pale blue or white closer to the horizon. Illustration by Yasmin of “Weird and Wonderful Science”.

Why is the sky not violet?

The wavelength of violet light is shorter than blue light, so why don’t we see the sky as purple? The answer lies with how our eyes react to light. Our eyes contain two types of cells that react to light – rods and cones. Rod cells are sensitive to brightness, and cone cells detect colour. We have three types of cone cells that detect either red, green or blue light, overall giving us our colour vision. The “blue” cones are more sensitive to blue light than violet, which is why to us the sky is a pale blue colour and not a bright purple.

Why are sunsets red?

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At sunrise or sunset when light has a longer distance to travel the sky appears to be red. Illustration by Yasmin of “Weird and Wonderful Science”.

As the sun gets lower in the sky, its light has to travel a much longer path through the atmosphere before reaching your eyes. This means that most of the blue light has already been scattered and re-scattered, and so is mostly removed, allowing the reds and yellows to shine through. This is what gives a colourful sunrise or sunset – and that perfect Instagram picture.

If you have time, check out this YouTube video that nicely summarises this post:

Header image: Dick Jones, Source
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