Why is the sky blue?

October 8, 2016 / The world around us / 0 Comments /

A yellowy sun sends out white light which makes the black of space look blue… Confused? You should be! The blue light that you see in the sky is a result of something called light scattering (or more specifically Rayleigh scattering), caused by the particles which make up our atmosphere, and in this article I’ll explain how this works.

 

Sunlight

Sunlight contains the full spectrum of visible light. If you look at the image of the spectrum of light, you can see that blue light has a wavelength of 450nm-495nm, and at the other end of the spectrum is red light with a much longer wavelength of between 620nm and 750nm The specific wavelengths of a particle of light (or rather, a photon) determines the colour we see, but it also dictates the amount of energy that particular photon has. Particles of light with high energy will have a shorter wavelength, so blue light contains more energy than red light for example.

visible-spectrum

 

Light scattering

Light scattering is quite a complex process where a photon of light interacts with atomic structure of a molecule and induces what is called an ‘oscillating induced dipole‘, which essentially means that electrons are displaced periodically in the molecule, making one part of the molecule more negatively charged that the other. This oscillation is done at the same frequency as the light that hits it, and emits light that is identical to the light which caused this oscillating induced dipole. 

Our atmosphere is full of lots of chemicals, but by far the most abundant are oxygen and nitrogen, which combined make up about 99% of our atmosphere. Oxygen and nitrogen are pretty unique in that their molecular structure is perfect for the scattering of blue light, but not so much the other frequencies, and so blue light is scattered around much more than the other frequencies. In the crude picture below, you can see the blue light frequency causes these oscillating induced dipoles, which then emit more blue light, but red light passes right through. It is hard to picture, so I have tried to help with a diagram below.

If you are interested in the details of how this works, you can read more about it here and here.blue-light-scattering

This means that when light from the sun hits our atmosphere blue is scattered down to us on Earth, so when we look up, we see lots of blue. blue-light-being-scattered-from-the-sun-to-earth

 

Red sunsets

When the sun is very low in the sky, the sky often turns red, and this is caused by the same thing. When the sun is low in the sky, the light must pass through lots of the atmosphere to reach you, and by the time it does reach you, much of the blue light would have been scattered out, effectively filtering it out. This leaves the longer wavelength particles of light left (because nitrogen and oxygen do not interact easily with these), and so it is a dominance of these colours which reach your eyes.

 

Summary

The sky is blue because of a complex interaction with the light from the sun and the molecules oxygen and nitrogen in our atmosphere, which cause something called Rayleigh scattering. Rayleigh scattering causes blue light to scatter when it interacts with these atmospheric molecules, and this scattering of blue light over our head makes the sky look blue.

Header image courtesy of … j e r e m y…

This Youtube video will give an overview of the information found on the article tab. If you want to know more about the topic, or want to see where the information came from, have a read of the article after you watch the video.

The sky is blue because the oxygen and nitrogen in the atmosphere scatter the blue light down, making the most abundant colour that reaches our eyes, blue.