• Question: why is the sky blue

    Asked by EmilyE on 11 May 2020.
    • Photo: Jean O'Dwyer

      Jean O'Dwyer answered on 11 May 2020:

      Sunlight reaches Earth’s atmosphere and is scattered in all directions by all the gases and particles in the air. Blue light is scattered more than the other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.

    • Photo: Aisling Ryan

      Aisling Ryan answered on 11 May 2020:

      Great question- I always wondered about this as well!!
      When we see colour, it is actually light of different sizes. Light is measured in wavelengths, and a certain wavelength corresponds to a certain colour.
      The wavelengths for the colour blue are much shorter than the wavelengths for other colours.
      When the sun shines light over our planet the light gets reflected off all of the gases in our atmosphere that we can’t see. Because the blue wavelengths are shorter and smaller they get scattered more and that’s why the sky appears to be blue!
      Of course this all depends on the time of day and how clear the sky is! If there are lots of clouds the sky will appear white or grey, and when the sun starts to set you will see other colours like red, orange and pink! I love to watch colourful sunsets 🙂

    • Photo: Achim Schmalenberger

      Achim Schmalenberger answered on 11 May 2020:

      Hi Emily, I haven’t had much contact with physics since I left school so my explanation is a bit more generalistic. In principle it comes down to the fact that when the sun light is entering our atmospheres, not all the light is being “treated” the same way. Blue light has a shorter wavelength than for instance red light. Blue light has also more energy than red light. When the sun light with all the different colours = wavelengths hit out atmosphere they get “separated” by the fact that they have different wavelengths. Therefore, it is possible that the sky on a different planet looks different if the atmosphere is different (see mars). Also, without an atmosphere there will be no colour in the sky at all (see pictures of earth taken from the moon) as the light is not encountering any atmosphere.

    • Photo: Ollie Otter

      Ollie Otter answered on 11 May 2020:

      Hi Emily,
      Excellent question!
      The short answer is that during most of the day all colours, except blue, get absorbed by our atmosphere. That is why we see the sky as being blue.
      The longer answer: visible light is a small part of the electro magnetic spectrum and is a form of radiation (completely harmless to us by the way). Light travels in waves and each colour has a certain wave length. Red light has the longest wave length and violet has the shortest wave length. Blue is somewhere in between them. The reason why any object has colour to our eyes is because light with the wave length of that colour gets reflected by that object and all other wave lengths get absorbed. That is also what is happening in our atmosphere and why the sky gets such a nice orangy red colour at sunrise and sunset; the light has to travel a bit further through the atmosphere at those times and light waves get stretched out a bit more. The wave lengths are a bit longer and only reddish hues get reflected. When the sun is higher in the sky light waves don’t have to travel so far and wave lengths are shorter, only blue light is then reflected and all other colours are absorbed.
      Hope that makes sense!

    • Photo: Anna Zakrzewska

      Anna Zakrzewska answered on 11 May 2020:

      I thought about it last Sunday, lying down on grass and just staring at the sky. It’s a very interesting phenomena. Have you ever played with a prism to see how many colours are visible in the in white light? It’s like a detangled rainbow, different colours are a result of different wavelengths, purple/blue being the shortest and orange/red the longest. On the way from the Sun, light gets absorbed and reflected (scattered) against the tiny particles in the Earth’s atmosphere, like oxygen, nitrogen and so on. This effect is different for different wavelengths, and guess what! It’s the strongest for shorter wavelengths, so the tones of blue. They become dominating and that’s why we observe the sky as blue.

    • Photo: Aengus O'Connell

      Aengus O'Connell answered on 11 May 2020: last edited 11 May 2020 11:18 am

      Hi Emily,
      That’s a really good question. The explanation requires you to understand a couple of things about light and colour first. Light is what gives everything its colour. Light is actually made up of all the colours moving very fast (ie. speed of light) together. To prove this as a concept you can run two experiments;

      One is by placing a glass of water on a window-sill where the sunlight is shining through and on the counter you will see a band of different colours. This experiment shows ‘dispersion’ where ‘white’ light is being split into the colours that make up light (this experiment is the same as how rainbows form).

      The second experiment shows how colours change and become white when moving at speed. It’s called Newtons disc, where you have a circle of white card about the side of a CD. Looking at the disc like a clock draw lines from 12-6, 1-7, 2-8, 3-9, 4-10, 5-11 so you are left with 12 equal segments. Colour each segment a different colour. Put a hole in the very centre of the disk and pass a pen or something similar through it and use your hands to rotate it rapidly. You will see that when the disc is moving quickly it appears white even though you know it is coloured.

      So now that you understand a bit about light, and how it is made up of all the different colours, in Physics ‘colour’ is the reflection of light. So everything has a colour because that is the part of the light that the object reflects (the other colours get absorbed by the object). For example, the leaves of a tree will appear green because they reflect the green part of the light. Every single object has a different molecular make-up, and so will absorb/reflect light differently and appear a different colour.

      So the sky is blue because the make up of the atmosphere (various gases) absorbs everything except the blue part of the light as it travels to earth from the sun.

      Hope this helps and wasn’t too boring of an answer!!!

    • Photo: Aruna Chandrasekar

      Aruna Chandrasekar answered on 11 May 2020:

      It’s a beautiful phenomenon called the scattering of light. We are able to see one wavelength of the visible light spectrum.

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