• Question: why is the sky blue ?

    Asked by n1ral to Louise, Zoe, Sharon, Jon, Daniel on 17 Jun 2010 in Categories: .
    • Photo: Louise Dash

      Louise Dash answered on 16 Jun 2010:

      Blue? Around here it’s mostly grey! But today is one of the lovely days when the sky is indeed a beautiful blue so this is a great question!

      You probably know that sunlight consists of lots of different colours of light, all mixed up to make white light. Each colour of light has a different wavelength – blue/violet light has the shortest wavelengths (around 500 nanometres) then through the spectrum (green, yellow, orange) it gets longer, and red light has longer wavelengths (about 700 nanometres).

      Our atmosphere is made up of different gases, mostly nitrogen and oxygen, which are in the form of molecules – and these molecules are much smaller than the wavelength of visible light. The light from the sun moves in a straight line, but if it hits a gas molecule it gets bounced off it – we call this “scattering”, and when light gets scattered by something much smaller than its wavelength, this is a special type of scattering called Rayleigh scattering (named after the person who first explained it!). Different wavelengths of light are scattered at different intensities, and with Rayleigh scattering the shorter wavelengths (blue light) become much more intense.

      So white sunlight that has been bounced off gas molecules in the atmosphere looks blue. Because this light is bounced in all directions, this is the light that you see if you look at the sky (the bit of the sky that’s not right next to the sun!) – and that is why the sky is blue.

    • Photo: Jon Copley

      Jon Copley answered on 16 Jun 2010:

      The sky looks blue because of something called Rayleigh scattering. Sunlight is made up of light waves of different wavelengths, which appear to us as different colours. The wavelengths that are red, yellow and green light pass pretty much straight through the atmosphere. But the molecules of gas in the atmosphere are just big enough to scatter the blue wavelengths of light (because blue light has a shorter wavelength – the molecules are too small to scatter the longer wavelength red, green, yellow and other colours). So the blue light from the Sun gets “spread” across the sky in all directions by the atmosphere, making the sky look blue.

      Although we also get blue wavelengths of light from stars, and from the Moon, their light sources are too faint to scatter across the sky (at least too faint for our eyes to see it), which is why the sky only looks blue during the day, and not at night.

      At sunset, the light from the Sun has to travel further through the atmosphere to our eyes, because the Sun is low on the horizon. As a result, larger particles in the atmosphere, like soot, can scatter the longer-wavelength colours, like reds and oranges, to give us the colours we see in the sky on the horizon at sunset.

      For the best sunsets, you need to go near to the poles, because it takes the Sun much longer to set at the poles than at the equator, so there’s more time for the sunset colours to develop. In the Antarctic, I’ve watched spectacular sunsets that last for a couple of hours – and they were some of the most beautiful things I’ve ever seen. Here’s a photo that I took of one, across the Antarctic ice, though it doesn’t really do it justice:

    • Photo: Sharon Sneddon

      Sharon Sneddon answered on 16 Jun 2010:

      The light from the Sun looks white. But it is really made up of all the colors of the rainbow. Sunlight reaches Earth’s atmosphere and is scattered in all directions by all the gases and particles in the air. Blue light is scattered in all directions by the tiny molecules of air in Earth’s atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.

      This process is called Rayleigh scattering.

    • Photo: Zoe Duck

      Zoe Duck answered on 17 Jun 2010:

      The sun’s light is made up of lots of differetn colours- you can see this when the light is split to form a rainbow. We see the sky as blue because the molecules in the air scatter blue light more than the other light colours.
      Sometimes at sunrise and sunset the sky appears red. This is due to the angle the sun makes with our eyes and the pollutants in the air causing us to see the red light from the sun rather than the blue that we see during the day

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