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| The Sun, sky and clouds over Birling Gap on Christmas day 2012 |
A few weeks ago my 4 year-old nephew asked
his mother why the Sun was yellow. Of course this got passed on to me, and I
found myself trying to explain the wave nature of light to a bemused toddler in
a moving car while turned around in my seat.
To a 4 year-old color is very important,
hey it’s still important twenty years later.
To understand
why the Sun is yellow, the sky blue, and the clouds white we first need to
understand how color works.
The different
colors we see are the result of different wavelengths of light being filtered out before reaching our eyes. Red light has
longer wavelengths, and blue light has shorter wavelengths. By combining all the
wavelengths of light together, red, orange, yellow, green, blue and purple, you
get white light.
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| NASA SDO AIA 304 solar image |
Refracted (or bent)
or
Each of these mechanisms can be seen as a
result of the Earth getting in the way of the Suns light as it makes its way
out across space, but as you will see it is predominantly the scattering of
sunlight that produces the colors of the Sun, sky, and clouds as we see them.
If you were to look at the Sun from space
without Earth’s pesky atmosphere in the way it would actually look white,
shortly before your eyes are burnt out. Even solar astronomers are in denial
though coloring the sun yellow or orange in a number of their space based
observations. We humans are creatures of habit after all.
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| Astronaut Sunita Williams holding the white Sun in her hand whilst on a spacewalk |
As light enters
our atmosphere it is interrupted on its journey by particles of gas. These
particles are much smaller than the wavelength of the light that it is
interrupting. This results in something called Rayleigh scattering. Rayleigh
scattering resorts in the light being redirected on its path towards the
surface and is strongly wavelength dependent. Meaning that the shorter the
wavelength, or the more blue the light is, the more it is scattered, while the
longer the wavelength, the more red the light is, the less it is scattered.
Leaving the sky its beautiful blue color.
That beautiful
blue sky is accompanied by a strange yellow orb so bright that you cannot look
at it directly even under the protection of our atmosphere. The reason for its
vibrant yellow color is directly related to the blueness of the sky. As the
atmosphere scatters out the blue light the remaining spectrum of the sun is
shifted towards the yellow part. As the Sun moves towards the horizon at sunset
the light passes through more and more of the atmosphere. This means that more
and more of the shorter wavelength light is scattered away making the Sun
appear redder as it sets in the evening.
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| Setting Sun (image credit Monika Landy-Gyebnar) |
If you live in
the UK, like I do, you will most likely be disappointed by the amount of time that the
bright sunshine and the beautiful blue sky is blocked by big white fluffy
looking objects.
No not sheep,
but clouds.
Clouds are drops
or frozen crystals of water and other chemicals in the Earths atmosphere. These
droplets or crystals scatter the light in all directions regardless of the
wavelength making them appear white. Dark clouds are created when they pass
into the shadow of another cloud, or when the top of that cloud casts a shadow
on its own base. They also look darker in contrast to a brighter sky. So a dark
cloud will not always mean rain. Though if you are in the UK it is the most
likely result.
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| Rainbow over the University of Exeter |
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| Double rainbow over the University of Exeter |
Other than scattering and reflection, we
can also observe the refraction in the atmosphere. As larger droplets of water
bend sunlight a rainbow forms. They appear in the sky opposite the sun as the
light is bent entering water droplets in the air and then reflected inside the
back of the droplet. This refraction, like in a prism, causes the light to be
split into different wavelengths and thus different colors; red, orange,
yellow, green, blue, indigo, violet. Reflecting
the light multiple times inside the water droplet can create double rainbows,
where the order of the colors are reversed.
Although our vision is restricted to just a
tiny portion of the whole electromagnetic spectrum the wave nature of light
puts on remarkable displays for all to see. So look up and enjoy the show
because, hey, even on a cloudy day the light is dancing.
Here are just a few helpful websites and blogs that also answer these questions.
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