1. Eye in the sky
When astronaut Chris Hadfield was in
space, did he miss sunsets? We’ve all
seen pictures of the sun from space and
I can’t help but wonder if those
astronauts missed the Earth’s beautiful
blue sky or its vivid crimson sunsets.
What is it about our planet that makes
these kinds of things even possible?
When I was younger I couldn’t stop
asking these kinds of questions. And
now that I’m older and have at least
some of the answers, they make my
evening runs that much more
enjoyable.
To fully understand this phenomenon we have to tackle the problem on three different levels: the
source, the medium, and the endpoint.
A Light Source
The light on Earth comes from the Sun. This beam carries just about every existing wavelength
of light to the Earth. The light ranges from the long wavelength radio wave to the miniscule x-ray. Once
it actually passes through the atmosphere all that is left is mostly visible, white light. If we use a prism,
we can break out white light into its separate components, the colors of the rainbow. The component
colors of light have varying wavelengths with violet-indigo light being the smallest wavelength at 400
nanometers and red light being the longest at approximately 700 nanometers. In physics, the Greek
letter lambda, λ, is used to denote wavelength.
Medium
Our atmosphere is unique in our solar system. It not only protects us from harmful radiation, it
also puts on quite a color show. The composition of the atmosphere is mostly nitrogen and oxygen but it
also has significant amounts of argon, carbon dioxide, and various other particles. Though these
molecules and elements are extremely small (picometers are one 10-12 of a meter), they do have an
effect on the incoming wavelengths of light- they scatter them. The name for this effect is Rayleigh
scattering. Rayleigh scattering was discovered in 1871 by John William Strutt. The scattering causes the
wavelengths of light to be deflected at random angles by the molecules. The formula used to calculate
this scattering is as follows:

2. Though this formula is often simplified to this:
Using this formula we can determine that the light with the shortest wavelength, blue and violet light,
scatters the most whereas light on the red side of the spectrum scatters least with green light scattering
intermediately.
The Endpoint
Lastly, we have to consider the endpoint: our eyes.
Evolution has made our eyes most sensitive in identifying the
three primary colors red, blue, and green. The cells that can
detect color are called cone cells. The additional colors we see
every day are varying combinations of those three colors, the
additive colors.
Worth note are the colors of cyan (a blue and green
combination) and orange (a mostly red and partial green
combination). It’s also important to note that the absence of
color is black while the presence of all colors is white. Lastly,
we can only perceive light if it actually interacts with the
cones in our eyes.
Noon:
Noon
Sunset
atmosphere to the cones in your eyes. Interestingly, our
eyes are not very specialized at detecting purple
wavelengths. The blue-detecting cone cells however are
firing like crazy! Furthermore, the sun doesn’t display all
At noon, the sun is high in the sky. At
this point in time, there is less atmosphere for
the light to have to go through to reach you.
This means that the majority of light being
scattered is the blue and violet light with some
green and little red light. So when you go
outside and stare at the sky, you’re seeing
blue light waves being reflected from the
incoming sun beam by molecules in our

3. wavelengths equally right from the start. The light spectrum of the sun favors blue and green light.
If you decided to briefly stare at the sun, you realize very quickly it’s not yellow but rather white
light that shines down on your retinas. This is because you’re still looking at mostly all the wavelengths
of visible light together.
Evening:
Fast forward six hours and now the sun is setting. The angle of the sun is now much closer to
being parallel with your line of sight. This results in a longer travel path for the light from the sun to your
eyes. This longer path means more blue, green, and red light will scatter compared to noon with the
blue scattering away almost completely. This gives the sky a more red and orange appearance. Similarly,
the sun is mostly left with just red wavelength light resulting in the red-orange sun we’ve all been
blinded by on the drive back from work.
Clouds:
Clouds are collections of condensed water droplets and
frozen crystals. They act roughly like a prism when light reaches
them and so scatter light in all directions. This phenomenon is
termed Mie Scattering. The scattered wavelengths reach our eyes
which then perceive the presence of all colors, white!
However once you start accumulating clouds, they start
blocking the light coming from the sun especially if they are very
dense with water. Absence of light results in the dark, ominous
color we associate with rain clouds.
Conclusion:
Every day we are greeted by sunrise gracing our retinas. It’s amazing to think that something
that seems so mundane goes through all of these steps to get here and that now we know all of these
steps and even have formulas to explain it all. That’s how science starts. People observed something and
tried to find a way to explain it. But as often happens, once you have your answers you end up with
more questions. The next time you’re out for a walk look around at what you see. Science is happening
everywhere. Shall I compare thee to a summer’s day? Please!

4. References:
Books
Tour of the Electromagnetic Spectrum http://missionscience.nasa.gov/ems/TourOfEMS_Booklet_Web.pdf
Arny, T. & Schneider, S. (2014). Explorations: an introduction to astronomy. New York, NY: McGraw-Hill.
Illustrations
NASA sunrise http://blogs.discovermagazine.com/imageo/2013/07/09/some-delicious-eye-candy-from-the-spacestation/#.UxkO8_ldXuw
Color Wheel http://www.d.umn.edu/~mharvey/th1501color.html
Noon/Sunset https://www.youtube.com/watch?v=0b1fqodmZJ0
Spectrum of the Sun https://www.youtube.com/watch?v=ro2MmmdARrs
Clouds http://eo.ucar.edu/webweather/cloud3.html
Videos
Veritasium https://www.youtube.com/watch?v=ro2MmmdARrs
The Curious Engineer https://www.youtube.com/watch?v=0b1fqodmZJ0
DCaulf Science https://www.youtube.com/watch?v=Asyzw3gMfb0
NASA http://spaceplace.nasa.gov/blue-sky/en/