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Eye in the Sky

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Jay Mejia
Jay Mejia

The document explains why the sky appears blue during the day and red at sunrise and sunset by describing how sunlight interacts with Earth's atmosphere. Shorter blue and violet wavelengths of light scatter more through the atmosphere, making the sky appear blue. At low sun angles like sunrise and sunset, light must pass through more atmosphere, scattering away the blue light and leaving the longer red wavelengths to reach our eyes and make the sky appear red.

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Eye in the sky When astronaut Chris Hadfield was in space, did he miss sunsets? Weve all seen pictures of the sun from space and I cant help but wonder if those astronauts missed the Earths 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 couldnt stop asking these kinds of questions. And now that Im 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, 了, 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:
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). Its 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 doesnt 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, youre seeing blue light waves being reflected from the incoming sun beam by molecules in our
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 its not yellow but rather white light that shines down on your retinas. This is because youre 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 weve 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. Its 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. Thats 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 youre out for a walk look around at what you see. Science is happening everywhere. Shall I compare thee to a summers day? Please!
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/

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Eye in the Sky

  • 1. Eye in the sky When astronaut Chris Hadfield was in space, did he miss sunsets? Weve all seen pictures of the sun from space and I cant help but wonder if those astronauts missed the Earths 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 couldnt stop asking these kinds of questions. And now that Im 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, 了, 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). Its 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 doesnt 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, youre 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 its not yellow but rather white light that shines down on your retinas. This is because youre 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 weve 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. Its 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. Thats 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 youre out for a walk look around at what you see. Science is happening everywhere. Shall I compare thee to a summers 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/