Lucien's blog

Lucien Rouse
Mr. Wirth
Regents Physics
2 March 2, 2008
Light’s Behavior in the Sky
We all know that a beam of light is typically white. So why then, does the sky appear blue when there is only atmosphere between the sun and us, and, even more confusing, is why does the sky change colors during the sunset? How do rainbows materialize when the sun reappears after a shower? Lastly, what causes mirages, a trick of the mind, or something more realistic? Simple wave characteristics can be used to explain all of these phenomena.
When a child asks why the sky is blue, we typically answer that, “it just is.” What the child really should be told is that it is because of earth’s atmosphere. Earths atmosphere is made up of roughly seventy eight percent nitrogen, twenty one percent oxygen, and about one percent of other various gases; most prevalent among them is argon. Although the atmosphere seems invisible to the human eye, it actually absorbs about sixty percent of visible light. Here is where it gets back to the color of the sky. When solar radiation from the sun strikes the atmosphere, waves with a lower wavelength get absorbed. So particles in the atmosphere absorb the lower wavelength blue waves and the higher wavelength red waves pass through. The low wavelength waves are absorbed by the gas molecules in the atmosphere, then scattered. So the blue light is scattered, in all directions and whenever we look up, the sky appears blue. For the same reason, the horizon appears lighter. The blue light travels further, and passes through even more atmospheric particles, and is thus scattered ever more. Rayleigh scattering, as well as Mie Theory, explains the phenomena perfectly.
http://www.sciencemadesimple.com/sky_blue.html
At sunset, the sky appears red because the sun is almost tangent to the earth’s surface. Blue wavelengths are still scattered, but the light has to travel further to reach you, so even more of it is scattered, and only red light reaches you directly, so the sun appears less bright, and more red. The sky around it changes colors if there are a lot of water or dust particles in the air. They reflect the sun’s light in all different directions, and again, the blue light is scattered, so red light reaches the observer.
http://www.sciencemadesimple.com/sky_blue.html

The next question is about how rainbows are formed. The answer has to do with water particles in the air, and how light reacts to them. When white light hits a water particle, it is refracted and dispersed at entry, reflected by the back of the water particle, and refracted a second time when exiting. The result is that the different wavelengths of light are spread out, forming the red, orange, yellow, green, blue, indigo, and violet arc.
http://en.wikipedia.org/wiki/Rainbow

No matter what part of the rainbow you look at, that part of the rainbow is at forty-two degrees from your horizontal viewpoint and that part of the rainbow. This is because the light ray is refracted twice and reflected once, so the rainbow exits the droplet at a one hundred and thirty eight degree difference than it’s starting direction. One hundred and eighty minus one hundred and thirty eight equals forty-two, so the resulting rainbow that we see is always forty-two degrees off of lights direct path towards us.
The last question is about mirages. The typical thinking is that it is the delusional hallucination of dehydrated travelers, but in reality, anyone can see a mirage. Mirages occur when there is a severe difference in the temperature of different mediums that light pass through. For example, the ground near a road is often much warmer than the air above it. Light travels slower through warmer air, so, therefore, when light travels from cold air to warmer air, it will refract away from the temperature gradient. This is called an inferior image, and it is the type of mirage that makes the sky appear to be on the ground.




http://en.wikipedia.org/wiki/Mirage

By contrast, when light goes from hot air to cold air, a superior image is produced. These mirages are more complicated (they can be upside down or right side up) but they are basically the result of light being bent towards the temperature gradient, and then going through other stages. Superior images are only interesting because of the earth’s spherical shape, and they are what allow islands to appear closer to the shore than they really are. Mirages can only occur when the temperature difference is at least two degrees Celsius, but they are most clear with about a four and a half degree Celsius difference. Mirages are real enough even to be photographed.
Despite the common answers to these even more common questions, each of these phenomena is a result of light waves and the mediums that it travels through. It all has to do with wave characteristics, the very same ones that we are studying in Physics right now.




Work Cited

"About Rainbows." Aug. 2005. University Corperation for Atmospheric Reseach. 2 Mar. 2008 .

"Why is the Sky Blue?" Science Made Symple. 1997. 2 Mar. 2008 . Young, Andrew T. "An Introduction to Mirages." 1999. 2 Mar. 2008 .