6.24 What produces the green and red flash?
A green flash is an optical phenomenon in which part of the sun changes colors from red or orange to green. This occurs at sunrise or sunset, and usually does not last for more than one or two seconds. Green flashes are a result of the dispersion of sunlight in the earth's atmosphere.
Dispersion deals with the refraction of light. More specifically it deals with white light turning into a full color spectrum via refraction. There are two different types of dispersion, waveguide and material. In this case our focus will be on material dispersion. It is important to note the material mentioned in the name “material dispersion” because the white light being refracted needs to encounter some sort of transparent material in order to refract. Any transparent material will suffice: water, glass, plastic… air. An example, and I can think of no finer example of refraction, is the Dark Side of the Moon album cover. It depicts light travelling through a prism, and on one side of the prism is white light and on the other side, the refracted light has been turned into a full color spectrum.
Refraction occurs when light encounters a material. Upon encountering a material the light undergoes a change in velocity. The degree to which the light’s velocity changes depends on the refractive index of the material it is travelling through. The refractive index is a number that describes the reduction in speed of light travelling through a material compared to the speed of light in a vacuum. The refractive index number is different for different materials (i.e. - water has a higher refractive index number than air, because light slows down more in water than air). Slower light will naturally have a reduced velocity. Thus the higher refractive index number of the material the light is passing through, the less velocity the light wave will have. An equation that describes the relationship between the velocity of a light wave, or phase velocity, and the refractive index is v= c/n where c is the speed of light and n is the refractive index quantity.
Since the phase velocity of the light wave is reduced when the light is refracted other parts of the wave are also affected. The wavelength, frequency and phase velocity are all tied together as seen in the equation λ = v/f. In this equation λ is the wavelength, v is the phase velocity, and f is the frequency of the wave. The change in phase velocity that occurs during refraction causes the wavelengths to increase or decrease, but the frequency remains constant. By looking at the above equation for wavelength it becomes obvious that if there is a decrease in phase velocity then there is going to be a decrease in wavelength.
The wavelengths that result from the refraction of sunlight in the earth’s atmosphere come in a full color spectrum. This makes since, because after all the sun produces white light and refracted sunlight produces a full color spectrum. One might expect the sky to be a rainbow of color all the time since a full color spectrum is being produced. The sky is not like this because different color waves have different wavelengths, and some wavelengths show up better in the sky than others. Some wavelengths like red and yellow are long and do not scatter particularly well. Other wavelengths like green and blue are short and scatter quite well.
The sky is actually blue during the day because blue wavelengths are short. Well, because of that and Raleigh scattering. Raleigh scattering is a type of dispersion that causes the scattering of wavelengths of light in air. Since air scatters short wavelength light more effectively than long wavelength light the blue light gets scattered the most, and causes the sky to appear blue.
Yet when evening comes our sky becomes a red or orange shade. This is because as the sun’s angle with the atmosphere changes the thickness of the atmosphere also changes, and along with it the refractive index number. During the day the sunlight is shining down onto the atmosphere. When the sunlight is coming down through the atmosphere from above the atmosphere has a lower refractive index number that allows for short wavelengths to be refracted and scattered across the sky. Light doesn't scatter through the atmosphere as well at sunrise or sunset because the sunlight is approaching the atmosphere from the side. The thicknesses of the atmosphere at this angle cause wavelengths like blue and green to get scattered on their way through. Only longer wavelengths, like red and yellow, make it through the atmosphere. Since the short wavelengths aren’t available Raleigh Scattering uses the long wavelengths to illuminate the sky.
It should be known that although the short wavelengths aren’t visible they are still present. In fact, they stay around longer than the long wavelengths because they are able to curve more. As the sun dips below the horizon the long wavelengths get blocked out by the curvature of the earth. The short wavelengths like blue and green are able to curve along with the curvature of the earth, and continue to get scattered on their path through the atmosphere after the red and yellow waves have disappeared below the horizon.
A green flash occurs when the short light waves are able to make it through the atmosphere and become visible to us. When the green flash appears the sun is below the horizon. The red and yellow wavelengths are no longer dominating the evening sky because their wavelengths do not curve with the curvature of the earth as well as blue or green wavelengths. Of course, the blue and green wavelengths are not dominating the sky either because their wavelengths are so short that they get scattered on their way through the thicker evening atmosphere. However, green wavelengths from the top of the sun are sometimes able to make it through right before the sun disappears completely below the horizon. The reason the green light waves can occasionally make it through the atmosphere at the last second is because they are the perfect combination of length and curve. Its wavelength is just long enough for it to make it through the atmosphere and be visible, and its frequency (which causes the wave to curve) is high enough that it is able to accommodate for the curvature of the earth.
Green flashes are a rare phenomenon, so to see one the conditions must be pristine. It is best to be standing at a vantage point where you can look down onto the horizon. Looking out at the horizon from a hot air balloon would be good. Standing on the shore and looking over the ocean to the horizon is an ideal place to see one as well. Beyond position, the weather needs to be clear and the air needs to be clean. Pollution and weather conditions increase the reactive index number, and make it more difficult for the light to get through.
–"Blue Sky and Rayleigh Scattering." Web. 29 Oct. 2009. http://hyperphysics.phy-astr.gsu.edu/Hbase/atmos/blusky.html
This website was very helpful to me in understanding the effects of Raleigh Scattering. It helped me realize that not only responsible for short blue wavelengths being scattered during the day, but also for the scattering of long red wavelengths in the evening. This is a university website, so I am sure it is reliable.
– Born, Max. Principles of optics electromagnetic theory of propagation, interference and diffraction of light. Cambridge: Cambridge UP, 1999. Print.
Max Born's book was invaluable in explaining refraction. I was having trouble seeing the difference between dispersion and refraction. The few pages I read from Born's book informed me that refraction is a type of dispersion, and went on to explain the difference. It had confused me for some time, so I was very grateful that his book was so informative.
–"Refractive index (physics) -- Britannica Online Encyclopedia." Encyclopedia - Britannica Online Encyclopedia. Web. 29 Oct. 2009. http://www.britannica.com/EBchecked/topic/495677/refractive-index.
This website told me what a refractive index was, how it affected light, and the different refractive index numbers for different things. The Encyclopedia Britannica is a reliable source.
–Young, Andrew T. "A Green Flash Page." San Diego State University Department of Astronomy. Web. 29 Oct. 2009. http://mintaka.sdsu.edu/GF/.
This was perhaps one of the best sources, because Young explains what a green flash is and how it occurs. He talks about mirages a lot, and I wasn't really including the mirage aspect in my paper, but his site had a host of other information and was very useful. This is another university associated website, so I am sure that the information I got off it is credible
–Wikipedia. Web. 29 Oct. 2009. http://www.wikipedia.org
I used wikipedia for a variety of information. Usually just because it was quick and easy to reference if I was looking up something specific. I know it doesn't have the greatest reputation for credibility but I checked everything, and it was good on everything I used it for.
–www.wikimedia.org
I don't know exactly how to cite this, but I got all of the photos for my webpage off of wikimedia.org. Below every photo I used it said that it was ok to use it.