May 11, 2004

My report is coming along. All the information is there but I just need to make it more visually appealing. I have learned how to create tables, put in links, and a wide variety of font manipulating techniques. This is the home stretch. I hope to have a finished product by the end of tomorrow.

May 3, 2004

I fine tuned my graphs today and Dr. Noé and I placed them on the web. I now need to finish and fine tune my report so that it is a complete and honorable piece of work that fully explains the plots I created.

April 29, 2004

I finally worked out all of the bugs in the spreadsheet equations and was actually able to have my plots match up exactly to those published by Paul D. Huibers and Dinesh O. Shah in 1997. Although their work was experimental in determining the thickness of a soap film using a spectrophotometer, my predictions matched their findings exactly. My report is finally starting to take shape. I have to incorporate some new information which contradicts old information, as well as build on what I have now.

April 28, 2004

I have stepped it up a notch today in learning how to work with the spreadsheet as well as navigate through the linux system. I am now able to work self-sufficiently in editing my webpage as well as manipulating equations using Quattro Pro to help create more precise plots. Becoming increasingly familiar with these programs has helped me concentrate more on the research itself without being sidetracked with technical difficulties.

Since I have been able to concentrate more on research details, I came across some new information. The 'reflectance' is not always the same according to Fresnel equations. This will change my data on the spreadsheet which I hope perfect by tomorrow.

April 22, 2004

I feel that I ran into a wall today with my research. I have been trying to create a plot using a spreadsheet (Quattro Pro) that shows the intensities of superimposed reflected rays as a function of their wavelength. If done correctly, this would result in a plot of relative thickness. However in creating the spreadsheet, I am having difficulty in finding the correct command to create a column of wavelengths in correct increments. The plot that I created looks just like the old plot of Intensity vs. Thickness. I asked for assistance from Jose, a soon-to-be graduate student who does research and helps out in the Center. We saved what information I had and transferred it over to Microsoft Excel thinking it might be easier to create a plot on a newer program but to no avail. Hopefully this will be cleared up on Monday when Dr. Noé returns.

April 21, 2004

Today I met with my program director Linda Padwa. With Dr. Noé being absent this week, I felt it was a good time to work on things such as my writing skills to help my research look more presentable. The main issue that Dr. Noé wanted me to address was the fact that my journal entries seemed to have characteristics of both a journal and a report. During the hours I spent with Linda, we were able to deconstruct my journal/report into its two proper components. As I briefly described the research I was conducting with Dr. Noé, Linda helped me channel those thoughts into a more meaningful and self expressed journal as well as a more technical, research based report. It has been made clear to me now that, these two components are better understood when split into two separate items. I hope that in light of this, my final work at the end of the semester will be more straight forward and understandable.

March 25, 2004

Many things have come to light in my recent research. This project has become increasingly complex as I have been pulling information from many sources to help put together a quantitative, clear focus on what it is I am trying to achieve. Thin films have an extraordinarily wide range of applications and trying to link together information for my research has become quite difficult.

During my last meeting with Dr. Noé in the Laser Teaching Center, we gathered my research data and my notes from journal articles, etc. We were looking for a way to link my observations with theory to help guide my research. Thus far I have been able to explain constructive and destructive interference of a thin film by studying a soap bubble. In previous journal entries I have discussed why there is an initial black band at the top of the bubble, i.e., because of its minute thickness. As gravity pulls downward on the water molecules of the soap bubble, the bubble itself becomes `wedge-like' in shape, getting thinner at the top as time passes. Therefore, the black band at the top increases in size until the bubble becomes too thin to support itself and pops.

I soon hope to be able to construct a 3-dimensional graph that will show the intensities of different colors spanning the entire spectrum of visible light as a function of thickness.

March 21, 2004

My recent research has led me to delve deeper into the topic of thin films. In trying to explain the black-band phenomenon on a soap bubble, I found myself reaching in many directions for information. What seemed to be a relatively simple physical demonstration has turned into a research project linking together wave phenomena, optics, thin films and many other realms of physics.

One of the most important new things I've learned about is the phase shift that sometimes occurs when light reflects off a surface. When light traveling through air strikes a surface with a higher index of refraction, there is a 180° phase shift upon reflection. One the other hand, if light reflects off of a surface while moving from a higher index of refraction to a lower one, there is no phase shift.

The black band at the top of a thin soap bubble is explained by these two types of phase shifts. When the thin layer of soap becomes much smaller than the wavelength of the light (this would be much smaller than 400 nm), the pathlength difference between the front and back surfaces becomes nearly negligible. So we just have the two reflections: the reflection on the outer surface (air/bubble), yields a 180° phase shift while the reflection on the inner surface (bubble/air) does not have a phase shift. Since the path length difference between the two surfaces is negligible we have near complete destructive interference when the two reflected waves recombine outside of the bubble. Hence the blackness we see at the top of the bubble.

March 3, 2004

I spent time today focusing on the 'black band' at the top of a soap bubble. I knew that it must deal with destructive interference in order for for us to perceive blackness when we know that something is physically present. I therefore did some research on what is actually happening at the outer surface of the soap bubble as well as the inner surface. I knew that light rays are reflected from both the inner surface as well as the outer surface, however they do not reflect in the same fashion. I soon learned that the vibrations of the light waves can change direction upon reflection depending on the change in medium. In dealing with the soap bubble, we have light rays emanating through air and reflecting off of a surface mostly made of water. When this happens, the vibration of the reflected wave is in reverse (180 degrees out of phase). On the other hand, when the rays are reflected off of the inner surface (water/air interface), the wave stays in phase. This was the clue I needed to understand why there is near total destructive interference. As a result of gravitational effects, the thickness at the top of the bubble becomes very thin. When the bubble thickness becomes much less than the wavelength of the light rays, the reflected light rays from the outer surface and inner surface completely destructively interfere giving the appearance of blackness.

I also researched what happens below this black band where the thickness of the soap bubble is much thicker. Below the black band is what people normally see as color bands or spectrums. However we do not always see the entire color spectrum. It turns out that when the two reflected rays interfere constructively, we see the entire color spectrum. When there is destructive interference, a wavelength is canceled out. This wavelength corresponds to a specific color in the spectrum and therefore this color is not seen by the eye upon reflection. What we do see is the color complimentary to the canceled color in its place.

I began wondering why we see many color bands as opposed to one color spectrum. The bands seemed to be thicker at the top of the bubble. This phenomena must have something to do with the thickness of the bubble. Initial thoughts on this seem to be that when the thickness of the bubble becomes thick enough so that another entire wavelength of light can enter and be reflected, we see another band. As for the thickness of the bands themselves, I believe it has to do with the rate at which the bubble becomes thicker. I hope to better explain these phenomena better in the near future.

February 25, 2004

Today was my first day of observations. I set out to observe two phenomena. One of them was the polarization of light as it reflects off of different materials. By using a polarizer, Dr. Noé and I set out to observe natural light and artificial light reflecting off of different materials from different angles. It became apparent very quickly that darker objects polarize light better than lighter objects. This can be shown simply by observing light being reflected off of a shiny plastic plate versus a white piece of paper. However while I was outside, I began looking at some of the different features of the campus buildings. The dark brown weather stripping made mostly of aluminum did not polarize the light so well. This caused me to think that the composition of a material can effect its ability to polarize light. In thinking this way, conductors would be 'bad polarizers' and insulators would polarize light better.

I began next to examine the angles at which I was observing the reflected light from the light source. I first noticed how much of a difference the angle of reflection can affect the polarization of light when I was observing the metal trim on a doorway and the tile wall directly next to it. The metal trim was polarizing the light better than the tile which did not agree with my observations previously. I then took step to the left and the light being reflected off of the tile was almost completely polarized.

I therefore came to the conclusion today that the angle of reflection and chemical composition of a material are key factors in determining its ability to polarize light.

The second phenomena I observed today was the appearance of a black layer at the top of a soap bubble formed on the rim of a glycerin bottle. Below this black layer were the usual spectral color bands that we normally see in bubbles, however this black layer at the top puzzled me. After visualizing a side view of what the bubble layer looked like (like a wedge), I realized that gravity is pulling down on the soapy bubble and therefore the thickness of the bubble is thiner at the top. When the bubble layer becomes thin enough at the top so that its thickness is thinner than the wavelength of light going through it, there is complete destructive interference and we see black. Soon I hope to mathematically show how this works.

Eric Tompkins
March 2004
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