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Week of June 9th, 2003This week was our first week here in the REU Physics program. We got acquainted with the laser teaching center. I am unsure what my research project will be this summer. It seems like I have many interests, however I am not sure how to turn them into projects. This first week was really an introductory week. I got familiar with the workings of the computer program Unix and learned how to manipulate the Unix system. Doctor Noe showed us different interesting things in the lab, like the effect a polarizer had on various types of light, such as lasers and incandescent bulbs. We also viewed different light sources through diffraction gratings.
I started a "mini"
experiment with a few of the other students in the lab. We played
around with the properties of two different lenses: a magnifying lens
and reading glasses. We brought the two lenses outside (on a sunny
day) and burnt holes in a piece of paper. Believe it or not burning
holes in paper did serve a scientific purpose; when the sunlight came
to a sharp focus and began burning the paper, the lenses distance from
the paper was the exact focal length of the lens. Therefore, by
measuring the distance at which the spot of sunlight came to it's
sharpest focus we could find the lenses focal length.
Week of June 16th, 2003The lens "mini" experiment that we started working on last week took much of my focus this week. The other two students that I was working with on the experiment last week broke off and worked on their own projects. The lens experiment, then, really became my own. Before I went further, though, I wanted to understand more about the way light moved through a lens. To do this I used a large workbench and set up a point source (which was a flash light) at one end and a screen at the other. I placed various types of lenses in between the source and screen and varied the distance of the source and lens from the screen. From this I found the focal length from the Thin-Lens Equation.
The Thin-Lens Equation enabled me to predict various kinds of information
from the orientations of the lens, screen and source, such as the
minimum lens-source or lens-screen distance to clearly view an image.
Before I really started working on the lens experiment I wanted to
clarify a few questions that I hoped to answer by the time everything
was done: why did paper burn under the lenses, or if it did not, why?;
what were the principles involved in the burning process; and how did
the size of the spot produced by the lens measure into everything?
Week of June 23rd, 2003I spent a great deal of this week outside taking down measurements and calculating various data. I used four lenses (three magnifying lenses and a pair of reading glasses) and projected the Sun onto a paper screen. It was difficult to record measurements because the spot was so bright, so I used two polarizing sheets to cut down the intensity at the spot. I recorded the lenses distance from the screen, the size or diameter of the lens and spot and eventually found thier areas.
I learned about the Sun's energy flux density or irradiance, which is
the power per unit area produced by the Sun. The solar irradiance
observed at the Earth's surface is approximately 1 kW/m2.
The paper screen does not spontaniously burn in the Sun, however it
does burn under a lens. The power per unit area at the paper must be
increased under the lenses and therefore the irradiance at the spot
must be larger than that when there is no lens present. With this
known, it took me a very long time, though, to understand the
relationship between the irradiance at the spot and the corresponding
areas of the lens and spot. A good way to visualize this is that the
lens is the large opening of a funnel and the spot is the small end of
it. The irradiance at the spot differs from that at the lens in that
at the lens the irradiance is simply the solar energy flux, while at
the spot it is the ratio of lens area to spot area.
Week of June 30th, 2003I pretty much started wrapping up the lens experiment this week and continued to preform a few measurements here and there. I put much of my observations on the web. I found that there was a minimum irradiance needed in order for the paper screen to burn. This minimum value was 43.6 kW/m2. Experimenting with the lenses I saw that the magnifying lenses burnt the screen but the reading glasses did not. Through my measurements I noticed that the reading glasses irradiance was less than the minimum and the magnifying lenses were much higher, about 27 kW/m2 and between 450 and 550 kW/m2 respectively.
I also found that if something was known about the lens being used and its relation to the Sun, such as its focal length and its distance from the Sun, it was possible to predict the size of the spot that would be seen on the screen. To predict the spot size all that we needed was the images distance (focal length), objects distance (the Sun to Earth distance), and the objects height or size, which was the size of the Sun. Comparing our predicted spot sizes to the actual ones I saw that they agreed fairly well with only slight discrepancies, which were caused by a few different reasons including lens spherical abberration and difficulty in viewing the bright spot completely acurrately.
Professor Metcalf also
came back from his spring visiting professorship in Holland this week.
Him, Dr. Noe and the rest of the group working in the Laser Teaching
Center got together and discussed our research and how things were
progressing. Since I was coming to an end with my "mini" project, I
asked him for some advice or suggestions on further work in similar
areas. Dr. Noe recommened the topic of solar power and energy, which
tied in the whole concept of solar irradiance that I was working on.
Professor Metcalf advised me to investigate reflectors, such as the
ones on the backs of bycicles or highway signs.
Week of July 7th, 2003Since I have always wondered about and been amazed by solar power, I decided to research about solar cells and solar energy before I went on to reflectors. I learned the basic principles and underlying concepts of how the Sun's radiation is harnessed as an energy source, however I found it quite difficult to think up a project in this area. My interest with physics lies in the actual real life application of it, yet I did not want to build a solar powered device because simply doing that would not involve the level of research that I was looking for. Even though it was related to my earlier work and I had an interest in the subject, I decided to drop the topic and went on to reflectors.
This subject was very interesting as well and really sparked questions that I have been thinking about for a long time. To see a reflective surface in work is really quite amazing; these three pictures, picture 1, picture 2, picture 3 are from an experiment that a previous student of the Laser Teaching Center was working on. The last two photos are taken at a distance in a dimly litten hallway and the reason that the reflector is bright is because it is picking up the light from the camaras flash. Through my brief investigation of reflective surfaces I learned the geometry that is integral for an understanding in this subject, for instance the way light can be incident on the surface from any angle and still reflect back to the source. The geometric design that is responsible for this process is called a corner cube.
Once I began to understand the workings of the corner cube, though,
Dr. Noe proposed an idea; his former colleague was working on a
project involving laser drilling and engraving that might be similar
to what I was originally researching on. Dr. Noe's associate, Richard
Migliaccio, is president of East Coast Optical
Technologies, an optical design, measurement and analysis
corperation. One of his current projects is in the field of laser
engraving. He is researching materials by shinning a powerful laser
beam on them and comparing the amount that is scattered with what is
absorbed, eventually finding one material that has the maximum
absorption. Laser drilling and engraving is desirable because it is a
much faster process than conventional methods, almost ten times
quicker than when using traditional means. Drill bits or parts never
wear down or need to be replaced when using the laser process, making
the overall process cheaper and a much more financially favorable
investment for the company.
|Thomas Cummings||Summer 2003|
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