PC 307 - Juniata College



PC 307W Advanced Laboratory in Physics - I

Syllabus 2003

Instructors: Dr. White Office: P232

Office Hours: T, W 2-4pm Th 2-3 and any other time in the afternoons that I am available.

Office Phone: 641-3545 E-mail: white@juniata.edu

Required text: Baird, D.C. (1995). Experimentation – An Introduction To Measurement Theory and Experiment Design 3rd. Engle Cliff, New Jersey: Prentice-Hall, Inc. We will be using this text in the second half of the term.

Reference texts: See attached Bibliography

Meeting: Your attendance is required twice a week on Tuesdays and Thursdays at 9:00 pm.

Objectives

The goals of this course include the following:

• Learn how to solve problems arising in the laboratory

• Learn how to keep a laboratory notebook

• Learn how to deal with numerical data, including statistical treatment

• Learn how to write a scientific report

• Learn how to use the library and literary search engines

• Learn how to think like a scientist

• Add concrete experience to the more abstract and theoretical experiences of the classroom

• Use computers for data collection

• Gain lab-group skills

Course Description

This course is conducted almost entirely in the laboratory. For three hours of credit, it requires about 9 hours (two-three afternoons) per week in addition to the meetings twice a week. You may schedule time in the lab whenever you wish, subject to the following restriction: You must schedule 6 hours in lab within the hours of a normal work week – Monday through Friday, between the hours of 8am and 5pm. I will occasionally pop-in to observe you and check on your progress during these times. Your notebook should reflect these, and other times you spend in the lab with a recorded time (along with the date).

For the first half of the term, each group (one or two students at most) will work on developing a laser-based experiment, see list below. During this time, you will need to complete a library search, equipment design and construction, multiple oral presentations to the class on your progress, multiple individual meetings with me, a completed report (with multiple reviews and rewrites). Near the end of this time, you will step your classmates through your experiment (and they will do the same for you).

The second half of the term is more open ended. Some of you will design your own work (for example, if you are in education, you may want to develop a lab curriculum for your use as a first year teacher.) Others will want to extend one of the laser projects toward a publication. Others will be given projects to complete that help them develop specific laboratory skills.

As you read this, I suspect you have figured out that I will expect a significant amount of self-motivation, and a significant amount of work. The course will be worth nothing to you (and to me) if you are not fully engaged, learning how to work on research projects, and willing to dedicate the needed effort and time to the projects. This will either be your most important course in physics, or your least. But you determaine which.

Group A George and Chris Atomic Spectroscopy

Group B Brandon and Anthony Thermal Lensing and Beam Profiling

Group C Bryan and Peter Fluorescence depolarization

Group D Jen and Devin Holograms

Group E Russ and Aaron Laser Tweezers

Lab projects

a) Atomic Spectroscopy of Rb, using an IR diode laser – including fine structure splitting, Doppler-free spectroscopy, and Zeeman effect

b) Thermal Lensing – investigating the changes in beam width and profile after passing through a dye-tinted solvent. Beam profiling – working with the modes of a HeNe laser

c) Fluorescence depolarization – effects of viscosity and temperature.

d) Holograms – make them and show us all how.

e) Laser tweezers – The first step will be to tweek the system to trap beads. You will measure the trap strength. Then we need to figure out the next step…

Descriptions of some of these projects are included at the end of this document. This document is also on my P-drive. I also e-mailed you the descriptions at the end of last term.

During the second half of the term, I might assign some of the following experiments if you are unable to design your own projects.

I) Bouyancy

II) Brownian Motion

III) Big G

IV) Gravity using a gravometer

V) LED’s in liquid Nitrogen

VI) Resonance in a solid

VII) Millikan – charge of an electron

VIII) Air resistance – using videopoint with coffee filters

IX) Chaotic Behaviors (bouncing, bouble pendulum, water drop, chaotic circuit)

X) Cv/Cp for a gas

XI) NMR

You may repeat this course. During subsequent enrollments, you will pursue a completely new, and generally more advanced, set of experiments.

Notebook

At this point in a budding physicist's career it is necessary to begin to behave like a professional scientist and this requires among other things, the keeping of a proper notebook. This notebook should be bound, with numbered pages. Data should be entered in ink directly at the time of observation. It is intolerable for a scientist to use either memory or scraps of paper for primary data recording. You should record phenomena that you do not understand as well as those that you do. Bad experiments should be recorded as well as good. This will tell you what not to do the next time around. Simply draw an X or single line (don't scribble out, white out or otherwise obliterate) through data which you judge to be bad along with an explanation of why the data are bad. The form of entry is not like that in the introductory lab where the notebook becomes the reporting device. The purpose of the notebook here is not to generate a final report, but rather recording and note-keeping. But it should still be intelligible.

The notebook should have a table of contents on the first page with a title for each experiment which is clearly descriptive of the measurement being performed. You may integrate experimental procedure with the taking of data since the book is a record of what you actually do, not necessarily what you intend to do. Each set of data should be dated. You may also use the notebook in the library to take notes from relevant sources if the information is crucial to the conduct of the experiment. Entries into the notebook are normally in chronological order independent of what kind of entry they are. This means that one experiment may continue in several pieces throughout the notebook. It also implies that there will be no blank pages. A relevant derivation may be placed before or after the data. You may put computation of results in the notebook unless the computations are done by computer, in which case you may tape the computer printout and code listing into the book if it is not too bulky. You should summarize results in tables, graphs or whatever form is appropriate for the measurement at hand. Tables should have units and titles.

Graphs may be done roughly in the notebook, but where good graphs are required, they should be done on the computer and then pasted in. Although computer generated graphs are permissible, one should be aware that using automated axis settings often leads to odd sized numbers along the axes, an undesirable situation. Graphs must have titles and the axes must be labeled. All additions to your notebook must be firmly attached! Anything that falls out of the notebook is forever gone and not considered a permanently part of your records.

For each experiment there should be sufficient references to the apparatus that you could come back to your notebook next semester and reproduce the experiment. This may mean, on accurate measurements, identifying the make, model, and serial number of critical equipment. If you do not refer to another source for your apparatus set-up, you will need to describe the apparatus in some detail. As you modify the apparatus, describe the modifications immediately in the notebook. Sketches or drawings are essential.

You should describe, with references, the relevant theory underlying each experiment. Number equations, and derive important ones.

The notebook should include a summary of results and a critical discussion of random and systematic errors, as well as a quantitative portrayal of the precision of the measurement. For experiments which require the taking of repeated data (probably most of them), you should give standard deviations along with the average measurement. Least squares straight lines and curve fitting where appropriate can be done with computer. Systematic errors are to be identified specifically and quantitatively, not with vague references to such imprecise notions as "instrumental error" or "human error." For example, you could state that you could read the scale to 0.1 division where the divisions were 1 mm. Then you could say that the scale was calibrated by comparison to such and such a standard. Finally, a brief conclusion can be put into the notebook the function of which is to remind you what would be discussed in great detail in a report.

There are books available which discuss laboratory notebooks if you need additional information. See the bibliography below.

Every week you are required to e-mail a page summary of the work you did. A copy should also be taped into your book.

Reports

For two of the experiments, you should write a major report in the format of a journal submission. See The AIP Style Manual () or for a shorter description of acceptable physics formats, see the back pages of any issue of the The Journal of Undergraduate Research in Physics (issues are in the lounge) . These must be in formal form, and (at least) one submitted before midterm. The remainder must be completely finished with at least one re-write by the end of classes at the end of the semester. More on deadlines will be given through out the term.

These reports should include the following: Title and author, abstract, theoretical/historical introduction, experimental description, computation of results, presentation of results in tables, graphs or whatever is best, along with statistical analysis, and a critical conclusion or discussion which goes into detail on the accuracy and precision of the measurement and identifies its significance. References should be placed in endnotes and there should be acknowledgment of any assistance you have had from other people. The form of the report is to be that of a physics article as submitted to a journal. It is useful simply to read several papers in physics archival journals to see the style used in physics report writing. I will also ask for two oral presentation of the results of your experiments.

In the theoretical/historical introduction to the experiment being reported I expect to see several references to similar type work that you have found in the library. This is not only a writing course to help you practice scientific writing, but it is a course which takes advantage of the physics journal literature to make connections between what you are doing and what has been done previously. I expect you to use the electronic catalog and to do electronic searches of the literature to find relevant references.

The reports will be graded not only for physical content, but also for proper scientific writing style. This means not only correct grammar, spelling, punctuation, but attention to audience, voice and other aspects of style appropriate to scientific writing. Scientific reports represent a literary genre as do works of fiction, magazine articles, or poems. There are conventions to follow which makes a person used to doing science comfortable about reading the report. If a report came in rather folksy, it would be rejected by the reviewers before it got into the journal just as a scientific report would be rejected in an anthology of poetry. The best way to latch onto the conventions of scientific writing is to peruse articles in scientific journals. In regards to the proper format for journal submissions, refer to The AIP Style Manual and the article by Adelberger listed in the Bibliography.

Grading

Formal paper #1 15

Timely submission of first draft

Quality of first draft

Timely submission of final draft

Quality of second draft

Formal paper #1 15

Timely submission of first draft

Quality of first draft

Timely submission of final draft

Quality of second draft

Weekly 1 page summaries that are e-mailed by Friday 3pm 14

Lab notebook completeness and quality 10

Lab cleanup

Individual work space 5

Total lab appearance 5

Five formal and informal presentations to the class 13

Test on our text 8

Overal lab participation/conduct/appropriateness/success 15

Collaboration

Students may work together to do experiments when it is advantageous to do so. Students are individually responsible for notebooks and reports.

Academic Honesty

The usual policies given online apply to this course. In particular, students may not use notebooks or reports of previous students in the course without the permission of the instructor.

Bibliography

1. Adelberger, R. E., Preparing a Manuscript for Publication, The Journal of Undergraduate Research in Physics (last page of each issue)

2. AIP Style Manual

3. Baird, D. C., Experimentation: An Introduction to Measurement Theory and Experiment Design, Prentice-Hall, 1962

4. Braddick, H. J. J., The Physics of Experimental Method, Wiley, 1954

5. Wilson, E. B., An Introduction to Scientific Research, McGraw-Hill, 1952

6. Cook, N. H. and Rabinowicz, E., Physical Measurement and Analysis, Addison-Wesley, 1963

7. Dunlap, R. A., Experimental Physics: Modern Methods, Oxford University Press, 1988

8. Melissinos, A. C., Experiments in Modern Physics, Academic Press, 1966

9. Barlow, R. J., Statistics: A Guide to the Use of Statistical Methods in the Physical Sciences, Wiley, 1989

10. Lichten, W., Data and Error Analysis in the Introductory Physics Laboratory, Allyn and Bacon, 1988

11. Lyons, L., A Practical Guide to Data Analysis for Physical Science Students, Cambridge University Press, 1991

12. Pugh, E. M. and Winslow, G. H., The Analysis of Physical Measurements, Addison-Wesley, 1966

13. Topping, J., Errors of Observation and Their Treatment, Reinhold, 1957

14. Young, H. D., Statistical Treatment of Experimental Data, McGraw-Hill, 1962

Revised: January, 2003

Some of the experiments (from an early draft of my latest grant proposal…sorry I could not get the latest draft back from the grant office files yet…sorry for any misspellings, etc.)

The effects of viscosity and temperature on the fluorescence depolarization

Electron transitions can be induced by illuminating a molecule (of an appropriate fluorescent sample) with light of both the correct frequency and polarization. If the polarization of the incoming light is perfectly perpendicular to the transition dipole of the molecule, the transition will not occur even if the transitional energy is perfectly matched. The probability of absorption of photons relates to the vector component of the transition dipole in the plane of polarization. When inducing a transition to a higher energy state by shining linearly-polarized light on the sample, we would expect the photons released due to the decay of these excited states also to exhibit polarization that is correlated with the orientation of the molecule. That the polarization of the incident and fluorescent light are directly related is the basis of fluorescent-depolarization spectroscopy.12,20

In the experiment, a student shines polarized laser light into a solution of fluorescent organic dye. The intensities of the fluorescence (both linearly polarized parallel, I((, and perpendicular, I( ) are measured at a 90( angle to the incident light. These intensities are used to calculate the anisotropy:

Anisotropy = r = [I((– I(]/[I((+2 I(]

If the fluorescence molecules are locked in position and orientation for the complete period of time between the absorption and emission of the photon (the excited state lifetime), the anisotropy will be 0.40. If tumbling is sufficiently rapid and random, r = 0. By varying the viscosity of the solvent and thereby varying the amount of tumbling of the dye molecules, we can probe the excited state lifetime by measuring the change in anisotropy. We have tested this experiment with three groups of students by having them study at the effects of various glycerin/alcohol mixtures on the depolarization of cresyl violet perchlorate using a HeNe laser with polarizing films. Despite the make-shift optical bench and an unstable laser, both physics students and chemistry students had some success with the experiment. To implement this experiment for the entire classes, we propose to use a more stable laser (LCM-T-11ccs at 532 nm, Power Technology, Inc.) and to have students investigate other fluorescent dyes (e.g., Rhodamine-560) composed of “stiffer” molecules that exhibit reduced bending effects. We will also introduce a chopper and lock-in amplifier to improve the signal-to-noise ratio dramatically.

In addition to determining viscosity effects on fluorescent depolarization, we will also use this technique to study glass transitions. In this case, a dye is placed in a material that exhibits such a transition (e.g., glycerin or some types of alcohol). As we lower the temperature, the dye becomes increasingly locked in place and orientation. The transition will be probed by measuring the anisotropy.

a) Atomic spectroscopy of Rb, using a IR diode laser – including fine structure splitting, Doppler-free spectroscopy, Zeeman effect, and two-photon spectroscopy.

Atomic structure, including hyperfine structure and Zeeman effect, are thoroughly discussed in Modern Physics (PC300) as students are introduced to quantum mechanics (Schrödinger wave mechanics) and calculations for the hydrogen atom. As a follow-up to this course, Advanced Physics Laboratory (PC307) students will use the diode laser system characterized by the student to measure hyperfine splitting in the 5S1/2 state of Rb by making a resonance between the 5S1/2 and 5P3/2 states.9,14 By varying the current supplied to the diode laser so that it sweeps wavelengths around the 780 nm transition, students can either detect the increase in fluorescence signal emitted perpendicular to the laser beam from the Rb gas cell or measure the decrease in transmitted laser light through the cell. Both of these measurements can be made with a pin diode detector as well as by an IR sensitive camera (security camera). The students will be able to make semi-classical calculations for this splitting for comparison. They will also see the effects of isotopes since our Rb cells have both of the naturally occurring isotopes, 87Rb and 85Rb. Doppler broadening of the fluorescence can be eliminated by reflecting the beam back on itself or using a “pump” and a second “probe” laser.6,9,22 Accurate measurements of the laser wavelengths will be made using a spectrum analyzer and computer-interfaced digital oscilloscope. A second diode-laser system will also improve the Doppler-free experiment.

Two extension projects that will be built around the Rb spectroscopy equipment are (1) the use of an external magnetic field to measure the Zeeman effect on the transitions, and (2) two-photon spectroscopy. The Physics department has a large, water-cooled electromagnet, so Zeeman effect experiments can easily be accomplished with only a small investment in purchase of a precise gauss meter. Two-photon experiments are possible for the same Rb system.23,24 (See Ryan 1993, Liu 1995) Two separate IR diode lasers at 795 and 776 nm can be used to excite Rb atoms through a two-step process from 5S1/2 to the 5D5/2 via 5P3/2. One possible relaxation route for the 5D5/2 will produce a visible 420 nm. The two-photon experiment requires a second diode laser.

b) Thermal Lensing – investigating the changes in beam width and profile after passing through a dye-tainted solvent.

As laser light travels through a material, some of the photons are absorbed. Heating due to the absorption is not equally distributed in the material because of the non-uniform intensity profile of the laser beam. This heating causes changes in the index of refraction of the material. Since the heating is not uniform, the change in the index of refraction is also non-uniform. Thus, sample acts as a lens. Since the index of refraction typically decreases with heating, and since the intensity of a laser beam is typically largest in the center of the beam, the material acts as a diverging lens. Therefore, the laser beam spreads out, creating a larger transmitted spot size. By chopping the laser beam prior to sending it through the lensing solvent, the spread of the beam can be monitored with a photodiode and a triggered oscilloscope. When one assumes a simple cross sectional geometry for the beam, typically gaussian, and one measures the growth of the lens as a function of time, information about the material can be calculated.

This lensing effect is often used to calculate heat capacity for different solvents and as an introduction to the use of lasers prior to using them in more complicated experimental situations.2,13,25 Although this experiment is a good introduction for students into the use of lasers, laser mounts, and time-resolved detection, there is a crucial step in the procedure that resembles witchcraft (no offense to referenced authors intended). This step involves using the known heat capacity of one solvent to work backwards through the experiment to calculate the width of the specific laser beam used in the laboratory. Then, with that calculated width, one can proceed with measuring the specific heat of another, unknown solvent. This extra step has introduced large amounts of error in this experiment – so much that physical chemistry students should question the usefulness of this technique when there are more straightforward methods of measuring heat capacity of a solvent. At Juniata, we hope to bypass this extra step, making the laboratory exercises more appealing to physical chemistry and physics students alike. By using a beam-profiler, a diode array-based instrument used to measure beam widths and intensity profiles across beam’s cross-section, students can measurement the beam width directly. Student will also be able to study the change in the beam’s during lensing. Although we know that thermal lensing has been implemented in many undergraduate programs, we have found no mention in the literature of using a beam profiler to improve accuracy. This addition will increase the relevance to physics students who have an interest in the optical effect for its own sake and may yield a new experimental design, worthy of publication.

We have already acquired a commercial beam profiler from Thorlabs. In addition to this thrmal lensing experiment, the beam profiler will be used to measure and fit to theory different transverse modes of a HeNe laser in the Modern Physics course (PC300). This past spring, an Advanced Physics Lab student briefly tested the new beam profiler for use in the thermal lensing experiment. Though these tests were far from extensive, the approach does look promising. Since we have already bought the most expensive part of this experiment, all that we need for implementation for the full class is an optical bench, basic lenses and holders, a thin cuvette to hold the lensing solvent, a cuvette holder and a stable laser. The laser that is presently used to test the feasibility of this experiment is a surplus tube bought from a discount laser supplier. It has been seen to be very unstable and therefore increases errors in this experiment.

Laser Tweezers: See

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