The weekly rhythm¶
A laboratory period is three hours. Plan the bench work around that time; some counting experiments require longer supervised or approved unattended runs. The schedule below gives a useful rhythm for the in-room work.
Before the period (roughly one hour). Read the experiment’s Theory section and answer the Pre-lab questions in your notebook. The pre-lab is not busywork: in most of these experiments it produces the number you need in order to know whether your first reading is sensible. A student who has not computed the expected fringe spacing cannot tell a good alignment from a bad one, and will spend the period taking data from a misaligned instrument.
First twenty minutes. Set up, and take a deliberately crude measurement of the headline quantity. Compare it to your pre-lab estimate. If they disagree by more than a factor of two, something is wrong with the apparatus or with the estimate, and now — not at the end of the period — is when to find out which.
Next two hours. Take the real data. Record it in the notebook as you go, in a table you drew before you started filling it in. Photograph the apparatus. Note the ambient conditions if they can matter.
Last twenty minutes. Do a quick fit before you leave. You have the apparatus in front of you for another twenty minutes; you will not have it next week. A fit that comes out obviously wrong is a reason to take ten more minutes of data, and the whole point of fitting in the room is to give yourself that chance.
After the period. Write the report.
What you hand in¶
Two things, on different schedules:
A notebook, checked at the end of every period and graded four times a semester. It is a contemporaneous record: what you did, what you read off the instrument, what went wrong, and what you did about it. It is not a fair copy. See The Laboratory Notebook and the Report.
A report, one per experiment. Short reports are due one week after the period; full reports are due two weeks after it. Reports use the structure and the voice described in The Laboratory Notebook and the Report — the same one used in the physics literature. Three of the fourteen reports each semester are designated full reports (typically Weeks 4, 10, and 14); the rest are short reports of two to three pages that omit the introduction and literature discussion.
Reading the notation¶
Conventions used throughout
Marker | Meaning |
|---|---|
[ ] Checkpoint | Show this to the instructor before continuing, and tick the box. It is usually the point past which a misalignment becomes expensive to undo. |
A Danger admonition | A hazard that can injure you: laser light, ionizing radiation, mains voltage, hot surfaces. |
A Warning admonition | A hazard to the apparatus, or a step that quietly ruins the data. |
A Tip admonition | A technique that makes the measurement work better. |
| Python you are expected to adapt, not to run unchanged. |
Working in pairs¶
Experiments are done in pairs, and both partners must be able to explain every part of the setup. A useful discipline: swap roles at the halfway point, so that whoever has been at the keyboard goes to the optics bench and vice versa. Data are shared; analysis and writing are individual.
When something does not work¶
This is the normal condition of experimental physics, and handling it is part of what the course teaches. In order:
Check that the instrument is measuring what you think. Disconnect the physics and see whether the reading goes where it should. A photodiode that reads the same with the laser blocked is not measuring the laser.
Change one thing. Note the change in the notebook before you make it.
Estimate before you adjust. If a fringe count is off by a factor of two, the likely cause is a factor of two in the geometry, not noise.
Ask. After you have written down what you tried.
A report that documents a measurement which failed, diagnoses why, and bounds what could be concluded anyway will earn more credit than one that reports a suspiciously perfect number with no error analysis.