Verify the path-difference condition at a bright and a dark fringe
Three steps, the way the exam actually works: work through the lab, write down your own measurements, then answer a 6-point free response. What you recorded goes to the grader with your writing, so a conclusion that does not follow from your own numbers will cost you the point — exactly as it would with a real reader.
Predict before you look
- Two sources emit in phase. What is the only thing that can put the arriving waves out of step at a distant point?
- A crest arriving with a crest reinforces; a crest arriving with a trough cancels. In wavelengths, what path difference produces each?
Nothing to submit here — these are to think through, so the prediction below is an informed one rather than a guess.
Commit to an answer now. It is not graded and being wrong costs nothing — the point is to have something specific to reconcile against once you have the data.
Answer every prediction to unlock the lab. A sentence is enough.
Run the investigation
Predictions first
The procedure and the simulation unlock once you have committed above. Observing before predicting is how a wrong intuition survives a lab intact.
Record what you measured
These are your numbers, not ours. The grader sees them, so your conclusions have to follow from what you actually recorded.
| Path difference on the central bright line | λ |
|---|---|
| Path difference on the first off-axis bright line | λ |
| Path difference on the first dark line | λ |
| Fringe spacing at d = 8.0 cm | cm |
| Fringe spacing at d = 4.0 cm | cm |
Answer the free response
Interference conditions are statements about path difference, and this lab lets you measure it directly. (a) Using your three probe readings, state the path-difference condition satisfied at a bright fringe and at a dark fringe, in terms of the wavelength. Cite your recorded values. (b) Explain physically why those particular path differences produce reinforcement and cancellation. (c) Using your two fringe-spacing readings, determine how the spacing depends on the source separation, and reconcile that with the relationship Δy = λL/d. (d) Predict what would happen to the fringe spacing if the wavelength were doubled while the separation stayed at 8.0 cm, then state what would happen to the pattern if one source were switched off, naming the condition satisfied at the dark fringes of the new pattern.
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