Young's Double Slit: Measuring Light's Wavelength

1 · Predict

Light passing through two closely-spaced slits creates a pattern of bright and dark fringes on a screen. Does moving the slits closer together make the fringes more tightly or more widely spaced?

• Narrower spacing for closer slits.
• Fringe spacing doesn't depend on slit separation.
• Wider — smaller slit separation spreads the fringe pattern out more.

2 · Set Up

1. Open the young-double-slit preset and press Reset. Light wavelength is 550 nm; the screen sits 100 cm from the slits.
2. Enable the fringe-spacing readout.
3. Set the slit separation for each trial and record the resulting fringe spacing on the screen.

3 · Collect Data

Plot fringe spacing y (y-axis) against 1/d (x-axis) for your three trials. Is the line straight through the origin?

4 · Analyze

1. For one trial, compute y = λL/d using λ = 550 nm, L = 100 cm (converting units carefully to get y in mm). Compare to the table.
2. Explain, using y = λL/d, why smaller slit separation d produces WIDER-spaced fringes — the opposite of what you might first guess.

5 · Extend

1. This experiment can run in reverse: measure the fringe spacing, slit separation, and screen distance, then solve for λ. This is how physicists first precisely measured the wavelength of visible light. Explain why a very small slit separation d (compared to visible-light wavelengths) is necessary to produce a fringe pattern wide enough to see and measure.
2. Young's original experiment needed the light from both slits to be coherent (a fixed phase relationship) to produce a stable interference pattern — using a single small source illuminating both slits, rather than two separate lamps. Why wouldn't two independent, uncorrelated light sources produce a stable, visible fringe pattern?

The Physics Behind This Experiment

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