Single-Slit Diffraction: A Different Kind of Pattern
1 · Predict
Light passing through a SINGLE narrow slit also spreads into a pattern on a screen (diffraction), with a bright central band flanked by dimmer fringes. Does a wider slit spread this diffraction pattern more or less than a narrower slit?
- Less — a wider slit produces a narrower, more tightly confined diffraction pattern.
- A wider slit spreads the pattern more.
- Slit width doesn't affect the diffraction pattern's spread.
2 · Set Up
- Open the single-slit preset and press Reset. Light wavelength is 550 nm; the screen sits 100 cm from the slit.
- Enable the first-minimum position readout.
- Set the slit width for each trial and record the position of the first dark fringe (minimum).
3 · Collect Data
| Slit width a (µm) | First minimum position y (mm) |
|---|---|
| 20 | |
| 40 | |
| 60 |
Plot first-minimum position y (y-axis) against 1/a (x-axis) for your three trials.
4 · Analyze
- For one trial, compute y = λL/a using λ = 550 nm, L = 100 cm (converting units to get y in mm — this is the position of the FIRST dark fringe, m = 1). Compare to the table.
- Compare this formula's shape to the double-slit fringe formula, but notice it uses the single slit's WIDTH a rather than a separation between two slits. Explain why a narrower single slit produces a WIDER central diffraction pattern — counter to intuition, but consistent with the same 1/width scaling.
5 · Extend
- This same physics (narrower apertures diffract light more) sets a fundamental limit on how sharply any lens or telescope can focus or resolve fine detail — the diffraction limit. Explain why a bigger telescope aperture (like the double-slit's wider spacing) gives sharper resolving power, not blurrier.
- A single slit's diffraction pattern has one central bright band with progressively dimmer side bands; a double slit's interference pattern has many closely-spaced bright fringes of similar brightness, all riding within a broader single-slit diffraction envelope. Explain why a real double-slit experiment (with slits of nonzero width) actually shows BOTH effects combined.
The Physics Behind This Experiment
Single-Slit First Diffraction Minimum
A single slit of width a produces its first dark fringe (minimum) at angle/position y = λL/a on a screen at distance L — the SAME formula shape as double-slit fringe spacing, but governed by the slit's own width rather than a separation between two slits.