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?

2 · Set Up

  1. Open the single-slit preset and press Reset. Light wavelength is 550 nm; the screen sits 100 cm from the slit.
  2. Enable the first-minimum position readout.
  3. 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)
20.00
40.00
60.00

Plot first-minimum position y (y-axis) against 1/a (x-axis) for your three trials.

4 · Analyze

  1. 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.
  2. 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

  1. 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.
  2. 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.

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