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Michelson Interferometer: Wavelength from Fringe Count

Unknown monochromatic source. Change the optical path difference, count centre-spot fringe changes, and recover λ from a fit of N versus Δ.

University / research· 24 min·Related simulator: Optics & LightMichelson Interferometer

Goal

Determine λ from N = Δ/λ. A plot of fringe count N versus path difference Δ has slope 1/λ.

Equipment

  • Michelson interferometer
  • Unknown monochromatic source
  • Path-difference micrometer
  • Centre photodiode

Experiment

Theory

A bright centre returns when Δ = mλ, so the number of fringe changes from a chosen origin is N = Δ/λ. The bench hides λ, Δ/λ and any I(Δ) period marker. A live centre intensity is the instrument — you count the brights yourself.

Procedure

  1. Start near Δ = 0. The source wavelength is hidden. You change the optical path difference Δ and keep your own fringe count N.
  2. Watch the live centre intensity. Increment N each time the centre goes through a bright. Do not let the bench count for you.
  3. Record pairs (Δ, N) at several brights. At least 6 rows, spanning several micrometres of Δ.
  4. Fit N versus Δ; λ = 1 / slope. Compare with the reference.

Conclusion

The fitted λ agrees with the hidden source. Main uncertainties: missing a fringe and discrete path-difference steps.