Michelson Interferometer
Move a mirror or change an arm’s optical medium to measure phase and fringe displacement in a Michelson interferometer.
Free theory articleMove a mirror or change an arm’s optical medium to measure phase and fringe displacement in a Michelson interferometer.
Concept & interpretation
Consider a Michelson interferometer illuminated by light of vacuum wavelength λ = 600 nm. Initially, the path lengths are equal, leading to constructive interference at the detector (intensity I = 1). If we move one mirror by u = 8 eighth-wavelengths, we can calculate the new state.
Experiment: Gas Cell Delay
In this experiment, we explore how changing the optical medium affects the interference pattern.
Setup
- Set the Vacuum wavelength lambda to 400 nm.
- Set the Mirror displacement eighth-wavelengths u to 1.
- Insert a gas cell by setting the Cell refractive index n to 1.25.
- Set the Cell length wavelengths l to 1.
- Observe the resulting Optical path difference and Normalized intensity in the results panel.
Observations
The mirror displacement alone creates an OPD of 2 · (1 · 400 / 8) = 100 nm. The gas cell has a physical length L = 1 · 400 = 400 nm. Because the light passes through it twice, the additional OPD is 2 · (1.25 - 1) · 400 = 200 nm. The total OPD is 100 + 200 = 300 nm. The signed fringe displacement is 300 / 400 = 0.75 cycles. The normalized intensity is (1 + cos(2π · 0.75)) / 2 = 0.5.