The Michelson Interferometer
Splitting a beam and recombining it after two paths turns a tiny difference in path length into a countable shift of fringes. The instrument measures displacement in units of the wavelength of its own light.
What a learner can do afterwards
- Relates the number of fringes passing to the distance a mirror is moved
- Explains how the instrument measures a refractive index by inserting a sample in one arm
- Describes why coherence length limits the path difference that still shows fringes
1 · Read
A half-silvered mirror splits one beam into two halves from the same source, so the halves stay coherent. The reflected half travels to the movable mirror M1 and back. The transmitted half travels to the fixed mirror M2 and back. When they recombine, the path difference decides the view: a whole number of wavelengths gives bright, a half number gives dark.
Move M1 by half a wavelength and each fringe steps into its neighbor's place, so exactly one fringe passes your mark. Count the passing fringes, call the count m, and the mirror moved m times half the wavelength. With a 630 nm red laser, one passing fringe means the mirror moved 315 nm.
A 2 cm glass chamber sits in one arm, first empty and then slowly filled with gas. The gas shortens the wavelength inside, so extra waves fit and 122 dark fringes slide past the mark with 632.8 nm light. From that count the gas index works out to 1.0019, far too close to 1 for Snell methods. A wider chamber stacks in more waves, so more fringes pass for the same gas.
Fringes fade when the arm lengths differ by more than the coherence length, so keep the paths nearly equal. And guard your count: missing 5 of 122 fringes shrinks the result, giving an index that is too small.
Split one beam, count the fringes as the mirror moves, and turn the count into distance with half wavelengths.
2 · Watch
Take it off screen
Where it sits
Learn first
This opens up
Nothing builds on it yet.
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.