Michelson interferometer – measure a wavelength, test the ether
PhysicsOpticsAges 16–17Virtual lab
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Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Optics
Move a mirror and count the circular fringes that pass the centre of the screen: Δd = Nλ/2 gives the wavelength of a laser, including an unknown one. A second screen rotates the Michelson–Morley apparatus in a supposed ether wind and compares the fringe shift that classical physics predicts with the null result.
Lesson: Michelson interferometer, measuring wavelength by counting fringes, the Michelson–Morley experiment, constancy of the speed of light
What it shows
A Michelson interferometer splits a beam in two, sends each part to a mirror and brings them together again. Moving one mirror by Δd changes the path by 2Δd, so one fringe passes for each λ/2 of movement and Δd = Nλ/2. In 1887 Michelson and Morley turned such an instrument in the supposed ether wind and expected a shift of about 0.4 fringe, but found less than one-twentieth of that. The result left no place for an ether wind, and special relativity later took the speed of light as the same for every observer.
How to use
On Count fringes, choose a Light source, a Fringes to count and a Speed, then press Move mirror. The mirror stops at that count and the run goes into the table. Calculate λ = 2Δd/N. Then measure Laser X. On Michelson–Morley, set Optical path L and Ether wind speed v, then press Rotate. The dashed curve is the prediction and the dots are the readings. The table gives the upper limit on the wind speed.
Parameters you can change
- Screen Count fringes, Michelson–Morley
- Light source Red laser, Green laser, Violet laser, Laser X
- Fringes to count 20–300
- Optical path L 1.2–11 m
- Ether wind speed v 5–60 km/s
Questions to explore
- How far does the mirror move for 100 fringes of red light, and what wavelength does that give?
- What wavelength do you measure for Laser X, and how does counting more fringes change your uncertainty?
- What fringe shift does classical physics predict for L = 11 m and v = 30 km/s, and what do the readings show?
