1. The problem statement, all variables and given/known data My personal question: What does a Michelson interferometer tell us? The actual problem: A Michelson interferometer is used to precisely measure distances of the order of 1 mm using a laser of wavelength 632.8 nm. a) If the motion of one mirror is 1 mm, what is the corresponding optical path length? How many fringes would have to be counted? b) A more convenient method may be to add a second wavelength which provides another set of independent fringes. Calculate the value of the second wavelength required to produce a maximum fringe coincident with the maximum fringes of the first wavelength at intervals of 0.1 mm and 1 mm. Comment on the practicality of this approach, and on which choice of wavelength would be preferable. 2. Relevant equations d*sin##\theta## = m*##\lambda## for constructive interference, m = 0, 1, 2, ... (?) 3. The attempt at a solution To my own question: I have read about the set-up of a Michelson interferometer. It consists of two mirrors, one fixed and the other movable. There is a third beam splitter mirror positioned in the middle. Incident light is split by the beam splitter, travels to the other two mirrors and back to the beam splitter where they recombine. Depending on the relative distances to each mirror from the beamsplitter, the light can constructively or destructively interfere when recombined, and this can be determined when viewed at a detector. But what does it actually do? From the intro of the actual problem, I guess it has something to do with length measurement. How? My lecturer spent just about zero time explaining anything about this device (and a lot of other things as well... it was basically 'you'll need to know this, now let's move on'). I have read up on a few websites (Wikipedia for example) but I still have no idea as to what sort of useful information it tells us. To the actual problem: a) If motion is 1 mm then a ray of light going to and from the mirror travels an extra 2 mm so OPL = 2 mm. (Maybe? From what I read on Wikipedia, anyway, since I can't remember the lecturer mentioning this either...) As to how many fringes are counted -- well, I still have no idea how fringes are done except that tilting the movable mirror produces these fringes. b) I really am quite lost. It would probably help if I understood the device first before trying to come up with... stuff... Please help.