1. The problem statement, all variables and given/known data Suppose you know the wavelength of light passing through a Michelson interferometer with high accuracy. Describe how you could use the interferometer to measure the length of a small piece of material. 2. The attempt at a solution - Sandwich the piece of material behind the movable mirror, and compare the intensity(amplitude) of the light on the detector with the intensity when the mirror is at the initial position. But if I know how to sandwich it without compressing it, that means I already know the length of it. Even if possible, I think this won't tell whether the length is x or x+mλ/2 for integer m, so I must make the wavelength of the light longer than twice the length of the material. I read that the wavelength of far-infrared light can be about 1mm, so the material should be smaller than 0.5 mm.(http://en.wikipedia.org/wiki/Infrared#CIE_division_scheme) (Is there a type of light with longer wavelength than infrared?) - Put the material in 1 of the 2 paths of light, and compare the light intensity on the detector with that when there is no material. But I realized I don't know any way of holding the material without restricting the path of light. Plus interference is irrelevant in this case. Please correct my mistakes and provide some hints!