An interferometer is not something that interferes with meters, nor a method for measuring between iron things. Instead, it is the use of phase differences (which cannot be easily measured) and the wave-like properties of light to form measurable changes in intensity (which can be easily measured).
Here I'll describe how I built a Michelson-Morley interferometer.
Is it useful? Maybe.
Is it awesome in that you can watch nanometer-scale phenomena? Ooh yes.
Step 1: The Parts List
1) A cheap laser diode - red is good, green is better.
I used a 5mW diode that I had bought from Roithner Lasertechnik in Austria many years ago - but I have no reason to believe that a cheapy 3 dollar laser pointer from the local dollar store wouldn't work as well. Actually, there will be reasons, but they lie outside the scope of this article and you can have a dig around with the keywords of 'spatial coherence' and 'astigmatism'.
2) Some single-surface mirrors - I bought half a dozen on eBay for a few US dollars.
These are fancy mirrors that have a highly reflective aluminium coating on one face of a glass slip. They prevent multiple internal reflections, which would occur with normal glass-faced mirrors.
3) A beam-splitter
I bought some de-lasered blue-ray drive heads on eBay, and found a pair of beam-splitter cubes among the teeny tiny spangly bits inside.
In the image you can see the parts.
The two single-surface mirrors are each glued to a piece of aluminium right-angle extrusion that hjave been spruced up with a black permanent marker.
The laser and beam-splitter are glued to two lengths of scrap aluminium, to make positioning a little easier and to ensure that the laser and beam-splitter are at the same height.