Sodium Fringes

Part 2 of the Michelson lab, where we examine interference fringes from a sodium lamp. Sodium has two bright, closely spaced emission lines in the yellow part of the spectrum, near 589 nm. Because there are two wavelengths present, there are also two interference patterns, right on top of each other. But depending on the relative lengths of the interferometer arms, sometimes the bright parts of one pattern fall on the bright parts of the other, and sometimes they fall on the dark parts of the other. This causes the contrast of the combined interference pattern to change from a maximum to near invisibility in a periodic fashion as the length of one of the interferometer arms is continuously changed.