Hi, I'm just wondering if someone could help me better understand how the Fizeau interferometer works... http://upload.wikimedia.org/wikipedia/commons/2/2f/Fizeau_interferometer_testing_optical_flat.svg From my understanding, the source shines light through a collimating lens and the light goes through the beam splitter. After the light goes through the beam splitter, in goes through a partially-reflective reference flat. The light that goes through the reference continues on and then bounces off of the test surface. As this light passes upward through the reference flat, it interferes with the light that is initially reflected off of the reference flat. This creates the interference waves. This is then reflected off of the beamsplitter and then passes through another lens. Finally you can analyze the interferogram. Is this understanding correct? I'm mainly confused about where the light waves really interfere. Does it interfere as the light passes through the reference or does it interfere where the two lines in the above diagram are merged? Or both? Also, What would the interferogram look like if I had a perfectly horizontal reference, and a slightly incline test flat? Could you tell what the angle of the test flat is based on this interferogram?