Determining optical rotary encoder RPM
Here visualization shows why there will be more counts per revolution than number of tooth - https://www.youtube.com/watch?v=v4BbSzJ-hz4&amp;feature=youtu.be&amp;t=61

And here is a video series on how to make DIY encoder whith explanations on how it works - https://www.youtube.com/watch?v=dPBKTZw_xi4

Hope this helps