Imagine you have a coherent light in a shoebox, and you cut two narrow slits out the side and shine the light on the far wall. The distance from one slit to the wall isn’t going to be exactly the same as the other, so there’s a “path difference”.

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The path difference refers to the difference in distance two waves from the same light source will travel to reach the same point. At this point, their crests line up in such a way as to interfere with each other. Here’s a more complicated approach to using it. (HINT: if you get lost, skip to the next lesson.. it’s much easier and niftier because it involves airplanes…)

Click here to go to next lesson on When Path Difference Matters

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