# What is a “lateral fringe displacement”?

I encountered the term "lateral fridge displacement" in my optics homework for a problem about inserting a thin plate of glass over one of Young's double splits.

So what does "lateral fringe displacement" mean in this context? I Googled but found nothing quite useful. Most of the results are about how this phenomenon emerges, but not specific about the exact definition.

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@AlexSu: every fringe spacing move is a phase shift of 2$\pi$. You can measure the displacement as a distance on the screen, an angular displacement or a phase change displacement. They are all different ways of measuring the same thing. –  John Rennie Jun 3 '13 at 5:36