# 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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In the usual Young's slits experiment the phase of the light at both slits is the same, so there is constructive interference and a bright line at the centre of the screen i.e. the point equidistant from both slits. When you insert a glass plate over one slit you change the phase of the light at that slit because the speed of light slows while it is travelling through the glass. This means the phase of the light from the two slits is no longer the same at the centre of the screen, so the bright line is displaced sideways on the screen. This the lateral displacement. I would guess you're supposed to calculate how far the bright line moves, or possibly given the line displacement calculate the phase shift and hence the plate thickness.

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thanks! some other guy told me "fringe" means this "lateral displacement" should be measured by the number of fringe spacing that the bright line in the center moves. – arax Jun 3 '13 at 2:17
@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