# The path difference when a block covers one slit in Young's double slit experiment

A modification of the simplest case of Young’s double slit experiment is when the path length for one of the slits is changed.

I've been told that if a strip of material of thickness $t$ and refractive index $n$ is placed over one slit then it adds a path difference of $(n − 1)t$, which results in the fringes being shifted. However, I am not sure how $(n − 1)t$ is derived and why this gives the path difference?

• It’s unclear what you’re asking. In the title it says block or cover the slit. In the follow up it looks like you’re talking about some transparent material with a refractive index blocking it. Commented Mar 21, 2018 at 21:55

Earlier that light had to travel the distance $t$ in vacuum with refractive index 1. Now the refractive index is $n$ so the path will be $n*t$ The additional path is $n*t - t$ which is $(n-1)*t$. So if the path difference between the two rays was some $x$, $(n-1)*t$ would be added to it.