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If the experiment was set up as shown in the image below, where the two slits are at different distances from the screen,

enter image description here

would the fringe spacing still be y = ($\lambda)*L/d$ where $\lambda$ is the wavelength of light, $d$ is the slit spacing (along the y-axis) and $L$ is the distance from the furthest slit to the screen? If not, what would the fringe spacing equation now be?

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  • $\begingroup$ Double of your previous Q but you got a better A :) $\endgroup$
    – Alchimista
    Commented Jan 1, 2019 at 9:50

2 Answers 2

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This problem is equivalent to set your screen slightly rotated an angle $\alpha$ (see the figure below).

If your spacing is $a$, then your new spacing with the rotated screen would be $a' = a \cos \alpha$.

enter image description here

Edit: You can take the approximation $\cos \alpha \approx 1 - \frac{\alpha^2}{2}$, which is valid for $\alpha \ll 1$. You can see that there isn't any first-order term, so this means that if you have small enough angles the $\alpha^2$ term is negligible. This proves that, at first order approximation, the spacing would be the same, but this is only true if $\alpha$ is small enough. This is the situation when the two slits are almost aligned. This justifies S. McGrew's answer.

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Offsetting one of the slits in the direction of L by a small amount just shifts the fringe positions, without affecting fringe spacing.

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  • $\begingroup$ is there any way to show this mathematically? $\endgroup$
    – zld123
    Commented Dec 31, 2018 at 18:52
  • $\begingroup$ I edited my answer, I show S. McGrew's answer mathematically. $\endgroup$
    – falgenint
    Commented Dec 31, 2018 at 19:43

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