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I don't understand what equation deals with this.

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Related (and may be be a possible dupe):… and links therein. – MAFIA36790 May 20 at 12:51

As a diagram screen $A$ is closer than screen $B$ and so the fringes are closer on screen $A$:

enter image description here

Fringe separation $\Delta x = \dfrac {\lambda \; D}{d}$ where $d$ is the slit separation, $\lambda$ is the wavelength of light and $D$ the distance from the slits to the screen.

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