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I don't understand what equation deals with this.

Pumpkinpeach
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  • Related (and may be be a possible dupe): http://physics.stackexchange.com/questions/203979/why-does-moving-the-source-slit-closer-to-the-double-slit-plane-decrease-the-sha and links therein. –  May 20 '16 at 12:51

1 Answers1

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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.

Farcher
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