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In a Young's Double Slit experiment, the separation of four bright fringes is $2.5\ mm$, the wavelength of light used is $6.2 \cdot 10^{-7}\ m$. If the distance from the slits to the screen is $80\ cm$, calculate the separation of two slits.

I applied $\frac{4\lambda D}{d}=1.5\cdot 10^{-3}\ m$ but the answer didn't match. It matches when I use $\frac{3\lambda D}{d}$ instead of $4$.

Shouldn't the separation of four bright fringes be $\frac{4\lambda D}{d}$ (after applying $\sin\theta = \tan\theta$ approximation)?

$D$ :distance from the slits to the screen

$\lambda$: wavelength of light used

$d$: separation of the slits

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If the four bright fringes are to one side of the maximum, that is the pattern is not-symmetric then the fourth fringe is actually the third order in the Young's formula since the first fringe is consider the zeroth order fringe. So for Young's the central fringe is 0 then the next brightest is 1 and so on. This would give you the 3.

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