A double slit $S_1-S_2$ is illuminated by a coherent light of wavelength $\lambda$. The slits are separated by a distance $d$. A plane mirror is placed in front of the double slit at a distance $D_1$ from it and a screen $\sum$ is placed behind the double slit at a distance $D_2$ from it (figure $17-E_2$ ). The screen E receives only the light reflected by the mirror. Find the fringe-width of the interference pattern on the screen.
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It can be seen from the figure that, the apparent distance of the screen from the slits is,
$D = 2D_1+ D_2$
So, Fringe width $=\frac{\text{D}\lambda}{\text{d}}=\frac{(2\text{D}_1+\text{D}_2)\lambda}{\text{d}}$
$D = 2D_1+ D_2$
So, Fringe width $=\frac{\text{D}\lambda}{\text{d}}=\frac{(2\text{D}_1+\text{D}_2)\lambda}{\text{d}}$
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