Two coherent monochromatic point sources S _ 1 and S _ 2 of...

At point ${P}_{2};$

$\Deltax = d = 1.8mm = 3000\lambda$

Hence a (bright fringe) will be formed at ${P}_{2}$

Now $\Deltax = dcos \theta = n\lambda$

$cos \theta = \dfrac{n\lambda}{d}$, $\theta$ is with the $x - axis$.

$- sin\theta\Delta\theta = \left(\Deltan\right)\dfrac{\lambda}{d}$

$\Delta\theta = - \left(\Deltan\right)\dfrac{\lambda}{dsin \theta}$

$\Delta\theta$ increases as $\theta$ decreases

At ${P}_{2},$ the order of fringe will be maximum.

For total no. of bright fringes $d = n\lambda ⇒ n = 3000$

$∴$ total no. of fringes $= 3000$