Q. In a double-slit experiment, green light $(5303 \; \mathring A )$ falls on a double slit having a separation of $19.44 \; \mu m$ and a width of $4.05 \; \mu m$. The number of bright fringes between the first and the second diffraction minima is :
Solution:
For diffraction
location of $1^{st}$ minime
$y_{1} = \frac{D\lambda}{a} = 0.2469 D\lambda $
location of 2nd minima
$ y_{2} = \frac{2D\lambda }{a} = 0.4938 D\lambda $
Now for interference
Path difference at $P$.
$ \frac{dy}{D} = 4.8 \lambda $
path difference at $Q$
$ \frac{dy}{D} = 9.6 \lambda$
So orders of maxima in between P$ $ & $Q$ is $5, 6, 7, 8, 9$
So 5 bright fringes all present between $P$ & $Q$.
