Question

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 :

• A. 09
• B. 10
• C. 04
• D. 05

Answer 1

Answer: (D)

For diffraction
location of $1^{st}$ minime
$y_1=\frac{D\lambda }{a}=0.2469D\lambda$
location of 2nd minima
$y_2=\frac{2D\lambda }{a}=0.4938D\lambda$
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 $P$ & $Q$.