Question

In a double-slit experiment, green light $\left(5303\stackrel{o}{A}\right)$ 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

The correct option is D $05$

For diffraction
location of $1^{st}$ minima
$y_1=\frac{D\lambda }{a}=0.2469D\lambda$
Location of $2^{nd}$ 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.