Question

In a Young?s double slit experiment, the two slits which are separated by $1.2\, mm$ are illuminated with a monochromatic light of wavelength $6000$ angstorm. The interference pattern is observed on a screen placed at a distance of $1m$ from the slits. Find the number of bright fringes formed over $1 \,cm$ width on the screen.

• A. $25$
• B. $12$
• C. $15$
• D. $20$

Answer 1

Answer: (D)

$20$

Answer 2

Given: $d=1.2\,mm$,
$\lambda=6000\, ?$
$=6\times 10^{-7}\,m$,
$D=1\,m$,
$x=1\,cm$
$=1\times 10^{-2}\,m$
For $n^{th}$ bright fringe, $x=n\frac{\lambda\,D}{d}$
$\therefore 1\times 10^{-2}=\frac{n\times6\times 10^{-7}\times 1}{1.2\times 10^{-3}}$
$\therefore n=\frac{1.2\times 10^{-5}}{6\times 10^{-7}}$
$=0.2\times 10^{2}=20$
There are $20$ bright fringes formed over $1\,cm$ width on the screen