Question

In Young's double-slit experiment, the two slits which are separated by $1.2 \, mm$ are illuminated with a monochromatic light of wavelength $6000 \, \overset{^\circ }{A}$ . The interference pattern is observed on a screen placed at a distance of $1 \, m$ 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)

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