Question

Two slits are separated by a distance of $0.5mm$ and illuminated with light of wavelength $\lambda =6000\stackrel{\circ }{A}$. If the screen is placed at $2.5m$ from the slits. The distance of the third bright fringe from the centre will be

• A. $1.5mm$
• B. $3mm$
• C. $6mm$
• D. $9mm$

Answer 1

The correct option is D $9mm$

Distance of $n^{th}$ bright fringe from the centre,
$x_n=\frac{nD\lambda }{d}$
So, $x_3=\frac{3\times 2.5\times 6000\times {10}^{-10}}{0.5\times {10}^{-3}}$
$=9\times {10}^{-3}m=9mm$.