Question

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

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

Answer 1

The correct option is D $9mm$

Given :

$\lambda =6000\stackrel{\circ }{A}=6000\times {10}^{-10}$ $m$

$d=0.5mm=5\times {10}^{-4}m$ $D=2.5m$

Distance of nth bright fringe from centre $y_n=\frac{n\lambda D}{d}$

For third bright fringe $n=3$

$\therefore$ $y_3=\frac{3\times 6000\times {10}^{-10}\times 2.5}{5\times {10}^{-4}}=9\times {10}^{-3}$ $m$

$⟹$ $y_3=9$ $mm$