Question

A beam of light consisting of two wavelengths 6500 $\dot{A}$ and 5200 $\dot{A}$ is used to obtain fringes in a Young's double slit experiment.Find the distance of the third bright fringe on the screen from central maximum for the wavelength 6500 $\dot{A}$ .The distance between the slits is 2mm and the distance between the plane of the slits and the screen is 120 cm -

• A. 0.117 cm
• B. 0.100 cm
• C. 0.90 cm
• D. None of these

Answer 1

The correct option is A 0.117 cm

Given : $D=120cm=1.2m$ $d=2mm=2\times {10}^{-3}m$
Wavelength $\lambda =6500A^o=6.5\times {10}^{-7}m$
Distance of third bright fringe from central maxima $y_3=\frac{n\lambda D}{d}$
where $n=3$
$\therefore$ $y_3=\frac{3\times 6.5\times {10}^{-7}\times 1.2}{2\times {10}^{-3}}=11.7\times {10}^{-4}m=0.117cm$