Q. In a double slit experiment, 5$^{th}$ dark fringe is formed opposite to one the slits. The wavelength of light is
A) $\frac{d^{2}}{6D}$
B) $\frac{d^{2}}{5D}$
C) $\frac{d^{2}}{15D}$
D) $\frac{d^{2}}{9D}$
Explanation
(d) For dark fringe,
$\frac{xd}{D} = (2m - 1)\frac{\lambda}{2}\ $
[image:10_image37.emf]
Here, $m = 5,\ x = \frac{d}{2}$
$\therefore\ \frac{d}{2}.\frac{d}{D} = (2 \times 5 - 1)\frac{\lambda}{2}\ $
Or $\frac{d^{2}}{D} = 9\lambda$
$\therefore\ \ \ \ \lambda = \frac{d^{2}}{9D}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.