Q. The two slits at a distance of $1\ mm$ are illuminated by the light of wavelength $6.5 \times 10^{- 7}m$. The interference fringes are observed on a screen placed at a distance of $1m$. The distance between third dark fringe and fifth bright fringe will be
A) $0.65\ mm$
B) $1.63\ mm$
C) $3.25\ mm$
D) $4.88\ mm$
Explanation
(b) Distance between $n^{th}$ bright fringe and $m^{th}$ dark fringe $(n > m)$
$\mathrm{\Delta}x = \left( n - m + \frac{1}{2} \right)\beta = \left( 5 - 3 + \frac{1}{2} \right) \times \frac{6.5 \times 10^{- 7} \times 1}{1 \times 10^{- 3}}$
$= 1.63mm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.