In a Youngs double slit experiment, the distance between the slits is $d$ and the distance of the screen from the slits is $D$. If the wavelength of light used is $\lambda$, the fringe width is given by which expression?

Solution

1. The fringe width $\beta$ is defined as the separation between two consecutive bright or dark fringes.
2. The path difference for the $n$-th bright fringe is given by $n\lambda =dx/D$.
3. For consecutive fringes, the separation is $\beta =\lambda D/d$.
4. Hence the answer is (B).