Question

In a double slit experiment, the distance between the slits is d. the screen is at a distance D from the slits. If a bright fringe is formed opposite to one of the slits, is order is:

• A. $\frac{d}{\lambda}$
• B. $\frac{\lambda^{2}}{dD}$
• C. $\frac{D^{2}}{2\lambda d}$
• D. $\frac{d^{2}}{2D\lambda}$

Answer 1

Answer: (D)

$\frac{d^{2}}{2D\lambda}$

Answer 2

: When a bright fringe is formed opposite to one of the slits, $x = \frac{d}{2}$

Path difference $= \frac{xd}{D} = \frac{d}{2} \times \frac{d}{D} = \frac{d^{2}}{2D}$

If it is $n^{th}$order bright fringe

Path difference $n\lambda = \frac{d^{2}}{2D}$or $n = \frac{d^{2}}{2D\lambda}$