Question

In the double-slit experiment, the distance of the second dark fringe from the central line is $3mm$. The distance of the fourth bright fringe from the central line is:

• A. $6mm$
• B. $8mm$
• C. $12mm$
• D. $16mm$

Answer 1

The correct option is B $8mm$

Answer is B.
The distance of 2nd dark fringe is given as $=(1+\frac{1}{2})\beta =3mm\Rightarrow \beta =2mm$
Therefore, distance of 4th bright fringe = 4$\beta =$ 8 mm.
Hence, the distance of the fourth bright fringe from the central line is 8 mm.