Question

Which fringe results from a phase difference of $4\pi$ between the light waves incident from the two slits?

• A. $2$
• B. $3$
• C. $4$
• D. $5$

Answer 1

The correct option is C $4$

Using $\varphi =\Delta x\frac{2\pi }{\lambda }$, where $\Delta x$ is the path difference.

For phase difference, $\varphi =4\pi$

Then $\Delta x=2\lambda$

This gives $n=2$ from $\Delta x=n\lambda$

That means second bright fringe (i-e fringe marked as 4) results in a phase difference of $4\pi$ .