Question

Find the phase difference between a particle at $1^{st}$ crest and a particle at $4^{th}$ crest.

Answer 1

The distance between the 1st and 4th crest is $3\times wavelength$.

Phase difference = $\frac{2\pi }{\lambda }\times distance$

In the problem we have phase difference = $\frac{2\pi }{\lambda }\times 3\lambda =6\pi$

The phase difference is $6\pi$.