The period of oscillation $=0.05\,s$

The velocity of the wave $=300\,m / s$

To find:- The phase difference between the oscillation of two points.

Solution:- We know that,

Path difference $=\frac{2 \pi}{\lambda} \times$ path difference

Path difference between two points,

$\Delta x =15-10=5\,m$

Time period, $T =0.5^{ s }$

$\Rightarrow$ frequency $v =\frac{1}{ T }=\frac{1}{0.05}=20\,Hz$

Velocity $v =300\,m / s$

$\therefore$ Wavelenght $\lambda=\frac{ v }{ n }=\frac{300}{20}=15\,m$

Hence, phase difference

$\Delta \phi=\frac{2 \pi}{\lambda} \times x =\frac{2 \pi}{15} \times 5=\frac{2 \pi}{3}$