Answer
phase difference$=\pi$ rad
Work Step by Step
We know that,
$T=\frac{1}{f}$
We insert the value of $f$ in the formula to find $T$:
$T=\frac{1}{500}=2\times 10^{-3}s=2ms$
The given interval $1$ ms is half of $T$ and it means that it is equal to half of one cycle i.e half of $2\pi$ rad.
Thus, the phase difference$=\frac{1}{2}(2\pi)=\pi$ rad
