Question: what is the phase difference between points c and d?
Answer: Since D is 2 concentric wave crests away from C, the phase difference is 2×2π = 4π rads. The figure shows the wave fronts of a circular wave. What is the phase difference between points C and D? Since E is on the trough before the crest F, the phase difference between the two is 1/2 (2π) = π.