Imagine that bob is on the surface of the sea when water waves move towards him from the left to right
as the crest of the wave approaches bob ⇒ is he moving up or down?
As the wave moves towards the right ⇒ Bob will be moving up and down but not sideways.
after passing of each peak and as the bottom of the wave approaches bob ⇒ is he moving up or down
As the wave propagates ⇒ we track Bob’s vertical displacement at a fixed horizontal position. this allows us to describe how the wave’s displacement varies with time.
from the moment bob first met the water wave until passing of 2 complete waves ⇒ his displacement time graph is shown.
Displacement time graph describes the displacement of Bob relative to his rest position over a time interval. ⇒ from his displacement time graph ⇒ we can able to deduce the period, and hence the frequency.