The sentence can be completed as follows:
"<span>When more than one wave is in the same location at the same time, then there is interference between the waves"
In fact, when there are two or more waves in the same location at the same time, their amplitude sum together. The two extreme possibilities are:
- costructive interference: the two waves arrive on phase at the same location (=their crests arrive at the same location at the same time). In this case, the amplitudes of the waves sum together and the resultant wave has greater amplitude.
- destructive interference: the two waves arrive out of phase at the same location. In this case, the amplitudes of the two waves cancel out, and the resultant wave has amplitude zero.</span>
In longitudinal waves, the motion of the particles in a medium is across the direction of the wave.
