2.2 wave by wave length (Longitudinal Wave) is a particle of the medium makes the wave moves through there is a move in the same direction the direction of movement of waves, such as sound waves, the waves in spring, etc.
2.2 longitudinal waves (Longitudinal Wave) is a particle of the medium makes the waves are moving back in the right direction, the direction of movement of waves, such as sound waves, waves in the spring onwards.