Answer:
Mechanical waves
Explanation:
Waves are periodic oscillations, that carry energy, but not matter.
Waves are classified into two types:
- Mechanical waves: these waves are produced by the oscillations of the particles in a medium, which can oscillate along the direction of propagation of the wave (longitudinal wave) or perpendicular to the direction of motion of the wave (transverse wave). These waves can only propagate in a medium, so they cannot travel in a vacuum. Examples of mechanical waves are sound waves.
- Electromagnetic waves: these waves are produced by the oscillations of electric and magnetic field. They are transverse waves. They are the only type of wave able to propagate through a vacuum (so, through space).
Therefore, the waves that need molecules in order to transfer energy are mechanical waves.
