Question

An illustration of a ringing bell and dots representing air molecules. The dots are close together, then far apart and close together again representing a longitudinal mechanical wave of sound going from the bell to an ear.
A ringing bell is sending a longitudinal wave to your ear. What else is true about the longitudinal wave? Check all that apply.

All sound waves are longitudinal.
Energy is transferred perpendicular to the wave’s motion.
Mechanical waves can be longitudinal.
Electromagnetic waves can be longitudinal.
Longitudinal waves require a medium to travel through.

Answer 1

Answers (with brief reasons):

- All sound waves are longitudinal. — False. In gases and liquids sound is longitudinal, but in solids you can also have transverse (shear) and surface sound waves.

- Energy is transferred perpendicular to the wave’s motion. — False. That describes transverse waves. In a longitudinal wave particle motion and energy transfer are parallel to the direction of propagation.

- Mechanical waves can be longitudinal. — True. Sound in air is a mechanical longitudinal wave (particles oscillate back and forth in the direction of propagation).

- Electromagnetic waves can be longitudinal. — False for ordinary EM waves in free space (they are transverse). (There are special contexts — e.g., plasma oscillations or guided modes — where longitudinal electromagnetic-like modes appear.)

- Longitudinal waves require a medium to travel through. — True for mechanical longitudinal waves such as sound (they need a material medium).