Questions: Longitudinal Wave Characteristics and Properties

Question 1

A student claims that sound waves could be polarized using a material with specially aligned structures, similar to how polarizing filters work for light. This claim is:

• A. Correct — sound, like light, has oscillations that can be restricted to one plane
• B. Incorrect — sound is longitudinal, so particle motion is already parallel to propagation with no perpendicular component to restrict
• C. Incorrect, but only because sound moves too slowly for polarization to be practical
• D. Correct, but only for ultrasound frequencies above 20 kHz

Answer: (B)

Polarization filters work by blocking one direction of transverse oscillation while passing another. But in a longitudinal wave, particle motion is already confined to the axis of propagation — there is no perpendicular oscillation to select or block. This is a fundamental geometric constraint, not a practical limitation. You cannot polarize something that has no transverse component.

Question 2

In a longitudinal wave traveling horizontally to the right through air, what are the air molecules doing?

• A. Moving up and down, perpendicular to the wave's travel direction
• B. Moving left and right, parallel to the wave's travel direction
• C. Staying stationary while pressure changes pass through
• D. Rotating in circles around their equilibrium positions

Answer: (B)

The defining characteristic of a longitudinal wave is that particle displacement is parallel (not perpendicular) to the direction of energy propagation. In a sound wave in air, molecules are pushed back and forth along the same axis the wave travels — creating alternating compressions (crowded together) and rarefactions (spread apart). Option A describes transverse waves like waves on a string or light.

Question 3

In a longitudinal wave, the wavelength is the distance from one compression to the next adjacent compression.

• T. True
• F. False

Answer: True

The wavelength of any wave is the distance over one complete cycle. In a longitudinal wave, one complete cycle goes from one compression to the next compression (or equivalently, from one rarefaction to the next). This is exactly analogous to measuring peak-to-peak distance in a transverse wave — the same wave property, just expressed through pressure regions rather than displacement peaks.

Question 4

A longitudinal wave and a transverse wave traveling through the same medium at the same frequency differ mainly in their propagation speed.

• T. True
• F. False

Answer: False

The fundamental difference is the direction of particle oscillation relative to energy propagation, not their speeds. Transverse waves have particles oscillating perpendicular to travel; longitudinal waves have particles oscillating parallel to travel. This geometric difference leads to distinct physical properties — including the fact that transverse waves can be polarized while longitudinal waves cannot — regardless of their speeds.

Question 5

Why can longitudinal waves not be polarized, while transverse waves can? What does this reveal about the fundamental difference in particle motion between the two wave types?

Model answer: Polarization restricts particle oscillation to one specific direction within the plane perpendicular to propagation. In a transverse wave, particles oscillate perpendicular to travel, so there are multiple possible oscillation directions to choose from. In a longitudinal wave, particles oscillate parallel to the direction of travel — there is only one possible direction, already fixed by the wave's geometry. With no perpendicular dimension to restrict, the concept of polarization simply does not apply.

This distinction is practically important: optical polarizers exploit the two-dimensional freedom of transverse oscillation (light). No analogous device can exist for sound. It also illustrates that polarization is not a general wave property but one specific to the transverse geometry.