Which of the following statements is true about longitudinal waves?

• A. The particles of the medium oscillate parallel to the direction of wave propagation.
• B. The particles of the medium oscillate perpendicular to the direction of wave propagation.
• C. Longitudinal waves cannot travel through solids.
• D. The velocity of a longitudinal wave is always equal to the speed of light $c$.

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the definition of a longitudinal wave. In a longitudinal wave, the particles of the medium oscillate back and forth in the same direction as the wave propagation.
2. Step 2: Contrast this with transverse waves, where particles oscillate perpendicular to the direction of wave propagation.
3. Step 3: Recognize that longitudinal waves can travel through solids, liquids, and gases because they involve compressions and rarefactions of the medium.
4. Step 4: Note that the velocity of longitudinal waves depends on the properties of the medium (such as elasticity and density) and is not related to the speed of light, which is a constant for electromagnetic waves.
5. Step 5: Conclude that the true statement is: 'The particles of the medium oscillate parallel to the direction of wave propagation.'