The particles oscillate parallel to the direction of propagation.
Explanation
Mechanical waves are classified into two primary types based on the direction of particle vibration relative to the direction of wave propagation: transverse waves and longitudinal waves. Sound waves are mechanical waves that propagate through a medium via the oscillation of particles.
Option Analysis
- The particles of the medium move from one place to another. is Incorrect: In wave motion, the particles of the medium do not physically travel from the source to the receiver. Instead, they oscillate back and forth around their mean (equilibrium) positions, transferring energy to adjacent particles.
- The particles oscillate perpendicular to the direction of propagation. is Incorrect: When particles oscillate perpendicular (at right angles) to the direction of wave propagation, the wave is classified as a transverse wave. Examples include light waves and waves on a string.
- The particles oscillate parallel to the direction of propagation. is Correct: A wave is termed longitudinal when the particles of the medium oscillate back and forth parallel to the direction of energy propagation. Sound waves in fluids (air, water) are longitudinal waves, characterized by alternating regions of high pressure (compressions) and low pressure (rarefactions).
- The wave travels in a straight line. is Incorrect: While waves generally travel in a straight line in a homogeneous medium (rectilinear propagation), this characteristic applies to both transverse and longitudinal waves and does not specifically define a longitudinal wave.
Key Takeaway: Sound waves are defined as longitudinal waves because the vibration of medium particles occurs parallel to the direction of wave propagation, resulting in the formation of compressions and rarefactions.