Sound waves are longitudinal waves, meaning that the waves propagate by compression and rarefaction of their medium. They are termed longitudinal waves because the particles in the medium through which the wave travels (air molecules in our case) oscillate parallel to the direction of motion. Alternatively, transverse waves oscillate perpendicular to the direction of motion. Common examples of transverse waves include light and, to a basic approximation, waves on the ocean.

Longitudinal waves transmit energy by compressing and rarefacting the medium in the same direction as they are traveling. Sounds waves are longitudinal waves and travel by compressing the air through which they travel, causing vibration.

Light, X-rays, and microwaves are all examples of electromagnetic waves; even if you cannot recall if they are longitudinal or transverse, they are all members of the same phenomenon and will have the same type of wave transmission. Transverse waves are generated by oscillation within a plane perpendicular to the direction of motion. Oscillating a rope is a transverse wave, as it is not compressing in the direction of motion.

An important distinction for the MCAT is the difference between transverse and longitudinal waves. Although both wave types are sinusoidal, transverse waves oscillate perpendicular to the direction of propagation, while longitudinal waves oscillate parallel to the direction of propagation.

The most common transverse and longitudinal waves are light waves and sound waves, respectively. All electromagnetic waves (light waves, microwaves, X-rays, radio waves) are transverse. All sound waves are longitudinal.

Transverse waves can be distinguished from longitudinal waves by the orientation of the oscillations to the direction of energy transfer. Transverse waves have oscillations perpendicular to the direction of energy transfer while longitudinal waves have oscillations parallel to the direction of energy transfer. The plucked guitar string may be tricky to think about because we use sound as a characteristic example of a longitudinal wave, and what does a plucked guitar string do but make sound? Well, the sound produced by the string is a longitudinal wave, but the string itself vibrates as a transverse wave. When the string is plucked, the energy is transferred down the string, yet the displacement is up and down or side to side. Meanwhile, an earthquake is a series of compressions that move underground due to shifting along a fault line for one. This is a longitudinal or compression wave. Another example of a transverse wave is those in the ocean. The wave oscillates vertically, causing rises and falls in the water level, but the waves are directed due offshore.