What's the Difference Between Mechanical Waves and Electromagnetic Waves?

Mechanical waves require a material medium through which to travel, whereas electromagnetic waves can travel through both matter and a vacuum. Mechanical waves involve oscillations of particles; electromagnetic waves involve oscillating electric and magnetic fields.

This distinction is part of C.2 Wave model and applies to both SL and HL.

How the Waves Differ

A wave transfers energy without producing an overall transfer of matter. For a mechanical wave, particles in the medium oscillate around their equilibrium positions and pass energy to neighbouring particles. Therefore, the wave cannot propagate if there are no particles present.

For example, sound travels through air because air molecules oscillate. Sound cannot travel through the vacuum of space because there are no particles to transmit the disturbance.

An electromagnetic wave consists of changing electric and magnetic fields. These fields sustain each other, allowing the wave to propagate without a material medium. In a vacuum, all electromagnetic waves travel at $c = 3.00 \times 10^8\ \text{m s}^{-1}$.

Both types obey the wave equation $v = f\lambda$, where $v$ is wave speed, $f$ is frequency, and $\lambda$ is wavelength. When a wave enters a different medium, its frequency remains constant, while its speed and wavelength may change.

A common misconception is that particles travel with a mechanical wave. They do not: particles oscillate locally while energy is transferred through the medium.

Exam Technique

For a Describe question, state the medium requirement and identify what oscillates. Do not claim that all mechanical waves are longitudinal: sound is longitudinal, but mechanical waves can also be transverse.