4PH1
Waves & The Electromagnetic Spectrum
Waves · 1 question type
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 14% of your exam marks.
stable
High
Stable14%
Wave equation (v = fλ), transverse vs longitudinal and EM spectrum properties tested consistently.
What a wave is
- A wave is a disturbance produced by an oscillating source that transfers energy (and information) from place to place without transferring matter
- The particles of the medium oscillate about a fixed position; the wave moves through them, but they do not travel with it
- A toy duck floating on water is the classic visualisation: a passing ripple lifts the duck up and lets it back down, but the duck itself stays roughly where it was
Two ways a wave can vibrate
- All waves fall into one of two categories, set by which direction the medium vibrates relative to the wave's travel:
- Transverse waves: the oscillation is perpendicular to the direction of energy transfer
- Longitudinal waves: the oscillation is parallel to the direction of energy transfer
Transverse waves
- A transverse wave shows alternating peaks (the highest points above the rest position) and troughs (the lowest points below it)
- Transverse waves can travel through solids and along the surface of a liquid, but a mechanical transverse wave cannot travel through the body of a liquid or any gas (the molecules have no lateral spring to vibrate against)
- Electromagnetic waves are transverse and can travel through solids, liquids, gases, and a vacuum; they are the only transverse waves not bound by needing a material medium
- Everyday examples of transverse waves:
- ripples on the surface of a pond
- a flicked rope or skipping line
- the vibrations of a plucked guitar string
- S- in earthquakes (one of the two main seismic wave types)
- all (light, radio, X-rays, microwaves and so on)
Longitudinal waves
- A longitudinal wave shows alternating compressions (particles bunched closer together than average) and rarefactions (particles spread further apart than average)
- The medium's density and pressure rise and fall as the wave passes
- Longitudinal waves travel through solids, liquids and gases because the medium can be compressed and expanded along the line of travel; they cannot travel through a vacuum because there are no particles to compress
- Everyday examples of longitudinal waves:
- sound waves in air, water and steel
- P-waves in earthquakes (the faster of the two main seismic wave types)
- pressure waves in any fluid (a sudden push on a piston of water)
Side-by-side comparison
| Feature | Transverse waves | Longitudinal waves |
|---|---|---|
| Direction of oscillation | Perpendicular to energy transfer | Parallel to energy transfer |
| Visible features | Peaks and troughs | Compressions and rarefactions |
| Density of medium | Stays roughly constant | Rises and falls as wave passes |
| Pressure of medium | Stays roughly constant | Rises and falls as wave passes |
| Travels in solids? | Yes | Yes |
| Travels in liquids? | On the surface only (or as EM through liquid) | Yes (right through) |
| Travels in gases? | EM only | Yes |
| Travels in a vacuum? | EM waves only | No |
| Common examples | Ripples, guitar strings, all EM waves | Sound, P-waves, pressure waves |
Exam tip
Transverse vs longitudinal waves
Describing the difference between transverse and longitudinal waves comes up (2–3 marks), so you need to know: in a transverse wave the vibrations are perpendicular to the direction of travel; in a longitudinal wave they are parallel to it. State both directions relative to travel — "up and down" alone isn't enough.