Wave Properties and the EM Spectrum
What waves are
Waves transfer energy (and information) from one place to another without transferring matter. There are two types:
- Transverse waves — the oscillations are at right angles to the direction of energy transfer (e.g. all electromagnetic waves, water ripples, waves on a rope).
- Longitudinal waves — the oscillations are parallel to the direction of energy transfer, creating compressions and rarefactions (e.g. sound waves).
Key wave properties
| Term | Meaning |
|---|---|
| Amplitude | Maximum displacement from the rest position (linked to energy/loudness/brightness) |
| Wavelength (λ) | Distance between two matching points on adjacent waves (e.g. crest to crest), in metres |
| Frequency (f) | Number of waves passing a point per second, in hertz (Hz) |
| Period (T) | Time for one complete wave: T = 1 ÷ f |
The wave equation
wave speed = frequency × wavelength (v = f × λ)- Wave speed (v) in m/s, frequency (f) in Hz, wavelength (λ) in m.
The electromagnetic (EM) spectrum
EM waves are all transverse, travel at the speed of light in a vacuum (3 × 10⁸ m/s), and form a continuous spectrum. In order of increasing frequency (and decreasing wavelength):
Radio → Microwave → Infrared → Visible light → Ultraviolet → X-rays → Gamma rays
(A memory aid: "Raging Martians Invaded Venus Using X-ray Guns.")
- Low frequency (radio) = long wavelength, lower energy.
- High frequency (gamma) = short wavelength, higher energy and more dangerous (ionising).
Uses and dangers
| Wave | Use | Danger |
|---|---|---|
| Radio | Broadcasting, communications | — |
| Microwave | Cooking, satellite/phone signals | Internal heating |
| Infrared | Remote controls, thermal imaging, heating | Skin burns |
| Visible | Sight, fibre-optic communication | — |
| Ultraviolet | Sun beds, security marking | Skin cancer, eye damage |
| X-ray | Medical imaging | Ionising — cell damage/cancer |
| Gamma | Sterilising, cancer treatment | Ionising — cell damage/cancer |
Worked example
A wave has a frequency of 50 Hz and a wavelength of 6 m. Find its speed.
v = f × λ = 50 × 6 = 300 m/s✓
Common mistakes
- Mixing up transverse and longitudinal — sound is longitudinal; EM waves are transverse.
- Confusing frequency and wavelength — high frequency means short wavelength.
- Forgetting the wave equation units (Hz, m, m/s).
Exam tips
- Learn the EM spectrum order and one use + one danger for each region.
- Practise the wave equation and rearranging it.
- Remember higher frequency EM waves (UV, X-ray, gamma) are ionising and more harmful.
Key facts to remember
- Waves transfer energy, not matter; transverse (perpendicular, e.g. EM/light) vs longitudinal (parallel, e.g. sound).
- v = f × λ; frequency in Hz, wavelength in m.
- EM spectrum (low→high frequency): radio, microwave, infrared, visible, ultraviolet, X-ray, gamma; all travel at 3 × 10⁸ m/s in a vacuum; higher frequency = more energy/ionising.