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Learn0625Papers 2, 4 · ExtendedWavesElectromagnetic spectrum
Waves · Subtopic 4 of 5

The Electromagnetic Spectrum

Electromagnetic waves are transverse waves that all travel at 3.0×1083.0\times10^8 m/s in a vacuum. In order of increasing frequency (decreasing wavelength): radio, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays. Each region has its own uses; the highest frequencies carry the most energy and are the most dangerous.

Cambridge 5054 Paper 1Cambridge 5054 Paper 2Cambridge 0625 Papers 2, 4Cambridge 0625 Papers 1, 3Edexcel 4PH1 Paper 1PEdexcel 4PH1 Paper 2P≈ 25 minRevision 5 · updated 2026-10-27
Section 1 of 14

Learning objectives

  • List the regions of the spectrum in order of frequency and wavelength.
  • State the properties shared by all electromagnetic waves.
  • Use c=fλc=f\lambda with c=3.0×108c=3.0\times10^8 m/s.
  • Describe uses and hazards of each region.
Pause and think

Radio waves, light and X-rays seem completely different. One goes through walls, one lets you see, one shows your bones. What do they have in common?

Section 2 of 14

Core idea

Start hereRadio and X-rays are the same thing

The radio in a car and the X-ray at a hospital seem unrelated, yet both are electromagnetic waves travelling at the speed of light. Only the wavelength differs, and that changes everything about what they can do.

The big idea

EM waves: transverse, 3×1083\times10^8 m/s in vacuum. Long to short λ\lambda: radio, microwave, infrared, visible, UV, X-ray, gamma. Shorter λ\lambda: higher ff, more energy.

1. The family

The electromagnetic spectrum
radiomicrowaveinfraredvisibleultravioletX-raysgammaTV, radiophones, ovensheaters, remotesseeingsun tans, security inkX-ray imagescancer treatmentlonger wavelength ← → higher frequency, more energy, more dangerous
The bigger the frequency, the more energy each wave carries and the more harmful it is to living cells (UV, X-rays and gamma can damage DNA).
Wavelength and frequency are inversely related: c = fλ
246810246810120wavelength (m)frequency (×10⁸ Hz)f = c/λ
Same speed for all, so a shorter wavelength means a higher frequency. A 3 m radio wave has frequency 10810^8 Hz.
c=fλc=f\lambda
where
cc
3×1083\times10^8 m/s for all EM waves in vacuum.
Worked exampleRadio vs green light

Radio λ=3\lambda=3 m; green light λ=5.0×10−7\lambda=5.0\times10^{-7} m.

Goal
Frequencies.
Strategy
f=c/λf=c/\lambda.
  1. 3×1083=1×108 Hz\frac{3\times10^8}{3}=1\times10^8\text{ Hz}
    Radio: 100 MHz.
  2. 3×1085×10−7=6×1014 Hz\frac{3\times10^8}{5\times10^{-7}}=6\times10^{14}\text{ Hz}
    Green light.
Answer100100 MHz; 6×10146\times10^{14} Hz.
✓ CheckRatio 6×1066\times10^6 = wavelength ratio ✓.

2. Uses and dangers

  • Radio: broadcasting. Microwave: cooking, satellites.
  • Infrared: remote controls, thermal imaging.
  • Ultraviolet: sterilising, fluorescence; sunburn.
  • X-rays: imaging bones; ionising.
  • Gamma: treating cancer, sterilising; ionising.
Common misconceptionFaster waves All EM waves travel at the same speed in a vacuum.
Pause and think

Why are UV, X-rays and gamma most dangerous?

Summary

  • Order and properties.
  • c=fλc=f\lambda.
Key words
Ionising
Able to knock electrons from atoms.
HistoryMaxwell's prediction. In 18641864 James Clerk Maxwell combined the laws of electricity and magnetism and found they predicted waves travelling at 3×1083\times10^8 m/s, the measured speed of light. He concluded that light itself is an electromagnetic wave, and predicted others would exist.
Fun factMicrowave ovens were an accident. In 19451945 engineer Percy Spencer was working on radar when a chocolate bar in his pocket melted. He tried popcorn kernels next: they popped. The first microwave oven, sold in 19471947, was nearly 1.81.8 m tall.
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