IGCSE §3: Waves · IGCSE
The Electromagnetic Spectrum
Revision notes on The Electromagnetic Spectrum for the OCR A-level Physics specification (H556). Free to read, with 4 practice questions in the app.
Electromagnetic waves — transverse waves consisting of oscillating electric and magnetic fields. They need no medium, so they travel through a vacuum.
All electromagnetic waves travel at the same speed in a vacuum:
c = 3.0 × 10⁸ m/s
This is the speed of light, and it is the same for radio waves and gamma rays alike.
The spectrum, in order of increasing frequency (and therefore decreasing wavelength):
| Wave | Wavelength, roughly |
|---|---|
| Radio waves | 1 m and above |
| Microwaves | 1 mm to 1 m |
| Infrared | 700 nm to 1 mm |
| Visible light | 400 to 700 nm |
| Ultraviolet | 10 to 400 nm |
| X-rays | 0.01 to 10 nm |
| Gamma rays | below 0.01 nm |
A mnemonic most people end up using: Roman Men Invented Very Unusual X-ray Gadgets.
The one direction to be sure of — radio has the longest wavelength and lowest frequency; gamma has the shortest wavelength and highest frequency. Getting the spectrum backwards turns every "which has the higher frequency" mark into a lost one.
Visible light is the narrow band our eyes detect, from red (longest wavelength, about 700 nm) through orange, yellow, green, blue and indigo to violet (shortest, about 400 nm). It is a tiny fraction of the whole spectrum.
Frequency, energy and danger go together. The higher the frequency, the more energy each wave carries, and the more harmful it tends to be. Radio waves pass through you harmlessly; gamma rays ionise the atoms they meet.
Ionising radiation — ultraviolet, X-rays and gamma rays carry enough energy per wave to knock electrons off atoms. That is what makes them able to damage DNA and cause cancer, and it is the dividing line in the spectrum that matters medically.
Example: an FM radio wave at 100 MHz has a wavelength of 3 m. A gamma ray at 10²⁰ Hz has a wavelength of 3 × 10⁻¹² m — smaller than an atom. Both travel at exactly the same speed.
4 questions on The Electromagnetic Spectrum
Reading is not revising. This topic has 4 questions in the app — multiple choice, calculations, and written answers marked against the mark scheme, point by point. Miss one and it comes back later with its options reshuffled, so you have to know it rather than remember where the answer sat.
- Every topic of Edexcel International GCSE Physics (4PH1)
- Flashcards and every equation, linked to the topic it comes from
- Full mock papers in the real paper structure
20 minutes a day is free. No card, nothing to cancel — and the paid version never renews itself either.
Start The Electromagnetic Spectrum