IGCSE Physics revision · Waves
The electromagnetic spectrum
The purest memory topic on the syllabus — the order, the uses, the dangers. EM-spectrum questions become straightforward mark opportunities when the table is secure, but confusing microwaves and X-rays is costly.
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What the syllabus demands
- —State the order of the EM spectrum by wavelength/frequency: radio, microwave, infrared, visible, ultraviolet, X-rays, gamma
- —Know that all EM waves travel at the same speed in a vacuum: 3 × 10⁸ m/s
- —Give the main uses of each region
- —Give the dangers of the high-frequency regions (UV, X-ray, gamma) and of microwaves and infrared
Definitions that earn marks
Clear definitions to practise — check your course mark scheme
- Electromagnetic spectrum
- The family of transverse waves that all travel at 3 × 10⁸ m/s in a vacuum, arranged by wavelength: radio, microwave, infrared, visible light, ultraviolet, X-rays and gamma rays.
- Monochromatic light
- Light of a single frequency (single wavelength/colour).
The equations
More equations to practise: the IGCSE formula sheet.
Where the marks die
Common mistakes to check
- 01
Reversing the order. From longest wavelength to shortest: Radio, Microwave, Infrared, Visible, UV, X-ray, Gamma. Pick one mnemonic and never change it.
- 02
Forgetting that as wavelength decreases, frequency increases — and energy (and danger) increases with frequency. Gamma is the most dangerous because it is the most energetic, not because it is 'radioactive light'.
- 03
Assigning uses to the wrong region: microwaves for satellite communication and cooking; infrared for remote controls and thermal imaging; X-rays for imaging bones; gamma for sterilising equipment and treating cancer.
- 04
Saying radio waves are dangerous — the low-frequency end carries too little energy to ionise. The dangers cluster at the high-frequency end: UV (skin cancer, eye damage), X-ray and gamma (cell mutation, cancer).
One worked example, done properly
Question
A radio station broadcasts at a frequency of 100 MHz. Calculate the wavelength. (c = 3 × 10⁸ m/s)
Method
- 1.λ = c ÷ f = (3 × 10⁸) ÷ (100 × 10⁶) = (3 × 10⁸) ÷ (1 × 10⁸).
λ = 3 m