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Topic 3 · 3.3

The electromagnetic spectrum

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Know the main regions of the electromagnetic spectrum in order of frequency and of wavelength.
  • Know that all electromagnetic waves travel at the same high speed in a vacuum, 3.0 × 108 m/s EXTENDED.
  • Describe the uses and the harmful effects of each region; know that satellites communicate mainly by microwaves.
  • Explain the choice of radiation for mobile phones, Bluetooth and optical fibres; digital and analogue signals and the benefits of digital EXTENDED.

📚The physics

The spectrum

The electromagnetic spectrum in order of increasing frequency and decreasing wavelength: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays, with uses listed under each. Below, the harmful effects: microwaves heat body cells internally, infrared causes skin burns, ultraviolet damages surface cells and eyes, and X-rays and gamma rays cause mutation or damage to cells.
Learn the order both ways: “Raging Martians Invaded Venus Using X-ray Guns”.

All electromagnetic waves are transverse and travel at the same high speed in a vacuum. EXTENDED That speed is 3.0 × 108 m/s, and approximately the same in air. The Sun radiates most of its energy as infrared, visible light and ultraviolet.

Communication

The Earth with a low orbit close to it and a much larger geostationary orbit. Communication satellites use microwaves. Low-orbit satellites serve some satellite phones; geostationary satellites carry satellite television and some satellite phones, staying above one point on the equator.
Communication with artificial satellites is mainly by microwaves.
  • EXTENDED Mobile phones and wireless internet use microwaves: they penetrate some walls and need only a short aerial.
  • EXTENDED Bluetooth uses radio waves: they pass through walls, though the signal is weakened.
  • EXTENDED Optical fibres (cable television, high-speed broadband) use visible light or infrared: glass is transparent to them, and they can carry very high rates of data.

Analogue and digital signals EXTENDED

An analogue signal varies continuously; after travelling it picks up noise, which is amplified with it. A digital signal has only two values, 1 and 0; after travelling it is noisy but still clearly readable, so it can be regenerated exactly.
EXTENDED Sound can be sent as either type of signal.

An analogue signal can take any value; a digital signal has only two values (1 and 0). Digital signals can be regenerated accurately, so they have a greater range, and they allow a higher rate of data transmission.

✏️Worked example

A television signal is sent up to a geostationary satellite 36 000 km above the ground and straight back down. (a) Name the region of the spectrum used. [1] (b) EXTENDED Calculate the time for the signal to go up and back. [2] (c) Explain why a geostationary satellite is used for satellite television. [2]

(a) Microwaves.

(b) Distance = 2 × 3.6 × 107 m = 7.2 × 107 m; \( t = \dfrac{s}{v} = \dfrac{7.2 \times 10^7}{3.0 \times 10^8} = \) 0.24 s.

(c) It orbits once a day above the equator, so it stays above the same point: home dishes can be pointed at it once and left fixed.

Check it. A quarter of a second is the delay you notice on live satellite interviews — so the answer is realistic.
Forgetting to convert km to m, which gives 0.000 24 s. And “up and back” means twice the height.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Theory.) Part of the spectrum reads: radio waves, ____, infrared, visible light, ____, X-rays, gamma rays. Name the two missing regions, and state which property increases from radio waves to gamma rays. [3] (Modelled on 0625/32 June 2026 Q6.)
Microwaves; ultraviolet. Frequency increases (wavelength decreases).
2. (Theory.) State the three regions of the spectrum in which the Sun emits most of its energy. [1]
Infrared, visible light and ultraviolet.
3. (Multiple choice.) Which region is used to detect forged banknotes? A: infrared. B: microwaves. C: ultraviolet. D: X-rays.
C. Security ink glows under ultraviolet.
4. (Theory.) Compare the speed of X-rays with the speed of radio waves in a vacuum. [1]
They are the same.
5. (Theory.) Give one harmful effect of excessive exposure to (a) ultraviolet, (b) gamma rays. [2]
(a) Damage to surface cells and eyes, leading to skin cancer and eye conditions. (b) Mutation or damage to cells in the body.
6. (Multiple choice.) EXTENDED Which statement about digital signals is correct? A: They cannot carry sound. B: They travel more slowly than analogue signals. C: They have only two values, 1 and 0. D: They cannot carry data faster than analogue signals.
C.
7. (Theory.) EXTENDED Explain why mobile phones use microwaves. [2]
Microwaves can pass through (penetrate) some walls, and they need only a short aerial to transmit and receive, which fits inside a phone.
8. (Theory.) EXTENDED Calculate the frequency of radio waves of wavelength 3.0 m. [2]
f = v/λ = 3.0 × 108 / 3.0 = 1.0 × 108 Hz (100 MHz).

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • NASA — “Tour of the Electromagnetic Spectrum”
  • BBC Bitesize — uses and dangers of electromagnetic waves