The Electromagnetic Spectrum: Question 8
Syllabus 3.3
A satellite television company sends a signal from a ground transmitter, up through the ionosphere (a charged layer of Earth's atmosphere), to a satellite in orbit, which then sends the signal back down through the ionosphere to a receiver dish elsewhere on Earth's surface. The company uses microwaves for this link, rather than radio waves of much lower frequency. Which statement best explains this choice?
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Worked solution
Step 1: Identify what actually changes across the spectrum here
All electromagnetic waves, including radio waves and microwaves, travel at the same speed through a vacuum (and through air), so speed cannot be the reason one region is chosen over another. Microwaves are also non-ionising, just like radio waves, so ionising interactions are not the explanation either. What genuinely differs between radio waves and microwaves is how each interacts with the ionosphere, a layer of charged particles high in the atmosphere.
Step 2: Apply this to the ionosphere
Radio waves of sufficiently low frequency are reflected or absorbed by the charged particles in the ionosphere, so they cannot pass through it out into space. Microwaves, being of much higher frequency, are able to pass through the ionosphere with little disruption, allowing a signal to travel between the ground and an orbiting satellite in both directions.
This matches option A.
Step 3: Why the other options are wrong
| Option | Claim | Error |
|---|---|---|
| B | Microwaves travel faster through the ionosphere | All electromagnetic waves travel at the same speed; speed is not what determines whether a wave can pass through the ionosphere |
| C | Microwaves are ionising and are “guided” by the charged particles | Microwaves are non-ionising; it is lower-frequency radio waves that are reflected/absorbed by the ionosphere, not microwaves being actively guided by it |
| D | Radio waves cannot travel through a vacuum at all | Radio waves, like all electromagnetic waves, can travel through a vacuum; the obstacle here is the ionosphere itself, not the vacuum of space beyond it |
Final answers
- Microwaves are used because they pass through the ionosphere, while lower-frequency radio waves are reflected or absorbed by it, option A.