The Electromagnetic Spectrum: Question 4
Syllabus 3.3
A WiFi router uses microwaves to send an internet signal to a laptop in a different room, through a closed door. The same laptop also has an infrared port, once used to transfer files directly to another device, but this only worked when the two devices were placed facing each other with a clear, unobstructed view between them. Which statement best explains why microwaves, rather than infrared radiation, are used to carry the WiFi signal to a device in another room?
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Worked solution
Step 1: Identify the key physical difference between the two regions
Microwaves and infrared are both non-ionising regions of the electromagnetic spectrum, and both travel at the same speed through air. The property that actually matters here is how each type of wave interacts with solid objects such as a wall or a closed door.
- Microwaves can penetrate through walls and doors, although the signal loses some strength as it does so. This is exactly why a WiFi router can still reach a laptop in another room.
- Infrared radiation cannot pass through solid objects at all. Sending and receiving infrared signals (as in an old-style infrared file-transfer port, or a TV remote control) requires an unobstructed line of sight between the two devices.
Step 2: Apply this to the WiFi scenario
Since the laptop is in a different room behind a closed door, a signal that needs a direct line of sight (infrared) could not reach it at all. A signal that can penetrate through the door, even with some weakening, (microwaves) can.
This matches option A.
Step 3: Why the other options are wrong
| Option | Claim | Error |
|---|---|---|
| B | Microwaves travel faster through air | All electromagnetic waves travel at the same speed through air/vacuum, regardless of wavelength or frequency; speed is not what allows microwaves to reach the laptop |
| C | Microwaves are ionising and pass through objects with no absorption at all | Microwaves are non-ionising, and they are only partly transmitted through walls. Some of the signal is absorbed, which is why WiFi signal strength drops in rooms further from the router |
| D | Infrared is reflected back through the door | Infrared is simply blocked by the door because it needs line of sight; it is not “reflected back” towards the router |
Final answers
- Microwaves penetrate walls and doors (with some signal loss), while infrared needs an unobstructed line of sight, option A.