The electromagnetic spectrum
Radio, heat, light, ultraviolet and X-rays are one thing at different wavelengths: what each band does, and the radio bands by name.
Home: Signals, Level 4 · Engine, level 1 of 2 in its track. Serves 4 steps across 2 subjects.
Radio waves, heat, visible light, ultraviolet and X-rays are all electromagnetic waves travelling at the speed of light, 299,792,458 m/s. They differ only in wavelength, and wavelength × frequency = the speed of light. A 7 MHz radio wave is 42.8 m long; green light is about 0.00055 mm. What a wave can do, and what it can harm, follows from where it sits on this scale.
The bands
From the longest waves to the shortest. The boundaries are conventions, not hard edges.
| Band | Wavelength | What it does |
|---|---|---|
| Radio | Longer than about 1 m | Carries speech and data across distance: see radio |
| Microwaves | 1 mm to 1 m | Radar, mobile phones and satellites; a microwave oven heats water at 2.45 GHz |
| Infrared | About 700 nm to 1 mm | Felt as heat from fires and the Sun; night vision |
| Visible light | About 380 to 700 nm | Red at the long end, violet at the short; a prism or raindrops spread them into a rainbow |
| Ultraviolet A | 315–400 nm | Most of the ultraviolet reaching the ground; with daylight, it helps the sun disinfect water in a clear bottle: see making water safe |
| Ultraviolet B | 280–315 nm | Sunburn and skin cancer; makes vitamin D in skin |
| Ultraviolet C | 100–280 nm | Stopped by the atmosphere; germicidal lamps at 254 nm kill micro-organisms and burn eyes and skin |
| X-rays | About 0.01 to 10 nm | Pass through flesh but not bone; damage cells |
| Gamma rays | Shorter than about 0.01 nm | From radioactive decay; the most penetrating and damaging |
Radio bands
The international names come from the ITU Radio Regulations. Which band travels how far decides what each is used for.
- HF (shortwave) reflects off the ionosphere, the charged upper atmosphere, so it can reach round the world with little power. It is the band for long-distance contact without infrastructure.
- VHF and above travel in straight lines and stop at the horizon, a few tens of kilometres from a mast on a hill.
- A simple half-wave dipole antenna is two wires, each a quarter-wavelength long; the total length in metres is about 143 ÷ the frequency in MHz. For 7 MHz, about 20.4 m.
| Band | Frequency | Used for |
|---|---|---|
| LF, low frequency | 30–300 kHz | Navigation beacons, time signals; follows the ground over long distances |
| MF, medium frequency | 300 kHz – 3 MHz | AM broadcasting; ground wave by day, farther by night |
| HF, high frequency | 3–30 MHz | Shortwave broadcasting, amateur and long-distance communication, by skywave |
| VHF, very high frequency | 30–300 MHz | FM broadcasting at 87.5–108 MHz, marine and aircraft radio; line of sight |
| UHF, ultra high frequency | 300 MHz – 3 GHz | Television, mobile phones, wifi at 2.4 GHz; line of sight |
| SHF, super high frequency | 3–30 GHz | Satellite links, radar, microwave relays |
Where to go next
This page covers what you are most likely to meet. It is not a field guide, and on identification that difference can kill you. These are the works that go further — chosen because they can be held offline.
Still to come
Illustrations carry a technique further than prose, but some things only move well. Recorded here rather than left implicit.
- A drawing of a half-wave dipole with its feed line and insulators, dimensioned for 7 MHz