Chapter 3: Waves — Part 6

Topic 3.3 · Electromagnetic spectrum

The family of EM waves

All electromagnetic waves travel at the same high speed in a vacuum.

That speed is 3.0 × 10⁸ m/s, and it is approximately the same in air.

Order of increasing frequency (decreasing wavelength): radio → microwave → infrared → visible → ultraviolet → X-rays → gamma rays.

Region	Typical uses	Harm from excessive exposure
Radio	Radio and TV, astronomy, RFID	—
Microwaves	Satellite TV, mobile phones, microwave ovens	Internal heating of body cells
Infrared	Grills, remote controls, intruder alarms, thermal imaging, optical fibres	Skin burns
Visible	Vision, photography, illumination	—
Ultraviolet	Security marking, fake-note detection, sterilising water	Damage to surface cells and eyes (skin cancer, eye conditions)
X-rays	Medical scanning, security scanners	Mutation or damage to cells
Gamma	Sterilising food and medical equipment; cancer detection and treatment	Mutation or damage to cells

Communication with artificial satellites is mainly by microwaves.

Some satellite phones use low-orbit satellites.
Some satellite phones and direct-broadcast satellite TV use geostationary satellites.

Mobile phones and wireless internet use microwaves because they can penetrate some walls and need only a short aerial.
Bluetooth uses radio waves, which pass through walls but the signal is weakened.
Optical fibres (visible light or infrared) are used for cable TV and high-speed broadband because glass is transparent to those wavelengths and they can carry high data rates.

A digital signal has a small number of discrete levels (often two). An analogue signal varies continuously. Sound can be sent either way.

Digital signalling allows a higher data rate and a longer range because the signal can be regenerated accurately (noise is less of a problem).

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