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FIND OUT MORE. Home Entertainment 351 • Internet 191 • Media 298–299 • Networks 191 • Satellites 28 • Sound 176–177

193

HOW ARE DIFFERENT WAVE BANDS USED?

Waves are grouped in wave bands by frequency.

Low- to medium-frequency wave bands travel far, and

are used for shipping signals. High-frequency waves

are used for radio and telephones. The ultra-high to

extremely high-frequency bands carry huge amounts

of information, including TV, mobile phone, radar

signals, and microwave communications.

HOW DOES DIGITAL TV WORK?

Digital TV converts sounds and images into binary

code. The code is carried on electromagnetic waves.

Digital pictures are clearer because binary code can

be compressed (made smaller) to send lots more

information about the picture to the receiver.

Broadcasters can also send more channels this way.

Have you seen television pictures as

clear as computer graphics? In a digital

broadcast, sounds and pictures are

converted into streams of binary numbers.

The result is higher-quality reception,

and many more channels.

WHAT ARE RADIO WAVES?

Electrons produce radio waves as they vibrate. They

are low-frequency, long-wavelength waves. Radio

broadcasts use the lower-frequency range of radio

waves, and television broadcasts use the higher range.

Electromagnetic rays are waves of electromagnetic force

that travel at the speed of light. The full range of rays

make up the electromagnetic spectrum.

ELECTROMAGNETIC SPECTRUM 3

The spectrum arranges the waves according to frequency (number of

waves passing a point per second) and wavelength (the distance

between the crest of one wave and the crest of the next).

1 DIGITAL BROADCASTING

A TV cameraman records a football match to be broadcast digitally.

The image is created with 25 still pictures (frames) each second,

30 in the USA. Only the changes from frame to frame are transmitted.

ELECTROMAGNETIC SPECTRUM

DIGITAL BROADCASTS

Gamma rays

have wavelengths

of up to about

0.01 nm

(nanometre —

one billionth

of a metre)

1 ULTRAVIOLET RAYS

These dangerous rays

from the Sun can

damage your skin.

1 GAMMA RAYS

Nuclear reactions and

radioactive decay emit

these high-energy rays.

1 X-RAYS

X-rays go through

materials that block light,

such as clothing and skin.

1 INFRARED

Night-vision cameras

detect infrared rays to see

missiles in the dark.

1 RADAR

Radar systems transmit

radio waves to detect

moving ships and aircraft.

1 RADIO WAVES

Radio waves are used for

radio, TV, telephone, and

satellite communication.

Visible light

390-

7,000 nm

Infrared rays

700 nm–1 mm

Radio waves

1 mm–1 km+

Ultraviolet rays

10–390 nm

Microwaves

0.3 m–0.001 m

X-rays

0.001–10 nm