2.3 Electromagnetic Waves Used for Communication

Question 1

Give 6 properties which all electromagnetic waves must have.

Answer 1

Consist of a varying electric and varying magnetic field vibrating at right angles to each other.
Travel at the same speed in a vacuum (3.00 x 10^8 ms -1)
Are unaffected by electric of magnetic fields
Obey the inverse square law in the absence of absorption
May be polarised
Have properties of reflection, refraction, diffraction and interference.

Question 2

What of limited number of waves can transmit and receive information without the need for connecting wires or fibres?

Answer 2

In the family of waves called ‘radio’ but include microwaves and, to some extent, light.

Question 3

What are some properties and/or uses of radio waves?

Answer 3

Radio and TV communications

Question 4

What are some properties and/or uses of microwaves?

Answer 4

Satellite communications
Wireless computer connections using blue tooth
Radar detection of ships and aircraft
Remote controllers for TV, hi fi etc
Thermographs

Question 5

What are some properties and/or uses of infrared waves?

Answer 5

Cooking or heating using radiant heater photography in the dark
Remote controllers for TV, hi fi etc
Thermographs

Question 6

What are some properties and/or uses of visible light?

Answer 6

Human and animal vision

Photosynthesis in plants

Question 7

What are some properties and/or uses of ultraviolet waves?

Answer 7

Produces vitamin D in the skin
Forgery detection
Can cause sun tan, eye damage and skin cancer
Causes fluorescence in certain mineral salts

Question 8

What are some properties and/or uses of X-rays?

Answer 8

Form images in medicine (radiography)
Non-destructive testing of welds or joints etc in metals
Measurement of thickness
Imaging internal structures of objects

Question 9

What are some properties and/or uses of gamma rays?

Answer 9

Sterilise food or medical equipment
Treat cancer (radiotherapy)
Kill bacteria

Question 10

The atmosphere is effectively opaque to all electromagnetic waves with the exception of what?

Answer 10

Some radio wavebands
The light in the optical window- the entire visible region (of wavelength from 390 to 780 nm)
The near ultraviolet
The near infrared and some far infrared

Question 11

Infrared of wavelengths between 1 and 20mm are absorbed by what?

Answer 11

Water, oxygen and carbon dioxide molecules in the air.

Question 12

What is most ultraviolet absorbed by?

Answer 12

The ozone layer

Question 13

What are X-rays and gamma rays absorbed by?

Answer 13

Gas molecules

Question 14

All forms of wireless communication rely on a carrier signal such as an electromagnetic wave with a particular frequency. What does modulating the carrier do?

Answer 14

We can encode the information to be transmitted; the higher the carrier frequency, the more information a signal can hold.
When the whole signal has been received, the carrier wave is filtered out leaving only the demodulated signal behind.
Tuning a radio to a particular station means that you are selecting the frequency of the carrier wave.

Question 15

What uses amplitude modulation (or AM)?

Answer 15

The medium wave (MW) and long wave (LW) bands on an analogue radio.

Question 16

How does amplitude modulation (AM) work?

Answer 16

Information is added to carrier wave, producing a signal of greater amplitude variation when the information is a loud sound and smaller amplitude variation for quiet sounds.
The modulated carrier wave is filtered by the radio leaving the information to be amplified and fed to the loudspeaker.

Question 17

How does frequency modulation (FM) work?

Answer 17

The information signal modifies the carrier wave by changing its frequency very slightly.
There are two changes possible: the overall change in frequency above and below the carrier frequency, and the number of times the frequency changes between these limits every second.
The first represents the amplitude of the information signal, the second the frequency.

Question 18

What is pulse code modulation?

Answer 18

A pulse modulation technique in which the amplitude of an analogue signal is converted to a binary value represented as a series of pulses

Question 19

What is time division multiplexing a technique used for?

Answer 19

To send several PCM or digital signals from a number of devices along the same transmission path at what seems to be the same time.
The signal from each device is split into packets of bits which are then sent in sequence to the receiver from the transmitter.
When the signals reach their destination they are reassembled in sequence by a demultiplexer.
They are then passed to the appropriate device for decoding.

Question 20

What is an electron volt?

Answer 20

Kinetic energy gained by an electron when placed in a p.d of 1V.