Waves in Industry

Year 10 Science

Fibre Optics, Mobiles and Microwaves

Why use Optical Fibres• Optical fibres are used as an efficient medium for transmission of

communication channels.

• A bundle of copper wires 10cm thick can carry 600 two-way conversations.

• A bundle of optical fibres just 1cm thick can carry nearly 30,000 conversations.

The fibre optic cable shown above has 50 times the carrying capacity of the bundled copper wires !!

Total Internal Reflection• Did you know that glass prisms, not mirrors, are used to

reflect light in high quality optical instruments?• Total Internal Reflection is the property used for this

efficient reflection of light.

• We all know that light can bend (or refract) when light travels from mediums of different density.

• Total Internal Reflection occurs when light rays hit the surface of a less dense medium at a large enough angle of incidence to make the light completely reflect back rather than bend.

(In experiment 4.6 you will look at Total Internal Reflection)

How do Optical Fibres work• Optical fibres use the property of Total Internal Reflection.• The optical fibre is made of an inner glass core and a less dense glass cladding.

It is these differences in density that allow the light to be bent so much that it is internally reflected (Note, your Core 4 book is wrong!!).

• The light totally internally reflects where the fibre and cladding join, continually bouncing along the inside, until it emerges from the end.

• Laser light is used as a light source – switched on and off to provide digital pulses of coded information.

Some properties of Electromagnetic Waves

• All electromagnetic waves can pass through a vacuum.

• Electromagnetic waves travel the vacuum of space at the common speed of light of 300 million metres per second (3.0 x 108 m/s)

What is the Electromagnetic Spectrum?• The Electromagnetic Spectrum is a continuum of electromagnetic waves with

artificial divisions based on the frequency and wavelengths of the waves.• There is no distinct point at which the frequency changes and no special

change in properties at particular waves boundaries (looking at a rainbow illustrates this).

Radio Waves• Wavelengths ranging from 10cm to 1000m.• Lowest energy waves in Electromagnetic Spectrum.• Radio Waves include: AM radio, FM radio, TV,

Microwaves and Radar.• Variety of uses – depends upon frequency (see below):

Producing Radio Waves• Radio waves are emitted by stars but can be produced artificially.• Radio waves can be produced when electrons in a metal rod, called

a transmitting antenna or transmitter, are made to vibrate rapidly.

• Radio stations use transmitters to transmit their own frequency of radio wave called the carrier wave.

• The voice or music from the radio station forms an audio signal.• The audio signal is then mixed with the carrier wave to produce a

modulated wave.• This modulated wave is what is transmitted by the radio station.

Detecting Radio Waves• The radio waves produced by the radio station can be detected using a receiving antenna (a metal rod just like the

transmitter).• In the receiving antenna the radio waves cause electrons in the metal to vibrate rapidly and produce an electrical signal.• The receiving antenna of your radio detects the modulated wave.• Your radio then subtracts the carrier wave from the signal, leaving just the audio signal to be amplified by an audio

amplifier and sent to the speakers.

AM and FM Radio• In AM (Amplitude Modulation) the audio signal

changes the amplitude of the carrier wave. • In FM (Frequency Modulation) the audio signal

changes the frequency of the carrier wave.

• AM radio waves have longer wavelengths than FM and can be received at greater distances.

• FM radio waves are less affected by electrical interference and hence provide a higher quality transmission of sound

Television• Television signals are transmitted on two separate carrier

waves– Visual signal is added onto one carrier wave using Amplitude

Modulation (AM)– Audio signal is carried on a separate carrier wave using Frequency

Modulation (FM)

• When you select a particular channel, you are selecting the respective visual and audio carrier waves for that channel.

• Your TV then completes the task of ‘stripping’ the carrier waves to produce the desired picture and sound.

• NOTE: In Digital Television visual and audio information is transmitted as a binary code using a variety of modulation techniques such as; pulse code modulation (PCM), pulse amplitude modulation (PAM), pulse duration modulation (PDM), and pulse position modulation (PPM).

Microwaves

• Wavelengths ranging from 1 millimetre to 30 centimetres.

• Were first used in World War 2 in Radar.• Used in microwave ovens (frequency of 2450 MHz) for

cooking. Produced by a magnetron when cathode rays (a beam of electrons) rotate past an electric field.

• Also, used in mobile phone communications at frequencies of around 900 MHz. Transmission can be across distances of up to 100 km, but there must be a direct ‘line of sight’

Infra-red Radiation• Wavelengths ranging from 700 nanometres (this is 0.0007

millimetre!) to 1 millimetre.• Emitted by hot objects• Variety of uses including:

– Remote controls– Security and burglar alarms– Medical treatments for soft tissue injury.– Thermal imaging applications.

Visible Light

• Wavelengths ranging from 400 to 700 nanometres.

• We see light of different frequencies as different colours.

• White light is light that contains all the colours of the spectrum

• A variety of applications including:– fibre-optic communications– Photography– Laser technology

Ultraviolet (UV) Radiation• Invisible to human eye• Wavelengths ranging from 10-400 nanometres.• Small doses beneficial to humans as it encourages

production of vitamin D.• Larger doses can lead to cell and tissue damage – possibly

causing skin cancer or eye cataracts.• Most types of glass absorb UV rays but clouds do NOT

absorb UV (that is why you can get sunburnt on cloudy days!)

• Variety of uses including:– Plastic curing in photo-initiator chemicals in polymerisation– Astronomical observations– Sterilisation of hospital equipment

X-rays

• Wavelengths ranging from 0.01-10 nanometres.• Have energy enough to pass through human flesh• Produced when fast moving electrons hit the heavy

atoms in a metal target• Variety of uses including:

– Cancer treatment by focussing the rays to kill cancer cells– Finding weakness in metals and analysing structures of

complex chemicals.– Imaging applications in medicine.

X-ray Images

Gamma Rays

• Wavelengths less than 0.01 nanometres.• Highest energy waves in Electromagnetic Spectrum.

• Produced when energy is lost from the nucleus of an atom during radioactive decay.

• Highly destructive to human tissue

• Can be used to kill cancer cells.

• Also used in finding fractures and weaknesses in metals.

• Can be detected with photographic film or a Geiger counter.