# Telecommunications

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• Created by: Kayleigh
• Created on: 21-05-11 14:54

## Speed of light, sound and waves

The speed of light is 3x10 to the 8 m/s

The speed of sound is 330m/s

Waves carry signals which contain information

The wavelength is the distance until the wave repeats itself

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## Optical fibres

Optical fibres travel at 2x10 to the 8m/s

They are cheaper and carry more signals than copper wires

Refraction is when the angle of incidence is less than the critical angle and so passes out the bottom of the glass. A small amount is reflected.

The critical angle- if the angle of incidence equals the critical angle the ray will emerge along the edge of the glass block.

Reflection is when the angle of incidence is greater than the critical angle and is totally internally reflected.

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aerial-tuner-decoder-amplifier-speaker

The power supply enters the chain through the amplifier.

Waves can bend around obstacles. This is called diffraction.

Waves with a long wavelength can diffact more than waves with shorter wavelengths.

Radio waves travel at the speed of light.

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## Television

The electron gun fires electrons through the tube. When the electrons hit the fluorescent screen and light is produced. The beam can be moved using coils of wire as electromagnets.

The electron beam is moved across the screen by the coils. The phosphor dots on the screen glow and give out light. The greater the number of electrons hitting a part of the screen, the brighter the spot of light produced. An image is built up line by line. The electorn beam scans across each line. 625 lines/picture, 25 pictures/second. A new picture arrives every 0.04 seconds and our eye holds the image for 0.1 seconds. This is called persistence of vision. As a new image arrives before the last one goes we see continous motion.

All colours on tv originate from red, green and blue.

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## Satellites and curved reflectors.

Microwaves diffract so little that it is said they only travel in straight lines. When these lines hit a curved reflector they are reflected back and all meet in one focus point.

Satellites send signals around the world. A geo-stationary satellite orbits the earth 36000km above the earth. It appears to be stationary above the earth but that is because it orbits at the same time as the earth one day.

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