P1d Flashcards
(11 cards)
Historically, how did the use of light greatly increase the speed of communication?
Historically, light was used to speed up communication over long distances. By creating a code of ‘on-off’ signals, a message could be relayed between stations far away, by flashing a light on and off in a way that could be decoded. This is the principle behind the Morse code.
Why is Morse code a digital signal?
The Morse code is a type of digital signal because the light pulse is only either ‘on’ or ‘off’.
What are the advantages and disadvantages of using light signals for communication?
Using light is a very quick way to communicate as the signals travel really fast. Multiplexing means that lots of different signals can be transmitted down a single optical fibre at the same time, so you don’t need as many cables. As it’s a digital signal, there’s little interference.
What are the advantages and disadvantages of using radio signals for communication?
Radio signals travel through the air, so they can be intercepted more easily. This is an issue for people using wireless internet networks.
What is total internal reflection?
A wave hitting a surface can experience total internal reflection. This can only happen when the light ray travels through a dense material like glass, water or Perspex towards a less dense substance like air.
If the angle of incidence is big enough, the ray doesn’t come out at all, but reflects back into the material. Big enough means bigger than the critical angle for the particular material - every material as its own, different critical angle.
If the angle of incidence is less than the critical angle, most of the light is refracted into the outer layer, but some of it is internally reflected.
If the angle of incidence is equal to the critical angle, the ray would go along the surface with quite a bit of internal reflection as well.
If the angle of incidence is greater than the critical angle, no light comes out. It’s all internally reflected.
How can light signals travel through optical fibres?
Optical fibres can carry data over long distances as pulses of light or infrared radiation. They work by bouncing waves off the side of a very narrow core which is protected by outer layers. The ray of light enters the fibre so that it hits the boundary between the core and the outer cladding at an angle greater than the critical angle for the material. This causes total internal reflection of the ray within the core. The pulse of light enters at one end and is reflected again and again until it emerges at the other end.
What are optical fibres used for?
Telephone and broadband internet cables, replacing the old electrical ones. They’re also used for medical purposes - to ‘see inside’ the body without having to operate.
What do properties of light produced by lasers allow them to be used for?
Surgery and dental treatment.
Cutting materials in industry.
Weapon guidance.
Laser light shows.
What does a laser produce?
A narrow beam of light of a single colour (monochromatic).
Why do most lasers produce an intense coherent beam of light?
1) All the waves in a laser beam are at the same frequency and wavelength. This makes the light monochromatic - all one, single colour.
2) The light waves are all in phase with each other - the troughs and crests line up, increasing the amplitude, so producing an intense beam. The waves in a laser beam are said to be coherent because they have a fixed phases difference.
3) Lasers have low divergence - the beam is narrow, and It stays narrow, even at a long distance from the light source.
How is a laser beam used in a CD player by reflection from the shiny surface?
The surface of a CD has a pattern of billions of shallow pits cut into it. The areas between the pits are called lands.
A laser shone onto the CD is reflected from the shiny bottom surface as it spins around in the player.
The beam is reflected from a land and a pit slightly differently - and this difference can be picked up by a light sensor. These differences in reflected signals can then be changed into an electrical signal.
The pits and lands themselves don’t represent the digital ons and offs. It’s actually a change in the reflected beam which represents on, while no changes represents off.
