P1* - EM waves, Communication, Communicating with Light, lasers,Infrared

Question 1

What is the wavelength of electromagnetic radiation?

Answer 1

There is no set wavelength, it can occur at many different wavelengths.
- There is a continuous spectrum of different wavelengths, but waves with similar wavelengths tend to have similar properties.

Question 2

What happens if two waves have similar wavelengths?

Answer 2

They will tend to have similar properties.

Question 3

What are the seven types of waves?

Answer 3

• Radio-waves
• Microwaves
• Infra-red
• Visible Light
• Ultra violet
• X rays
• Gamma Rays

Question 4

How much of the EM radiation we receive from the sun is visible light?

Answer 4

About half of the EM radiation we receive the sun is visible light.
The remainder is Infrared radiation ( heat ) and some is UV.

Question 5

What happens to the EM radiation as frequency/wavelength are changed?

Answer 5

As frequency and wavelength of EM radiation changes, its interaction with matter changes, I.e. the way a wave absorbed, reflected or transmitted by any given substance changes.

Question 6

What is the key rule of the EM wave spectrum?

Answer 6

As a rule, the EM waves at each end of the spectrum tend to be able to pass through material, whilst those nearer to the middle are absorbed.

Question 7

Why are waves with higher frequencies ( shorter wavelengths ) more dangerous?

Answer 7

• They have more energy, e.g. X rays in large amounts can cause mutations that lead to cancer.

Question 8

Why is absorbing EM’s dangerous?

Answer 8

When any EM radiation is absorbed it can cause heating and ionisation, if the frequency is high enough.

Question 9

What is ionisation?

Answer 9

Ionisation is where an atom or molecule either loses or gains electrons, and it can be dangerous.

Question 10

What is a use of EM waves besides cooking food?

Answer 10

A use of EMs besides keeping us warm and cooking food is using them for communication.

Question 11

What happens to Information when you communicate it?

Answer 11

• Before you communicate information, it EM waves are changed into an electrical signal, which is then sent off on its own ( like you get in an ordinary phone line ) or carried on an EM wave.

Question 12

Why are light, radio and electrical signals useful for transmitting information?

Answer 12

• Using light, radio and electrical signals is great because the signals travel really fast.

Question 13

Why may you use electrical wires?

Answer 13

Electrical wires and optical fibres can carry loads of information very quickly.

Question 14

What is an advantage of using electrical and optical wires over radio?

Answer 14

• Information sent through optical fibres and electrical cables is pretty secure, they are inside a cable and so can’t easily be tapped into.
• Radio signals travel through the air, so they can be intercepted more easily, this is an issue for people using wireless internet networks.

Question 15

What are advantages of using wireless signals over cables?

Answer 15

• Cables are difficult to repair if they are broken, which isn’t a problem for wireless networks.
• Wireless communication is portable, e.g. mobile phones, laptops, wifi

Question 16

What does wireless communication rely upon?

Answer 16

• It relies on an aerial to pick up a signal, and signal strength often depends on the location.

Question 17

How do optical fibres carry data?

Answer 17

• Optical fibres can carry data over long distances as pulses of light or infrared radiation.

Question 18

How do optical fibres work?

Answer 18

• Bouncing waves off the sides 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 and is reflected again and again until it emerges at the other end.

Question 19

What are some uses of optical fibres?

Answer 19

• Optical fibres are increasingly being used for telephone and broadband internet cables replacing the old electrical ones.
• They are also used for medical purposes, to see inside the body without having to operate.

Question 20

How fast does light travel in a vacuum?

Answer 20

300,000,000.

It is slowed down by about 30% when travelling in an optical fibre.

Question 21

What is multiplexing?

Answer 21

• 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.

Question 22

What is an advantage of using optical fibre over analogue?

Answer 22

• It is a digital signal, there is very little interference.

Question 23

What is Ordinary visible light?

Answer 23

Ordinary visible light is a combination of waves of different frequency and wavelength ( and so colour ) that are “out of phase” with each other.

Question 24

What is a laser beam?

Answer 24

A laser beam is just a special ray of visible light that has a few extra properties which make it special

Question 25

What extra properties does a laser beam?

Answer 25

• All the waves in a laser beam are at the same frequency and wavelength, this makes the light monochromatic, it is a single pure colour.
• Light waves are all in phase with each other, the troughs and crests line up, increasing the amplitude, so it produces an intense beam.
• The waves in a laser beam are said to be coherent, because they have a fixed phase difference, in this case zero because they are all in phase.
• Lasers have low divergence, the beam is narrow and it stays narrow, even at long distances it doesn’t diverge.

Question 26

What are lands?

Answer 26

• The area between pits cut onto a CD.

Question 27

How do CD players use lasers to read information?

Answer 27

• 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.
• This difference can be picked up by a light sensor
• These differences in reflected signals can be changed into an electrical signal.
• An amplifier and a loudspeaker then convert the electrical signal into sound of the right pitch ( frequency ) and loudness.

Question 28

What represents the digital “on” and “off” signals on a CD?

Answer 28

• The change in reflected beam represents on, while no change represents off.

Question 29

What signals do remote controls use to transmit information?

Answer 29

Remote controls use IR to transfer information.

Question 30

What is a drawback of using IR to transfer information?

Answer 30

It can only be used over short distances.

Question 31

How are Infrared signals used in security systems?

Answer 31

• They can be used in burglar alarms and security lights.

- The sensors detect heat from an intruders body.

Question 32

What can you use instead of light to transfer information along an optical fibre?

Answer 32

• You can use IR instead of visible light to carry information through optical fibres.

Question 33

What is infrared radiation given out by?

Answer 33

• Infra-red radiation is also known as heat radiation, it is given out by hot objects.
• The hotter the object the more IR radiation it gives out.

Question 34

How can infrared be used to monitor temperatures?

Answer 34

• Heat loss through a house’s insulated roof can be detected using infra-red sensors.

Question 35

How is infra-red used by night vision equipment?

Answer 35

• Infrared is detected by NV equipment, the equipment turns it an electrical signal, which is displayed on screen as a picture.
• The hotter the object is, the clearer the image.

Question 36

How is IR used in remote controls@?

Answer 36

• Remote controls can emit pulses of IR to control electrical devices such as TVs
• The pulses act as a digital on/off code.
• The device will detect and decode the pattern of pulses coming from the remote control and follow the coded instruction.
• A CD player might be programmed to know that a certain sequence of pulses at a particular speed means play, so when it receives the signal, it will play the CD, a different sequence may tell it to pause.
• IR signals are used the same way to transfer information between mobile phones and computers over short distances ( so they are another form of wireless communication)

Question 37

what is the drawback with using IR?

Answer 37

• You need to be close to the device you are operating, because the IR beam from a small low powered remote is pretty weak.
• You also need to point the beam straight at the detector on the device, because the IR waves only travel in straight lines.