1
Q
What is a progressive wave?
A
A wave that travels through a substance or space, transferring energy
2
Q
What is the amplitude of a wave?
A
The maximum displacement from the undisturbed position
3
Q
What is the wavelength of a wave?
A
The distance between the wave peaks (they may give the distance between Half a wavelength so make sure double it)
4
Q
What is the frequency of a wave?
A
The number of cycles per second, measured in Hz
5
Q
What is a hertz?
A
The unit of frequency, equivalent to s-1
6
Q
What is a period?
A
The time for one complete cycle, measured in seconds
7
Q
What does ‘in phase’ mean?
A
- Particles along a wave that move in phase move in the same direction with the same speed
- The particles have the same displacement from their mean position
- Particles, in phase are separated by a whole number, n of wavelengths, nlamda
8
Q
What does ‘out of phase’ mean?
A
Particles along a wave that move out of phase are at different points in their cycle at a particular time
9
Q
What does ‘anti phase’ mean?
A
- Particles along a wave that move in anti phase move in opposite directions at the same speed
- The particles have opposite displacements from their mean position
- Particles moving in anti phase are separated by a distance of a whole number, n of wavelengths plus an extra half wavelength, nlamda + lamda/2
10
Q
What is a phase difference?
A
- A phase different is measured as a fraction of the wave cycle between two points along a wave, separated by a distance x
- phi = 2pix/lamda rad
- One complete rotation involves turning through 2pi radians
- When a wave is reflected off the surface of a denser medium, it undergoes a phase change of 180 degrees
1. If two points say P and Q are both at the point of no disturbance and are half a wavelength separated their phase difference is 180 degrees
11
Q
What is a longitudinal wave?
A
- In longitudinal waves particles vibrate parallel to the direction that energy travels in
- Example: sound waves
12
Q
What is a transverse wave?
A
- In transverse waves vibrations are at a right angles to the direction that the energy travels in
- Example: EM waves
13
Q
What is an electromagnetic wave?
A
Electromagnetic waves are transverse waves all of which travel at the speed of 3x108 mpersecond in a vacuum
14
Q
What is a mechanical wave?
A
Mechanical waves cannot travel through a vacuum, but need a medium to travel through e.g. sound waves and seismic waves and waves on a string
15
Q
Describe sound waves
A
- Sound waves are longitudinal waves produced by vibrations that move backwards and forwards making the line of wave progression
- These vibrations produce regions of high and low pressure
- The regions of high pressure are called compression
- The regions of lower pressure are called rarefactions
- The wavelength of a longitudinal wave is the distance between successive compressions, or the distance between successive rarefactions
16
Q
What happens when sound travels through a solid?
A
- Energy is transferred through inter-molecular and inter-atomicbonds
- Sound travels quickly through solids because the bonds are stiff and the atoms are close together
- In gases the energy is transferred by molecules colliding, so the speed of the sound depends on the speed of the molecules
- Sound waves travel fastest in solids, less quickly in liquids, and even slower in gases
- Sound waves are mechanical waves, so they cannot travel through a vacuum
17
Q
What are properties similar to all electromagnetic waves? What is the order?
A
- All travel at the speed of light
- Their energy is carried by oscillating electric and magnetic fields
- Radio waves, microwaves, infrared, visible light, ultraviolet, x rays, gamma rays
18
Q
What is ionisation radiation and which EM cause it?
A
- Radio waves, microwaves and infrared radiation do not cause ionisation but may have a heating effect
- Ionising radiation includes shorter wavelength EM radiation: ultraviolet x rays or gamma rays
- If living cells absorb ionising radiation, DNA molecules in the cells be damaged, which can lead to mutations or cancers
19
Q
What is the effect of radio waves, microwaves, infrared radiation and visible light on living cells?
A
- Radio Waves: have little effect on living cells
- In microwave ovens, water and fat molecules in food absorb microwaves effectively. The molecules are forced to vibrate, heating the food. Microwave ovens used in many homes and businesses produce the frequencies most strongly absorbed by water, 2450MHZ
- Molecules that absorb infrared radiation vibrate more, increasing their internal energy. The heating effect is used for cooking and heating
- We cans ee because light-sensitive cells in the retina at the back of our eyes absorb visible light
20
Q
What is the effect of ultraviolet, gamma rays and X-rays on living cells?
A
- Ultraviolet Radiation: absorbed by skin causes a tan as melanin develops in these cells. Exposure to ultraviolet radiation raises the risk of skin cancer but, if a person is tanned, the melanin absorbs some UV radiation, reducing the amount of UV radiation reach cells deeper in the tissues, Skin cells exposed to UV radiation also produce vitamin D
- Gamma rays and x rays have similar properties, but gamma rays come from radioactive decay whilst x-rays are created when electrons strike a metal target. X rays are used for medical imaging as they penetrate soft tissues but are absorbed bu dense cells such as bones. High does of radiation kills cells, for example from exposure to high-intensity, high-energy radiation or exposure for a long duration. Gamma rays are used in radiotherapy and to sterilise surgical instruments
21
Q
How are radio waves used for communication?
A
- Radio waves transmit TV and radio programmes by superimposing information from the programme on to a carrier wave, changing (modulating) either its amplitude or its frequency
- The receiver is tuned to the frequency of the carrier wave and converts the signal into sound and images. Radio telescopes are massive, land based telescopes that receive radio signals from bodies in space that can penetrate through the atmosphere
22
Q
How are microwaves used in communication?
A
- Some microwaves penetrate the atmosphere and are used for satellite communicator and in mobile phone networks
- Microwaves have a shorter wavelength than radio waves, and diffract less so transmitters and receivers are a straight line of sight
23
Q
How is infrared, visible, gamma rays, x rays and UV radiation used in communication?
A
- Infrared and visible radiation can carry data in optical fibres. Remote controls often use infrared radiation as it only travels a short distance in air
- Gamma rays, X-rays and short-wavelength UV radiation do not penetrate the atmosphere, so space-based telescopes are needed to investigate these wavelengths in the universe
24
Q
What needs to be thought of with EM for communications?
A
In deciding which wavelets to use for a particular from of communication, consideration must be given to
- How much a wave will be absorbed by the atmosphere
- Or how much a wave will spread out due to the effects of diffraction
25
How are EM waves transmitted?
1. Electromagnetic waves that are transmitted only have their electric fields oscillating vertically (and their magnetic fields oscillating horizontally)
2. When the field of an EM wave only oscillate in one direction the wave is said to be a polarised wave
3. In this case we say that the electric fields are confined to the vertical plane (by convention the direction of polarisation is defined by the direction of the electric field)
26
What is a polarised wave?
-When the oscillations of the wave are confined to one plane, the wave is a polarised wave. For example, in an EM wave the electric field might be confined just to the vertical. plane. These waves are said to be vertically polarised
27
What is an example of an unpolarised wave?
1. Light is an example of an EM wave that is usually unpolarised when it is transmitted
2. Light waves are produced when electrons socially win atoms producing electromagnetic waves of a frequency of about 5x1014Hz
3. Since electrons in atoms can vibrate in any direction, the electric and magnetic fields of light osmically in any direction, so such light is unpolarised
28
Describe an unpolarised microwave incident on a metal grille
1. The electric fields of the waves have vertical and horizontal components
2. The horiztontal electric field components of the wave are absorbed by the wires of the grille because they cause electrons to osciallte along this direction
3. The vertical components of the electric field are not absorbed by the grille, so the waves that emerge from the grille are polarised with heir electric field vertical
4. Polarisation is a party of transverse waves only
5. Longitudinal waves cannot be polarised as the particles in a longitudinal wave always oscillate along the direction of energy transfer
29
What is the effect of polarisation?
1. Light is polarise sheen it passes through a polarising filter
2. The filter only allows through electric field oscillations in one plane because the filter absorbs energy from oscillation in all other planes
3. The metal grille is a good model for a polarising light filter
4. Long molecules of quinine iodosulphate are lined up and electrons in these molecules affect and electrons in these molecules affect the light in the same way as the microwaves are affected by the grille
30
Is polarise light or unpolarised light more intense and why?
1. Polarised light is less intense than unpolarised light because only half of the energy is transmitted through the filter
2. If a second polarising filter is held at right angles to the original filter, all oscillations are blocked and no light is transmitted
3. This is called crossing the polarisers
4. Light reflecting from a surface can be polarised by reflection. At some angles if incidence, the only reflected rays are rays whose electric field oscillate in one direction
31
What does it mean to cross polarisers?
If a second polarising filter is held at right angles to the original filter, this is called crossing the polarisers
32
How is polarisation used in sunglasses?
1. Polarising sunglasses include lens with polarising filters that are oriented so that they reduce the glare of reflected light
2. When light has been reflected it is partially polarised
3. The filters are orientated so that they cut our light reflecting from horizontal surfaces, such as water and snow
4. This reduces eye strain for skiers and make it much safer for drivers
5. Using polarising filters to reduce the flare of reflected light also makes it far easier to see into water
33
How is polarisation used in radio?
1. Outdoor television and FM radio ariels must be correctly aligned for the best reception
2. Transmitters generate plane-polarised electromagnetic waves, which are picked up most effectively by a reciter with the same alignment
3. It is possible to reduce interference between nearby transmitters if one fo the two transmitters is aligned vertically, and the other is aligned horizontally
34
What is refraction?
Refraction is the change in direction at a boundary when a wave travels from one medium into another
35
What is the angle of incidence?
The angle of incidence is the angle between the incident ray and the normal. The total is an imaginary line at right angles to the boundary between two materials
36
What is the angle of refraction?
The angle of refraction is the angle between the refracted ray and the normal
37
What is the refractive index?
It is the ratio of a wave's speed between two materials. It is normally quoted for light ravelling from a vacuum into a material
38
What is the law of refraction?
sini / sinr = constant (1n2)
39
What is total internal reflection?
The total internal reflection of light is the complete reflection of light at a boundary within a material that has a higher refractive index than its surroundings (angle of incidence is greater than critical angle)
40
What is the critical angle?
The angle of refraction for which the angle of incidence is 90 degrees
41
What is an optical fibre?
1. A thin glass (or plastic) fibre that transmits light (or infrared radiation)
2. These waves travel through the glass but are trapped inside by repeated TIR
3. This is possible if each reflection has an angle of incidence larger than the critical angle
4. The critical angle depends on the ratio between the refractive index of the optical fibre and its cladding (coating)
42
What is a step index optical fibre?
1. An optical fibre with a uniform refractive index in the core and a smaller uniform index for the cladding
2. By choosing a suitable material for the core and cladding, only certain wavelengths of light or IR radiation can travel though the fibre by TIR
43
What is dispersion?
- A spectrum seen using a prism is caused by dispersion
- Different colours of light travelling through the glass slow down by different amount
- The refractive index varies with wavelength
- A similar effect happens inside optical fibres
- As a single travels within an optical forbore, it disperse
- Two types of dispersion occur in step index optical fibres
44
What is material dispersion?
1. The spreading of a signal caused by the variation of refractive index with wavelength
2. It occurs because of the refractive index of the optical fibre varies with frequency
3. Different wavelengths of light in the signal travel at different speeds
4. This causes a sharp pulse to be spread into a broader signal
5. Therefore the duration of each pulse increases
6. This is called pulse broadening
7. Pulse broadening is a problem because it limits the maximum frequency of pulses and therefore the bandwidth available
45
What is pulse broadening?
Pulse broadening occurs when the duration of a pulse increases as a result of dispersion in an optical fibre
46
What is modal dispersion?
1. The spreading of a signal caused by rays taking slightly different paths in the fibre
2. It occurs when the rays inside an optical fibre take slightly different paths
3. Rays taking longer paths take longer to travel through the fibre, so the duration fo the pulse increases and the pulse broadens
4. Modal dispersion is signification in multimode fibres, because these fibres are broad enough to allow the rays to take different paths
5. For communications, monomode fires are used and these have a very narrow core, so that light is very early confined to one single both along the axis of the cable
47
What is absorption?
1. Absorption occurs when energy from a signal is absorbed by the optical fibre in which it travels
2. Some wavelengths of light are absorbed strongly in materials that are used to make optical fibres, so the signal strength falls
3. It is important to make an optical fibre from a material with low absorption at the wavelength used to send signals
4. The wavelengths commonly used are 650nm, 850nm and 1300nm.
5. It may also be necessary to amplify the signal if it travels long distances though the optical fibre
48
What is a wave?
- A wave is the oscillation of particles or fields
- A progressive(moving) wave carries energy from one place to another without transferring any a material
- A wave is caused by something making particles or fields oscillate (or vibrate) at a source
- These oscillations pass through the medium (or field) as the wave travels, carrying energy with it
49
What are some ways that waves can carry energy?
1. Electromagnetic waves cause things to heat up
2. X rays and gamma rays knock electrons out of their orbits, causing ionisation
3. Loud sounds cause large oscillations of air particles which can make things vibrate
4. Wave power can be used to generate electricity
5. Since waves carry energy away, the source of the wave loses energy
50
Define the different parts of a wave
1. Cycle - one complete vibration of the wave
2. Displacement, x, metres - how far a point on the wave has moved from its undisturbed position
3. Amplitude, A, metres - maximum magnitude of displacement
4. Wavelength, lamda, meters - the length of one whole wave cycle, from crest to crest or trough to trough
5. Period, T, seconds - the time taken for a whole cycle (vibration) to complete, or to pass a given point
6. Frequency, f, hertz - the number of cycles (vibrations) per second passing a given point
7. Phase - a measurement of the position of a certain point along the wave cycle
8. Phase difference - the amount one wave lags behind another
- Phase and phase difference are e,asured in angles (in degrees or radians) or as fractions of a cycle
51
How can waves by reflected and refracted?
1. Reflection: the wave is bounced back when it hits a boundary E.g you can see the reflection of light in mirrors. The reflection of water waves can be demonstrated in a ripple tank
2. Refraction: the wave CHANGES DIRECTION as it enter a DIFFERENT MEDIUM. The change in direction is a result of the wave SLOWING DOWN OR SPEEDING UP
52
What is the frequency of a wave?
-The frequency is the inverse of the period
1. Frequency = 1/period
2 f=1/T
3. 1Hz = 1s-1
53
How do you measure wave speed and the wave equation?
1. Wave speed (c) = distance travelled (d) / time taken (t)
| 2. Speed of a wave (c) = wavelength (lamda) x frequency (f)
54
What are transverse waves?
- Transverse waves vibrate at right angles to the direction of energy transfer
1. All electromagnetic waves are transverse. They travel as vibrations through magnetic and electric fields, with vibration perpendicular to the direction of energy transfer
2. Other examples of transverse waves are ripples on water or waves on a string
3. Two main ways of drawing transverse waves (DIAGRAM)
55
What are longitudinal waves?
- Longitudinal waves vibrate along the direction of energy transfer
1. The most common example of a longitudinal wave is sound
2. A sound wave consigns of alternate compressions and rarefactions of the medium it is travelling through (that is why sound can't go through vacuum). Some types of earthquake shock waves are also longitudinal
3. It is hard to represent longitudinal waves graphically and you will usually see them plotted as displacement against time (these can be confusing though, because they look like a transverse wave)
56
What is a polarised wave?
- A polarised wave only socialites in one direction
1. If you shake a rope to make a wave you can move your hand up and down or side to side or in a mixture of directions, it still makes a transverse wave
2. But if you try to pass waves in a rope through a vertical fence, the wave will only get through if the vibrations are vertical. The fence filters out vibration in other directions. This is called polarising the wave
- POLARISATION CAN ONLY HAPPEN FOR TRANSVERSE WAVES
57
How is polarisation evidence that electromagnetic waves are transverse?
1. In 1808, Etienne-Louis Malus discovered that light was polarised by reflection
2. Physicists at the time through that light spread like sound, as a longitudinal wave, so they struggled to explain polarisation
3. In 1817, Young suggested light was a transverse wave consisting of vibrating electric and magnetic fields at right angles to the transfer of energy and this explained why light could be polarised
58
What do polarising filters do?
- Polarising filters only transmit vibration in one direction
1. Ordinary light waves are a mixture of different directions of vibration (the things vibrating are electric and magnetic fields). They can be polarised using a polarising filter
2. If you have two polarising filters at right angles to each other then no light will get through
3. Light becomes partially polarised when reflected from some surfaces - some of it vibrates int eh same direction
4. If you view reflected partially polarised light through a polarising filter at the correct angle you can block out unwanted flare. Polarised sunglasses make use of this effect
59
How are television and radio signal polarised?
1. If you walk down the street and look up at the TV signals on people's houses, you'll see that the rods on them are all horizontal
2. The reason for this is that TV signal are polarised but he orientation of the rods on the broadcasting aerial
3. To receive a strong signal, you have to line up the rods on the receiving aerial with the rods on the transmitting Ariel and if they are not aligned, the signal strength will be lower
4. It is the same with radio, if you try tuning a radio and then moving the Ariel around, your signal will come and go as the transmitting and receiving aerials go in and out of alignment
60
What is the refractive index of a material?
- The refractive index of a material measured how much it slows down light
1. Light goes fastest in a vacuum and it slows down in other material, because it interacts with the particles in them. The more optically dense a material is, the more light slows down when it enters it
2. The absolute refractive index of a material, n, is a measure of optical density, It is found from the ratio between the speed of light in a vacuum, c, and the speed of light in that material, cs
- n = c / cs
3. The relative refractive index between two materials 1n2 is the ratio of the speed of light in material 1 to the speed of light in material 2
- 1n2 = c1 / c2
4. Combining these two equations gives
- 1n2 = n2 / n1
61
What is the absolute refractive index of a material?
1. The absolute index of a material is a property of that material only
2. A relative refractive index is a property of the interface between two material and it is different for every possible pair
3. Because you can assume nair = 1, you can assume the refractive index for an air to glass boundary equals the absolute refractive index of the glass
62
How can you use snell's law to calculate the refractive index?
- Snell's law uses angles to calculate the refractive index
1. The angle of the incoming light makes to the normal is called the angle of incidence, theta1
2. The angle the refracted ray makes with the normal is the angle of refraction, theta2
3. The light is crossing a boundary, going from a medium with refractive index n1, to a medium with a refractive index n2
4. When light enters an optically denser medium it is refracted towards the normal
5. n, theta1 and theta2 are related by Snell's law n1sintheta1 = n2sintheta2
63
What happens to light leaving an optically denser material?
- Light leaving an optically denser material is refracted away from the normal
- When light goes from an optically denser material into an optical less dense material (e.g. glass to air)
1. Shine a ray of light at a boundary going from refractive index n1 to n2, the gradually increase the angle of incidence
2. The light is refracted away front eh normal, so as you increase the angle of incidence, the angle of refraction gets closer and closer to 90 degrees
3. Eventually theta1 reaches a critical angle theta for which theta2 = 90 degrees. The light is reflected along the boundary
4. As sin90 = 1, Snell's law, n1sintheta1 = n2sintheta2 becomes n1sinthetac = n2 x1 so:
- Sinthetac = n2 / n1 = 1n2
5. At theta1 greater than the critical angle, refraction is impossible and all the light is reflected back into the material, this is called total internal reflection
64
How do optical fibres use total internal reflection?
- An optical fibre is a very thin flexible tube of glass or plastic fibre that can carry light signals over long distances and round corners. You only need to know about step-index optical fibres
1. Step-index optical fibres themselves have a high refractive index but are surrounded by cladding with a lower refractive index to allow total internal reflection
- Cladding also protects the fibre from scratches which could let light escape
2. Light is shone in at one end of the fibre. The fibre is so narrow that the light always hits the boundary between he fibre and cladding at an angle bigger than the critical angle
3. So all the light is totally internally reflected from boundary to boundary until it reaches the other end
65
What does dispersion and absorption do?
- Dispersion and absorption cause signal degradation
- A signal (a stream of pulses of light) travelling down an optical fibre can be degraded by absorption or by dispersion. Signal degradation can cause information to be lost
66
What does absorption cause?
- Absorption causes loss in amplitude
1. As the signal travels, so of its energy is lost through absorption by the material the fibre is made from
2. This energy loss results in the amplitude of the signal being reduced
67
What does dispersion cause?
-Dispersion causes pulse broadening
-There are two types of dispersion that can degrade a signal:
1. Modal dispersion - light rays enter the fibre at different angles, and so take different paths. The rays which take a longer path take longer to reach the other end that those that travel down the middle of the fibre
-A single mode fibre only lets light take on path, so it stops modal dispersion
2. Material dispersion - light consists of different wavelengths that travel at different speeds in the fibre - this causes some light wavelengths to reach the end of the fibre faster than others
-Using monochromatic light can stop material dispersion
(DIAGRAM)
68
What do both types of dispersion lead to?
1. Both types of dispersion lead to pulse broadening. The signal sent down the fibre is broader at the other end
2. Broadened pulses can overlap each other and confuse the signal
3. An optical fibre repeater can be used to boost and regenerate the signal every so often, which can reduce signal degradation caused by both absorption and dispersion
Z A Level Physics
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