unit 1: waves in communication Flashcards
1
Q
what is a wave?
A
- a wave is a movement of energy from one place to another
- where something called the displacement oscillates
2
Q
what is a longitudinal wave?
A
- a wave where particles oscillate back and forth ( parallel ) along the direction of the propagation
( the direction the wave is travelling ) - the oscillations cause compressions and rarefactions that move the wave pattern
3
Q
what is meant by displacement?
A
- how far the particles move from the middle ( rest position )
4
Q
what is meant by a compression and rarefactions?
A
- compressions are where the particles are packed together
- rarefactions are where the particles are further away from one another
5
Q
what is a type of longitudinal wave?
A
- sound waves
- microphones detect pressure variations caused by compressions and rarefactions of the air in a sound wave
- ultrasound is used for medical diagnosis and for under water echo location
6
Q
what is a transverse wave?
A
- a wave where particles oscillate at right-angles to the direction of propagation- up and down or side to side
7
Q
what are some types of transverse waves?
A
- light waves and water waves
- transverse waves in water or in stretched strings can be visible as travelling ripples or as standing waves
8
Q
what is an electromagnetic wave?
A
- these waves travel through a vacuum, no medium is needed
9
Q
what is a medium?
A
- a medium is a substance or material that contains particles
10
Q
what are the main features of a transverse waves?
A
1) wavelength- the distance between two consecutive points where oscillations are in phase with one another, one wavelength = one cycle e.g. peak to peak or trough to trough
2) propagation- the direction the wave is travelling
3) peak / crest- the top of the wave
4) trough- the lowest part of the wave
5) amplitude- the height of the peak or the depth of the trough measured from the central rest or the equilibrium point
( middle )
11
Q
what is the focus of a displacement-time graph?
A
- focuses on oscillations at just on point in space
12
Q
what is the focus of a displacement- distance graph?
A
- focuses and shows the wave shape over the whole space at that instant
13
Q
what is meant by frequency?
A
- describes the number of complete waves that pass a point per second e.g. if the wave takes 1 second to pass, the frequency is 1Hz
- however, if the wave takes 2 seconds to pass, then the frequency of the wave is 0.5Hz
- so if the wave did 6 cycles in 2 seconds, you do 6/2 = 3Hz
14
Q
what is time period?
A
- time period is the time it takes to complete one full wave
15
Q
how do we calculate time period?
A
1/T = f
T = time period
f = frequency
16
Q
what is light made up of?
A
- light is made up of transverse waves that oscillate at different angles relative to the direction of propagation
17
Q
what is polarisation?
A
- this is when a polarising filter will remove any oscillations/ light that are at 90 degrees to the filter and not wanted, electromagnetic waves can be polarised
- e.g. if the first filter is vertical the wave will pass through, but if the second wave is horizontal it will stop the light from passing through, as it is 90 degrees relative to the first filter
18
Q
what is a real life application of polarisation?
A
- if light enters a pond, it will reflect off the bottom of the pond and the surface of the water
- we can polarise the light to remove the light that reflects from the surface of the water ( and the oscillations that are 90 degrees ), to clearly see the bottom of the pond
19
Q
how do we convert to or from standard form on a calculator?
A
- press the ENG button, which makes the power more negative
- pressing shift and then ENG will make the power more positive e.g. type 50 and then equal, then press shift ENG which changes the value to 0.05 x 10³ which is the same as 50
20
Q
what are the prefixes in physics?
A
kilo (k) = 1000 = x10³
mega (M) = 1,000,000 = 10⁶
Giga (G) = 1,000,000,000 = 10⁹
milli (m) = 0.001 = x10⁻³
micro (μm) = 0.000001 = x10⁻⁶
nano (n) = 0.000000001 = x10⁻⁹
21
Q
how do we calculate the SI unit of mass in kg?
A
- you must divide the number by kg which is x10³
e.g. 0.01 x 10⁻⁶ g to kg = 0.01 x 10⁻⁶ / 10³ = 0.01 x 10⁻⁹ kg
22
Q
how do we calculate wave speed?
A
wave speed (m/s) = frequency (Hz) x wavelength (m)
v = f x λ
23
Q
how do we convert from cm to m?
A
- if you convert from cm to m, then your result will always be smaller
- cm to m is / 100 e.g. 20 cm = 0.2m
24
Q
what is the correlation between frequency, wave speed and wavelength?
A
- if we increase the frequency, the wavelength decreases, meaning the speed of the wave stayed the same
25
example of wave speed question
1) calculate the speed of the wave with a frequency of 10kHz and a wavelength of 2m
f = 10kHz, k= x10³
λ = 2m
v = ?
v = (10x10³) x 2 = 20000 m/s
26
how do we calculate speed?
speed = distance travelled (km) / time taken (s)
27
give an example of calculating speed.
e.g. the transverse wave travels 6000km in a time of 1010 seconds. calculate the speed of the transverse wave in km/s⁻¹
s = ?
d = 6000km
t = 1010s
s = 6000 / 1010 = 5.94 km/s⁻¹
28
what is phase difference?
- when comparing two or more waves, the phase difference indicates how much one wave is shifted relative to another wave
- they can be in-phase ( aligned peak to peak or trough to trough), out of phase (aligned peak to trough), or somewhere in between
- he phase difference is commonly expressed as an angle or a fraction of a wavelength
29
what are the main concepts of phase difference?
- one wavelength is the same as one full circle
- half a wavelength is the same as half a circle
- one quarter of a wavelength is the same as one quarter of a circle
30
how do we convert between degrees and radians?
- radians is a unit of angle, just like degrees, however they are more accurate than degrees
- 1 quarter of a circle is 90 degrees, 2 quarters is 180 degrees, 3 quarters is 270 degrees and a full circle is 360 degrees
- radians is always the number of quarters you have counted x π/2 e.g. 810 degrees is 9 quarters and so it would be 9π/2 rads
31
what is the phase difference in degrees/rad and number of wavelengths of 1/4, 2/4,3/4 of a cycle, 1 cycle, 1.5 cycle and 2 cycles?
1/4 = 90°, π/2, λ/4
2/4 = 180°, 2π/2, 2λ/4
3/4 = 270°, 3π/2, 3λ/4
1 = 360°, 4π/2, 4λ/4
1.5 = 540°, 6π/2, 6λ/4
2 = 720°, 8π/2, 8λ/4
32
convert 3π/2 radians into degrees.
3π/2 is the equivalent to 3 quarters of a circle which equals 270 degrees
33
how do we calculate the phase difference between two points on a wave?
- e.g. what is the phase difference between point M and N
- start at point M on the wave and count from trough, middle, peak, marking each point with a cross, until you reach point N
- then count how many crosses there are on your wave between each point including the cross at point N
- e.g. 3 points = 3/4 which is 3π /2 rads
34
what is meant by coherence?
two waves are coherent when they have:
- the same frequency and a constant phase difference
35
what is a meant by a constant phase relationship?
- when the phase difference between two waves never changes
- this is also known as in-phase
36
what is meant by diffraction?
- when waves spread out as they pass through a gap or go past an obstacle
- a narrower gap causes more diffraction and if the gap is too large, most of the waves pass straight through the middle so diffraction is ineffective
37
what is a wavefront?
- these are lines drawn to join points in a wave where all the oscillations are in-phase
- these are one wavelength apart and perpendicular to the direction of propagation
38
when does diffraction work best?
- diffraction works best when the gap size is equivalent to the wavelength
39
what is superposition?
- this is when two or more waves occupy the same space and their amplitudes add, as waves have no charge and so occupy one another's space
40
what makes a wave in phase?
- when two waves are in-phase their phase difference is 0, 2π, 4π, 6π... radians
- the waves are aligned peak-peak or trough-trough
41
what makes a wave anti phase?
- two waves are anti phase of their phase difference is π radians ( 180 degrees )
- the waves are aligned peak-trough
42
what makes a wave out of phase?
- if the waves phase difference is anything else / any other angle then it is out of phase
43
what is constructive interference?
- when two waves meet, they interfere with each other
- if they meet each other in phase the amplitudes 'add up' to produce large peaks and troughs, meaning a large wave is formed
44
what is destructive interference?
- when two waves meet, they interfere with each other
- if they meet each other in anti phase the amplitudes 'subtract' and cancel out to produce no peaks and troughs, meaning no wave is formed
45
what type of fringe does constructive and destructive interference produce?
constructive interference = bright fringes
destructive interference = dark fringes
46
how is diffraction affected by different wavelengths?
- the diffraction pattern will become more spread out if a larger wavelength is used e.g. the larger the wavelength, the larger the angle at which the light spreads out
47
what happens when white light is diffracted?
- if white light is diffracted, then each bright fringe becomes a rainbow, this is because white light is made up of all the visible colours ( wavelengths ) and different wavelengths spread out of different angles
48
what is path difference?
- this is when light from different slits will travel different distances to get to the same point
- path difference can be used to determine whether constructive or destructive interference will occur
49
how does path difference link to interference and wavelength?
- if the path difference is equal to a whole number of wavelengths, constructive interference will occur and we will get a bright fringe
- if the path difference is equal to a half a wavelength, destructive interference will occur and we will get a dark fringe
50
how do we calculate path difference?
- this is when we subtract the difference between slit one and slit two of a wave, and if the path difference is always 0, then it gives constructive interference. if the path difference is half of the wavelength in the question then destructive interference will occur
- e.g. the light in this question has a wavelength of 1m
- slit 1 has a length of 10m and slit 2 is also 10m so 10-10 = 0, and is constructive interference
- e.g. slit 1 is 20m and slit 2 is 19.5, so 20-19.5 = 0.5 which is half the wavelength of 1, meaning destructive interference will occur
51
what are diffraction gratings?
- these are flat arrays of regularly spaced lines, which are designed to break up a place wave-front into a set of seperate wave sources
52
what is order of diffraction?
- this is when white light is made up of a mixture of different wavelengths of light so each bright fringe will be rainbow
- the middle single section, where all wavelengths are added together, is labelled zero
- the first time you see the light fringes, this is labelled first order
- the second time you see them, they are labelled second order
- the third time you see them, they are labelled third order
53
which colour is diffracted the most?
- red is diffracted the furthest away from zero as it has the longest wavelength
- at each colour all the wavelengths are added e.g. red shows all the red wavelengths
54
explain how dark and bright interference fringes are produced.
- a path difference is created by dividing a light source so that seperate rays of light travel different paths
- the difference in path causes a phase difference
- the path difference increases as the angle through which a grating scatters the light is increased
- when the path difference equals a whole number of wavelengths, the light rays will be in phase and constructive interference will occur, meaning the amplitudes of the wave will add up forming large peaks and troughs which produces a bright fringe
- in between, at a path difference of λ/2, 3λ/2, the path difference is equal to half a wavelength meaning the light rays will be in antiphase and destructive interference occurs, meaning the amplitudes of the wave will cancel one another and produce no peaks or trough, giving a dark fringe
55
what is an optical fibre?
- optical fibres are used to transmit light, they are long, thin, cylindrical cores of glass encased in a cladding of glass of lower refractive index
- they are used for communications and for medical applications
56
what happens when light is fed into the cut end of the fibre?
- light is fed into the cut end of the fibre, so when it hits the sides of the fibre, it almost always does so at angles greater than the critical angle
- this means that all the rays of light get totally internally reflected and keep bouncing down the length of the fibre
- no wave energy gets lost through the walls of the fibre, although as glass is not perfectly transparent, some is gradually absorbed
- when light waves arrive at the far end of the fibre, up to a few km away their intensity is still large enough to measure as a signal
57
what is the normal line?
- a line at right angles to the surface of a transparent medium ( e.g. glass or water ) that passes through the point where a ray enters or exits the medium
58
what is the angle of incidence?
- the direction of the incoming ray
59
what is refraction?
- the direction of an outgoing ray after bending due to it passing from one medium to another
60
how does the speed change when light enters a glass block?
1. light slows down and bends towards the normal due to entering a more dense medium as the angle of incidence is larger than the angle of refraction ( light going into the block )
2. light speeds up and bends away from the normal due to entering a less dense medium, as the the angle of incidence is less than the angle of refraction ( light going out of the block )
61
what is total internal reflection?
- when a wave that is already in an optically dense medium hits a boundary with a less dense medium and energy is reflected back into the denser medium
62
what is the critical angle?
- the least angle of incidence at which total internal reflection occurs
- this only applies when the light tries to leave an an optically dense medium
( higher refractive index) at a boundary into a less dense medium ( lower refractive index )
- the higher the refractive index the lower the critical angle
63
what happens to the critical angle at a glass-air interface?
- when the light ray comes to the other side of the glass block and tries to leave, back out into the air, or into a vacuum, the light wave will speed up, making the wave front turn back towards the direction in which it was travelling before it entered the glass
- however, during the speed up some of the wave energy gets reflected back inside the glass, this is internal reflection
- the larger the angles involved the more light is reflected and the less energy gets through in the refracted beam to escape from the glass into the air
64
how do you calculate the critical angle at a glass-air interface?
sinC = 1 / n
C is the critical angle
n is the refractive index of the material
65
(a) the refractive index of a material is 1.67. calculate the critical angle.
- to calculate the critical angle you must press shift first on your calculator to got sin⁻¹
sinC = ?
1
n= 1.67
sinC = 1 / 1.67 = 5.988
sin⁻ ¹= 36.78°
66
(b) the critical angle of a material is 52°. the speed of light in air is 3x10⁸ m/s. calculate the speed of light using c/v = 1/sinC
c= 3x10⁸
v=?
1
sinC= sin(52)
1) find n using n= 1 / sinC
n= 1 / sin(52) = 1.27°
2) then find v using c / n
v = ( 3x10⁸) / (1.27) = 236.4 x 10⁶ m/s
67
(c) the speed of light in air is 3x10⁸ m/s and the speed of light in Pyrex is 1.70x10⁸ m/s. calculate the critical angle of pyrex.
c= 3x10⁸
v= 1.70x10⁸
1
sinC= ?
1) find n using n= c /v
n= (3x10⁸) / (1.70x10⁸) = 1.76
2) then find sinC using sinC = 1 / n
sinC= 1 / 1.76= 0.57
sin⁻¹ = 34.58°
68
how do we measure the critical angle of a semi-circular glass or perplex block using a ray box?
1) lie a piece of A3 paper on a bench and draw a straight line in a little longer than the block near the paper
2) place a ruler on the line and place the straight side of the semi-circular block against the ruler
3) mark the centre of the circle on the paper
4) aim a ray of light from the ray box through the curved surface of the block so that is meets the straight surface at the centre of the circle
5) notice that the ray passes through the curved surface along the normal. this is so that there is no deviation at the curved surface as the ray enters the block
6) rotate the paper and block about the centre of the block so that the angle of incidence inside the block increases
7) continue increasing the angle until the emerging ray just disappears. mark the path of the ray inside the block and the normal
8) measure the angle between the ray inside the block and the normal ( angle of incidence inside the block ), this is the critical angle
- you can use the formula n=1/sinC to find the refractive index of the block
69
how do you calculate the refractive index?
n = c / v = sin i / sin r
n = the refractive index of the material
c = the speed of light just before entering the material ( 3 x 10⁸ )
v = the speed of light in the material
i = the angle of incidence
r = the angle of refraction
70
(a) give an example of how to calculate the refractive index when the speed of light in the air is 3x10⁸ and the speed of light in water is 2.25x10⁸.
n = c / v
n= ?
c= 3x10⁸
v= 2.25 x 10⁸
n = (3x10⁸) / (2.25x10⁸) = 1.3 and has no units
71
(b) the speed of light in air is 3x10⁸, the refractive index of glass is 1.45. what is the speed of light in glass?
v = c / n
c = 3x10⁸
n = 1.45
v = (3x10⁸) / 1.45 = 206.9 x10⁶ m/s
72
(a) a laser passes through a material with an angle of incidence of 41° towards a boundary with air. the index of refraction of the material is 0.81, calculate sin(r).
n = sin(i) / sin(r)
sin(r) = sin(i) / n
sin(r) = sin(41) / 0.81 = 0.81
73
(b) calculate the angle of refraction for the above laser.
shift = sin⁻¹(0.81 = 54.1°
74
light in air approaches the boundary of oil at angle of 28.9° with respect to the normal. the light travels at a speed of 2.3 x 10⁸ m/s through the oil. determine the angle of refraction.
use c / v = sin(i) / sin(r)
1) n = ?
c = 3x10⁸
v = 2.3x10⁸
n= (3x10⁸) / (2.3x10⁸) = 1.3
2) sin(r) = ?
sin(i) = 28.9°
n = 1.3
so sin(r) = sin(28.9) / 1.3 = 0.37
the press shift sin⁻¹(0.37) = 21.7°
75
what is the inverse square law?
- the intensity ( I ) of an electromagnetic wave decreases the further away it is from it's source
76
what equation is used to calculate the inverse square law?
I = k / r²
I = intensity in watts / metre²
k = constant in watts ( W )
r² = distance from the source in metres or metres²
77
(a) a light emits visible light. what is the intensity 2m from the source if k = 1?
I = 1 /2²
I = 1/4
I = 0.25 W/m²
78
(b) a mobile phone emits a microwave. at what distance is the intensity 7.5 W/m², if k = 3?
r² = k / I
r² = 3 / 7.5 so r = √(3 / 7.5 ) = 0.63m
79
(c) an infrared remote control has an intensity of 3.5 W/m² at 10cm away. at what distance is the intensity 8.0 W/m²?
I one= 3.5W/m²
k= ?
I two= 8.0W/m²
r²=10cm -> / 100 = 0.1²
1) k = 3.5 x 0.1² = 0.035m
2) r = √(0.035 / 8.0 ) = 0.066m
80
what equation can you use to compare the intensity of two sources?
I1 / I2 = (d2 / d1 )
I1 = initial intensity
I2 = final intensity
d2 = final distance
d1 = initial distance
e.g. a light is 6m. how many times smaller will the intensity be from a light 0.5m away?
I1 / I2= (0.5/2)² = 0.007 x smaller
81
what is cladding?
- one or more layers of materials with a lower refractive index in intimate contact with a core material of higher refractive index
82
why is glass fibre surrounded with glass cladding?
- cladding is used to protect the core of the fibre
- it causes light to be confined in the core of the fibre by total internal reflection and prevents light from 'escaping' the core
83
how does cladding affect the loss of energy from the ray of light travelling along the optical fibre?
- as cladding has a lower refractive index and is less optically dense than the core so the energy will remain inside the fibre due to total internal reflection
- as well as this the angle of incidence has to be bigger than the critical angle so TIR can happen and the light remains inside, if not the light will go outside
84
what two types of things can be used for broadband?
- copper wires
- fibre optics
85
how is information transmitted in copper wires compared to fibre optics in broadband?
copper wires- copper broadband uses electrons for data transmission to minimise interference
optical fibres- transmit info through fast travelling pulses of light which are converted from electronic signals
86
how do you hack copper wires compared to optical fibres?
copper wires- they rely on electromagnetic signals to hack a wire
optical fibres- place a bend in a fibre and aim a detector at the light that will make it leak due to stress
87
what are the advantages of using a copper wire vs a fibre optic for broadband?
copper wires- they are easier to install, test and maintain, they are also cheap as they are simple to produce as well as being durable and compatible
optical fibres- they have less interference and keeps the signal strength over greater distances at a higher frequency making it more efficient
88
what are the disadvantages of using copper wires vs optical fibres in broadband?
copper wires- have restricted bandwidth which is unsuitable for transmitting large amounts of data over long distances
fibre optic- thinner and lighter so can gain physical damage, they are also sensitive and expensive to produce and install
89
compare how copper wires and fibre optics effect the quality of data.
copper wires- experience significant data loss as they can lose up to 94% of the signal over distances greater than 100 metres
fibre optics- fibre optics typically provide a better performance as they only lose 3% of signal over long distances
90
what are the similarities and differences between fibre optics and satellite transmission?
similarities- both methods are used to transmit data over long distances and both support high data rates, making them suitable for large amounts of data
differences- fibre optics use light signals transmitted through glass and plastic fibres, whereas satellites use radio frequency signals transmitted between satellites in space and ground stations
- as well this optical fibres can carry data across continents but satellites can relay signals globally
- optical fibres have a frequency of between 1THz and 1000THz and satellites have a frequency between 4GHz and 8GHz
- satellites are easier to hack but have a slower broadband where optical fibres ate faster
91
how are optical fibres used in medicine?
- bundles of optical fibres make it possible to send light to and receive it back from places that would otherwise be inaccessible e.g. inside the human body
- a small fibre bundle is used for piping light from an external source down to the distal
( remote ) end, where it illuminates the area being investigated
92
what is an endoscope?
- optical instruments with long tubes that can be inserted into a body organ through an opening such as the throat, nose, ear canals or anus
- these allow trained medicals to see inside a body organ without undertaking surgery
93
how is an endoscope used to form an image?
-light is piped in from an external source using a small bundle of optical fibres down to the distal (remote) end, where it illuminates and reflects off the area being investigated
-an analogue image is passed up through the second bundle of optical fibres and each fibre collects and carries the light for one pixel of the image seen on the screen
-light passes down the optical fibre by total internal reflection as it strikes the sides at angles greater than the critical angle, and the less dense cladding increases the critical angle so that there are less reflections down the length of the fibre and thus less energy is lost
94
compare the frequencies of fibre optics in broadband vs in medicine
in broadband- typically have higher frequencies as they need to be able to transfer data over long distances
in medicine- typically have lower frequencies ( visible light or infrared ) to allow high resolution imaging
95
do fibre optics in medicine and broadband use analogue or digital signals?
in broadband- use digital signals in broadband to reduce interference when conveying information
in medicine- use both digital and analogue signals to be able to transmit light to illuminate the internal body and capture images
96
what optical fibres do fibre optics in broadband and in medicine use?
in broadband- a multimode fibre optic cable is used as it works well over great distances
in medicine- low-lose optical fibres are used to transmit multicolour lase light
97
how do optical fibres in medicine and broadband effect the quality of data?
in broadband- typically provides a high quality of data as barely any data is lost when travelling over distances
in medicine- provides worse quality as it is used for short distances only, so less time for signal loss
98
what is an analogue signal?
- a signal that mimics the variation in size and quality of the physical quantity it represents
99
what is a digital signal?
- a signal that can encode that information as a series of numbers ( 0's and 1's )
100
explain one advantage of using digital signals over analogue signals
- digital signals can be regenerated so interference can be removed and they can travel over longer distances, so provides a better quality of data
101
how far do analogue and digital signals travel?
analogue- typically 2000-3000 feet
digital- no limitation
102
how susceptible are analogue and digital signals to signal loss and what is the quality like?
- digital signals are more susceptible to interference than analogue signals as they can compress and transmit data easier
- so digital signals have a better quality of data than analogue signals, as they can be easily interfered by noise
103
can the amplitude of digital and analogue signals vary?
analogue- completely free to vary
digital- the amplitude does not vary as the sound is converted into a digital code
104
how is an analogue and digital signal encrypted and stored on a computer?
analogue- the signal is digitalised and compressed to generate a data signal at a suitably low bit rate which the bit stream is then encrypted. they are stored in an analogue memory device
digital- transmitted over networks and algorithms convert it to a secure format using cryptographic keys. they are stored in memory using binary code or chips
105
what devices use analogue signals?
clocks- inside clocks there are oscillators that generate a clock signal that moves the gear and hands if the clock
switches- in a switch, a signal is passed when it is on and blocked when off
106
what devices use digital signals?
smartwatch- a precise signal is created and this oscillator signal is then sent to a counter circuit that generates a response
cell phone- digital phones convert your voice into binary information and then compress it
107
how is an analogue signal converted to a digital signal?
1) select a transducer, a device that produces an analogue electrical voltage signal proportional to the quantity you want to measure
2) connect the output of the transducer to the input of a A to D converter, using a screened cable that is well earthed
3) set up the A to D converter to sample the analogue signal
4) select an appropriate sampling rate, which is your sensitivity on the time axis
5) select an appropriate sensitivity for the conversion of the voltage signal into a number, the smallest difference you will be able to convey with digitally converted data is one unit
6) connect the A to D converter output to a switch / transmitter to send the digital info along a fibre network
108
what is a sampling rate?
- the number of times per second that the quantity is measured
109
explain the importance of sampling rate in the conversion of analogue signals to digital signals
- as when using a higher sampling rate, it tend to deliver a better quality representation as it allows more accurate capture of peak values
110
what is the sampling sensitivity?
- the smallest increment in the quantity that is measured and recorded
111
what is multiplexing?
- when we have multiple channels for light, keeping signals seperate
112
what are the similarities and differences between all electromagnetic waves?
similarities- all waves travel with the same speed ( 3 x 10⁸ ) through a vacuum
differences- they all have widely differing wavelengths, frequencies and photon energies that are used and detected differently
113
what parts of the electromagnetic spectrum can we visibly see?
visible light- this is the colours from deep red (740nm ) to indigo blue ( 370 nm ) that we can detect with our eyes
114
what equation is used to calculate the intensity of a wave?
I = k / r²
I = intensity
k= intensity 1m from the centre of the source
r² = radius from the source
- if you double the distance, you quarter the intensity
115
order the electromagnetic spectrum from increasing frequency.
lowest frequency-
- radio waves
- microwaves
- infrared
- visible light
-ultra violet
- x-rays
- gamma rays
highest frequency-
116
order the electromagnetic spectrum from increasing wavelength.
longest wavelength-
- radio waves
- microwaves
- infrared
- visible light
- ultra violet
- x-rays
- gamma rays
shortest wavelength-
117
what is a microwave signal?
- refers to the electromagnetic waves that fall between infrared and radio waves on the spectrum
118
what is a radio wave signal?
- refers to the electromagnetic wave that is used for wireless communication
119
what are the similarities between microwave and radio wave signals?
- both signals are transverse waves and apart of the EM spectrum, both towards the longer wavelength end
- both are used in wireless communication
120
what are the differences between microwave and radio wave signals?
1) wavelength- microwaves have a shorter wavelength ( 1mm- 1m ) and radio waves have a longer wavelength ( 100-10mm )
2) frequencies- microwaves have a higher frequency ( 1-40GHz ) and radio waves have a lower frequency ( 3Hz-1GHz
3) band with- microwaves have a large band with and radio waves have a low band with
4) microwaves cannot easily pass through through objects, where as radio waves are able to pass through many objects including solid ones
5) microwaves can be interfered by rain, for, mist and damp conditions, whereas radio waves can be interfered by geographical features such as hills and mountains
6) microwaves can carry more energy and travel longer distances as they aren't absorbed by the earths atmosphere, whereas radio waves carry less energy due to it's lower frequency
7) microwaves can successfully pass through the ionosphere, whereas radio waves cannot pass through the ionosphere but instead it reflects off it
121
what is a visible light signal?
- a range of electromagnetic radiation that can be detected by the human eye
122
what are the differences between visible light and microwaves?
1) wavelength- microwaves have a longer wavelength ( 1mm-1m ) and visible light has a shorter wavelength ( 400-700nm )
2) frequencies- microwaves have a lower frequency ( 1-40GHZ ) and VL has a higher frequency ( 405-790THz )
3) band withs- microwaves have a smaller band with compared to VL with a larger band with
4) microwaves can pass through some objects e.g. glass and paper but lose momentum fast, where as light is not absorbed or reflected can but can be transmitted through objects that are opaque, transparent or translucent to light
5) microwaves are interfered by rain, fog, mist and damp conditions where as VL can be interfered when light waves from the inner and outer surface combine, causing destructive ( anti-phase ) or constructive ( in phase ) interference
6) microwaves carry more energy and travel longer distances as they aren't absorbed by the earths atmosphere, where as VL carries less energy than microwaves as they have a lower frequency and a shorter wavelength
7) microwaves can successfully pass through the ionosphere, where as V; can penetrate the earths atmosphere, except when it is blocked by clouds
123
what is an infrared signal?
- these are electromagnetic radiation with wavelengths longer than visible light but shorter than microwaves, so are visible to the human eye
124
what is a Bluetooth signal?
- this is a short range, low power radio transmission technology that enables exchange of data between devices within a short amount of distance
125
what are the similarities between infrared and Bluetooth signals?
- both signals are transverse waves and part of the EM spectrum
- both carry energy and travel at 3 x 10⁸ m/s in a vacuum
126
what are the differences between infrared and Bluetooth signals?
1) wavelength and frequency- infrared has a shorter wavelength than Bluetooth and a higher frequency of around 320THz compared to Bluetooth with around 2.4-2.4835GHz
2) band withs- infrared has a larger band with than Bluetooth
3) infrared can pass through solid objects as they must be 'line of sight / unobstructed', whereas Bluetooth signals are transmitted via radio waves and so can travel through obstacles, but will weaken the signal
4) infrared can be interfered from sunlight, in which a longer wavelength band is used to reduce this, where as Bluetooth can be interfered from other radio signals where frequency hopping is used to reduce this
5) infrared carries more energy and information than Bluetooth because it has a higher frequency
6) infrared has a higher frequency so can pass through the ionosphere without being reflected, whereas Bluetooth has a lower frequency so is reflected off the ionosphere
127
what is the ionosphere?
- the ionosphere is a layer of electrons and electrically charged atoms and molecules that surrounds the earths, extending from about 50km to 1000km in height
- it is primarily formed due to the ultra violet radiation from the sun, and plays an important role of reflection and modifying radio waves used for communication
128
how are phone signals transmitted from one phone to another?
- cell phones do not just send signals, instead they communicate via radio waves, which travel through the air to a nearby cell tower
- this sends your voice to the person you are calling and the process is reversed so the person on the other end can hear
129
what is the importance of microwaves during phone communication?
- microwaves are important as they are neither refracted, reflected or absorbed by the ionosphere, allowing satellites to relay signals around the earth, enabling 24 hour communication
- microwaves also have a higher frequency so can carry more information over longer distances
130
why are radio waves only used by low orbits?
- radio waves are only used for low earth satellites as high orbit satellites are above the ionosphere in which radio waves cannot pass through ( 36000km ), because they are reflected
131
why might we not get a phone signal when in range of a transmitter?
- we can lose a signal due to the occurrence of superposition such as when light is reflected from large buildings and overlaps them, causing destructive interference
- large buildings can also absorb the signal
132
why does Wi-fi not interfere with Bluetooth signals?
- Bluetooth and Wi-Fi try to stay away from interfering with each other by operating on different channels and frequencies
( multiplexing )
- Bluetooth operates and hops between 79 channels, whereas Wi-Fi hops between 11
- they are both in the same frequency band, but Bluetooth frequency hops many times a second
133
give some examples of devices that use Bluetooth and how.
1) headphones- transmit audio signals through a low-powered 2.4Ghz
2) printer- your device ( phone ) has Bluetooth capabilities that pair with the printer, when you want to print the device sends the print job to the printer via Bluetooth
134
what is an upload signal?
- indicates the maximum date transfer rate at which a device can send information to a server or another device
135
what is a download signal?
- this is the amount of time it takes for data to download from a server
136
why do upload and download signals have different frequencies?
- they have different frequencies to prevent the constructive interference of signals
137
why does the upload signal need amplifying?
- as upload signals are weak and have less power, so need amplifying
138
what is the electromagnetic spectrum?
- this is a continuous range of wavelengths, the types of radiation that occur in different parts of the spectrum have different uses and dangers- depending on their wavelength and frequency
- the shorter the wavelength, the higher the frequency and energy
139
what is the ground state?
- this is when electrons occupy the orbital closest to the nucleus in whish there is space
- it is the lowest energy state possible
140
how is an emission spectrum formed?
example- hydrogen atom
1) the electron starts off in it's ground state where it is closest to the nucleus
- each shell has a specific amount of energy needed to reach them from the ground state
2) an incoming wave that carries energy enters, giving the electron the correct amount of energy needed for it to jump from the ground state to a different shell, so the electron absorbs the energy and jumps ( the more energy absorbed the higher the electron will jump )
- the specific amount of energy that one electron absorbs is called the discrete energy level
3) now that the electron is above it's ground state the electron is excited, however negative electrons do not like to be excited so need to de-excite
4) to stop being excited, the electron must emit the same amount of energy as it absorbed, as a light wave so it can move back down to it's ground state
- when the electrons fall back down to it's ground state it loses energy, this energy is emitted as light ( photon )
- we detect the light waves emitted in the form as an emission spectrum
141
how does an electron absorb the energy?
1) through an incoming photon of light
2) by electricity- it gains energy by a free electron colliding with an electron in the atom
142
how are multiple emissions of one atom observed?
- if you have multiple samples of one atom then you will observe several possible emissions
143
why do we have different colours in an emission spectra?
- as one electron may jump up 1 shell and another may jump up 3 shells so when they de-excite the emit different discrete energy levels which code for one different colour
144
what does a singular line on an emission spectrum show?
- a singular coloured wavelength e.g. green shows one discrete jump from one shell to another
145
what equation is used to calculate energy of light?
- the amount of energy in light is proportional to the frequency ( high frequency = high energy, low frequency = low energy )
E = h x f
E= energy in Joules
h = planks constant ( 6.63 x 10⁻³⁴ )
f = frequency in Hz
146
give an example of how to calculate the energy of frequencies.
e.g. what is the energy in J of 0.5 x 10¹⁴ Hz?
E = f x h
E = ( 0.5 x 10¹⁴ ) x ( 6.63 x 10⁻³⁴ ) = 3.315 x 10⁻²⁰ J
147
how do we rearrange to calculate the frequency?
f = E / h
148
give an example of how to calculate the frequency of light.
e.g. what is the frequency of light with 5.32 x 10⁻¹⁹ J?
f = E / h
f = ( 5.32 x 10⁻¹⁹ ) / ( 6.63 x 10⁻³⁴ ) = 8.02 x 10¹⁴ Hz
149
how else, apart from shells, can different energy levels be shown?
- energy level diagrams
- when calculating you ignore the negative signs e.g. -5.45 x 10⁻¹⁸ would become 5.45 x 10¹⁸, as it shows that you need to give the electron the correct amount of energy for it to escape the positive attraction of the nucleus
150
how do you calculate the frequency of the wave absorbed and re-emitted by the electron, using the energy level diagram?
e.g. -5.45 x 10⁻¹⁹ and -2.18 x 10⁻¹⁸
1) find out the difference between the first and second energy level
2) ( 5.45 x 10⁻¹⁹ ) - ( 2.18 x 10⁻¹⁸ ) = 1.6 x 10⁻¹⁸, ignoring the minuses
2) calculate the frequency of the light using f = E / h
f = 1.6 x 10⁻¹⁸ / 6.63 x 10⁻³⁴ = 2.4 x 10¹⁵ Hz
3) then calculate the wavelength using c = f x λ
c = 3 x 10⁸
λ = 3 x 10⁸ / 2.4 x 10¹⁵ = 1.25 x 10⁻⁷ m
- to convert into nm you press ENG = 125nm
151
what is a progressive wave?
- a waveform travels, transferring energy from on position to another e.g. sound in this room, ocean waves, light from the sun
152
what is a stationary / standing wave?
- wave motions that store energy, rather than transfer energy to another location
- they are formed when two waves are travelling in opposite directions and are of the same frequency and speed, with a fixed end at both sides
- these waves superpose producing points of constructive and destructive interference, the combined superposition gives an impression of a wave that is not progressing and so is called a stationary wave
- stationary waves do not transfer energy along the wave, they store it
153
what is a node?
- a point that always has zero amplitude along a stationary wave, caused by destructive interference
154
what is an anti-node?
- a point of maximum amplitude along a stationary wave, caused by constructive interference
155
how are nodes and anti-nodes formed?
- an incident wave is sent out
- wave reflects back once it hits the boundary, so they travel in opposite directions
- nodes = amplitudes subtract
- antinodes = amplitudes add
156
how many nodes and antinodes does the 1st, 2nd and 3rd harmonic have?
1st harmonic- has 2 nodes, 1 antinode
2nd harmonic- 3 nodes, 2 antinodes
3rd harmonic- 4 nodes, 3 antinodes
157
what are resonators?
- store wave energy by reflecting the wave back on itself to form a stationary wave pattern
- only efficiently receive energy from an external source that has a frequency close to one of their own natural frequencies
- they have a fundamental ( lowest ) natural mode of oscillation, and higher harmonics
158
what is a natural frequency?
- a series of harmonics each of which corresponds to an exact number of half wavelengths fitting within a normal boundary
159
what is a driving frequency?
- this is a frequency of energy from an external source such as a violin bow
- the bow will only transfer energy to the violin strings if it's frequency is similar to the natural frequency of the violin strings
- if this happens, the strings will resonate and a musical harmonic will be heard
160
what is a harmonic?
- the set of all possible standing waves are known as the harmonics of a system
- the simplest of the harmonics is called the fundamental or first harmonic
- harmonic waves will form at specific wavelengths and the simplest standing wave that can form under these circumstances has one antinode in the middle
- this is half the wavelength and is also written as L = 1/2 x λ
161
how do we know which harmonic we are in?
- on a wave you count how many antinodes there are and that number is what harmonic you are in
162
what equation is used if the pipe if fixed at both ends?
L = λ / 2 x harmonic ( 1,2,3 or 4 etc. )
163
what equation is used if the pipe is open at one end?
L = λ /2 x odd numbers ( 1,3,5 )
164
what equation is used if the pipe is open at both ends?
L = λ / 2 x harmonic ( 1,2,3,4 etc. )
165
what is the equation used to calculate node-to-node?
λ/2
166
give an example of how to calculate the wavelength of a wave in the 3rd harmonic.
L = 3/2 x λ
rearrange for λ
λ = L / ( 3/2 )
167
give another example of how to calculate the wavelength of a wave in the 4th harmonic and work out node-to-node.
L = 4/2 x λ
λ = L / ( 4/2 )
node-to-node = 1/2 x L/2
168
what is the equation to calculate frequency?
f = V / λ
frequency = wave speed / wavelength
169
(a) calculate the wavelength of the waves on the string if the length is 1.2m.
the wave is in the 3rd harmonic
L = 3/2 x λ
λ = 1.2 / ( 3/2 ) = 0.8m
170
(b) calculate the frequency if the speed of the wave is 200m/s.
f = v / λ
v = 200 m/s
λ = 0.8 m
f= 200 / 0.8 = 250Hz
171
how do we calculate the mass per unit length of a wave?
μ = m / L
mass per unit length = mass (g) / length (m)
172
how do we calculate wave speed using tension and mass per unit length?
v = √T / μ
wave speed = √ T (N) / μ (kg)
173
how do we convert from g-> kg?
g -> kg = / 1000
174
a guitar string is plucked multiple times. calculate the wave speed when the tension is 2N and the mass per unit length is 3.2g
v = √ T/ μ
T = 2N
μ = 3.2g -> / 1000 = 3.2 x 10⁻³
v = √ 2 / ( 3.2 x 10⁻³ ) = 25 m/s
175
how do we rearrange to calculate the tension?
T = v² x μ
176
how do we convert from cm -> m?
cm -> m = / 100
177
(a) a guitar is plucked and has a wavelength of 50cm, it produces a frequency of 350Hz. what is the wave speed?
λ = 50cm
f = 350Hz
v= 350 x 0.5 = 175m/s
178
(b) the mass per unit length is 0.001 kg/m. calculate the tension of the string
T = v² x μ
T = 175² x 0.001 = 30.6N
179
(2a) a violin string has a tension of 45N and a mass per unit length of 0.0025 kg/m. what is the wave speed?
v = √ T / μ
v= √ 45 / 0.0025 = 134 m/s
180
the wavelength of the wave produced is 7cm. calculate the frequency.
λ = 7cm / 100 = 0.007 kg/m
f = v / λ
f = 134 / 0.07 = 1.92 x 10³Hz
