CGP AS Section 3 - Waves Flashcards
1
Q
Define a progressive wave:
A
A wave which carries energy from one place to another without transferring any material.
2
Q
What causes a wave?
A
- something making particles or fields oscillate at a source
- these oscillations pass through medium as the wave travels, carrying energy with it
3
Q
How can you tell waves carry energy?
A
- EM waves cause things to heat up
- x-rays and gamma knock electrons out of their orbits causing ionisation
- loud sounds cause large oscillations of air particles which can make things vibrate
- wave power can generate electricity
- since waves carry energy away, the source loses energy
4
Q
Define cycle:
A
one complete vibration of the wave
5
Q
Define displacement:
A
how far a point on a wave has moved from its undisturbed position
- represented by x
- units: metres
6
Q
Define amplitude:
A
maximum magnitude of displacement
- represented by A
- units: metres
7
Q
Define wavelength:
A
length of one whole wave cycle, from crest to crest or trough to trough
- represented by lambda
- units: metres
8
Q
Define period:
A
time taken for a whole cycle to complete or to pass a given point
- represented by T
- units: seconds
9
Q
Define frequency:
A
number of cycles per second passing a given point
- represented by f
- units: Hertz
10
Q
Define phase:
A
a measurement of the position of a certain point along the wave cycle
-units: it is measured in angles (degrees or radians) or as fractions of a cycle
11
Q
Define phase difference:
A
the amount one wave lags behind another
-units: it is measured in angles (degrees or radians) or as fractions of a cycle
12
Q
What can occur to waves?
A
reflection
refraction
13
Q
Define reflection:
A
the wave is bounced back when it hits a boundary
-e.g. reflection of water waves can be demonstrated in a ripple tank, reflection of light in mirrors
14
Q
Define refraction:
A
the wave changes direction as it enters a different medium (the change of direction is due to the wave slowing down or speeding up)
15
Q
What SI unit is Hertz?
A
1/s (seconds to the power of -1)
16
Q
what is the equation for frequency?
A
frequency = 1 ÷ time period
17
Q
What is the equation for wave speed when you have distance and time?
A
wave speed = distance travelled ÷ time taken
18
Q
What is the equation for wave speed when you have wavelength and frequency?
A
wave speed = wavelength x frequency
19
Q
What type of waves are electromagnetic waves?
A
All EM waves are transverse
20
Q
How do transverse waves travel?
A
- travel as vibrations through magnetic and electric fields
- vibrations perpendicular to direction of energy transfer
21
Q
What are examples of transverse waves?
A
- all EM waves
- ripples on water
- waves on strings
22
Q
What are the two main ways of drawing transverse waves and representing them graphically?
A
- can be shown as graphs of displacement against distance along the path of the wave
- can be shown as graphs of displacement against time for a point as the wave passes
(both give same shape)
23
Q
What is an example of longitudinal waves?
A
- sound waves
- secondary earthquake shock waves
24
Q
How do longitudinal waves travel?
A
-consist of alternate compressions and rarefactions of the medium it is travelling through (why sound can’t travel through a vacuum)
25
How can you represent longitudinal waves graphically?
-it is hard to represent longitudinal waves graphically, you will usually see them plotted as displacement against time (can be confusing because they will look like transverse waves)
26
Can a transverse wave go up and down and left and right as a mixture?
yes
27
What is a polarised wave?
A polarised wave only oscillates in one direction
28
What waves can be polarised?
transverse waves
29
How is polarisation evidence that electromagnetic waves are transverse?
Malus discovered that light was polarised by reflection. Physicists at the time thought light spread like sound as a longitudinal wave so struggled to explain polarisation. Young suggested light was a transverse wave consisting of vibrating electric and magnetic fields at right angles to the transfer of energy.
30
What does ordinary light waves consist of?
A mixture of different directions of vibration (both electric and magnetic fields are vibrating)
31
What is used to polarise a wave?
a polarising filter
32
What happens when you have two polarising filters at right angles to each other?
No light will get through
33
When does light become partially polarised?
when it is reflected from some surfaces (as some of it vibrates in the same direction)
34
How do Polaroid sunglasses work?
when you view partially polarised light through a polarising filter at the correct angle you can block out unwanted glare
35
Why are all TV aerial rods horizontal?
This is because TV signals are polarised by the orientation of the rods on the broadcasting aerial
-to get a strong signal you have to line up the receiving aerial with the rods on the transmitting aerial (if they aren't aligned the signal will be lower)
36
What signals are polarised?
television signals
| radio signals
37
What happens when you move a radio aerial around?
the signal will come and go as the transmitting and receiving signals go in and out of alignment
38
When does superposition happen?
when two or more waves pass through each other
39
What happens when two waves cross?
At the instant when they cross, the displacements due to each wave combine, then each wave carries on
40
What is the principle of superposition?
when two or more waves cross, the resultant displacement equals the vector sum of the individual displacements
41
What types of interference can you have?
constructive
| destructive
42
What is constructive interference?
when a crest and a crest meet and give a bigger crest or a trough and a trough meet and give a bigger trough
43
What is destructive interference?
- a crest and a trough of equal size gives nothing, they cancel each other out completely
- if a crest and trough meet but aren't equal destructive interference isn't total, for the interference to be noticeable the two amplitudes should be nearly equal
44
When are two points on a wave in phase?
- if they are both at the same point in the wave cycle
| - points in phase have the same displacement and velocity
45
Graphically, when are two points of a wave in phase?
when they have a phase difference of 0 or 360° or 2π rads or any other multiple are in phase (any full cycle apart)
46
Graphically, when are two points exactly out of phase?
when they have a phase difference of odd-number multiples of 180° (π radians or a half cycle)
47
In practice when will you get two different waves being in phase?
when both of the waves come from the same oscillator
| -in other situations there will nearly always be a phase difference between them
48
When are two sources coherent?
Two sources are coherent when they have the same wavelength and frequency and a fixed phase difference between them (two sources will be in phase or have a fixed phase difference)
49
What do two coherent sources give?
clear interference patterns
50
What is path difference?
the amount by which the path travelled by one wave is longer than the path travelled by the other wave
-whether it is constructive or destructive interference at a point depends on the path difference
51
Where will you get constructive interference from two coherent sources?
- at any point an equal distance from two coherent and in phase sources
- at any point where the path difference is a whole number of wavelengths (this is when the two waves are in phase and reinforce each other)
52
Where will you get destructive interference from two coherent sources?
-at points where the path difference is half a wavelength (or any other unit and a half) the wave arrives out of phase and you get destructive interference
53
Mathematically when does constructive interference occur?
when path difference = n wavelengths (where n is an integer)
54
Mathematically when does destructive interference occur?
when path difference = (2n + 1) ÷ 2 wavelengths
| or when path difference = n + 0.5 wavelengths
where n is an integer in both cases
55
What is a standing wave?
a stationary (standing) wave is the superposition of two progressive waves with the same frequency (wavelength), moving in opposite directions
56
What is the difference between progressive and standing waves?
Standing waves have no energy transmitted
57
How can you demonstrate stationary waves?
by setting up a driving oscillator at one end of a stretched string with the other end fixed
-the waves created by the oscillator is reflected back and forth (most frequencies resultant pattern is jumbled but if the exact number of waves produced in the time it takes for a wave to get to the end and back again then it is reflected and the waves reinforce each other)
58
What are resonant frequencies?
When you get a stationary wave where the pattern doesn't move (just stays there bobbing up and down)
-at resonant frequencies an exact number of half wavelengths fit onto the string
59
What are nodes?
where the amplitude of the vibration is zero
60
What are antinodes?
points of maximum amplitude
61
what is the first harmonic?
the stationary wave is vibrating at the lowest possible resonant frequency
- has one 'loop' with a node at each end
- half a wavelength
62
What is the second harmonic?
twice the frequency of the first harmonic
- has two 'loops' with a node in the middle and one at each end
- one full wavelength
63
what is the third harmonic?
three times the frequency of the first harmonic
- has three 'loops'
- 1.5 wavelengths fits on the string
64
What are examples of uses of stationary waves?
in microwaves
| in sounds
65
How do microwaves use stationary waves?
- microwaves reflected off a metal plate set up a stationary wave
- you can find nodes and antinodes between a microwave transmitter and a metal plate by moving the probe between them
66
How do sounds use stationary waves?
powder can show stationary waves in a tube of air
- stationary sound waves are produced in a glass tube
- lycopodium powder laid along the bottom of the glass tube is shaken away from the antinodes but left undisturbed at the nodes
67
Describe how you can investigate factors affecting the resonant frequencies of a string:
(describe the overall outline basic for the experiment)
- start by measuring the mass and length of strings of different types using a mass balance and ruler
- find the mass per unit length of each string by doing mass ÷ length (units kg/m)
- set up your string fixed to a vibration transducer and signal generator at one end and a pulley with some masses hanging off the end creating a tension force
- record mass per unit length, length between the fixed point and the pulley of the string and work out the tension by using tension = total mass of masses x g
- turn on the signal generator and vary the frequency until you find the first harmonic
68
How can you investigate how length of the string affects the resonant frequency from a basic experiment set up?
- keep the string type and tension the same and alter the length
- do this by moving the vibration transducer towards or away from the pulley, find the first harmonic again and record f against l
69
How can you investigate how tension of the string affects the resonant frequency from a basic experiment set up?
- keep the string type and length the same and adding or removing masses to change the tension
- find the first harmonic again and record f against T
70
How can you investigate how mass per unit length of the string affects the resonant frequency from a basic experiment set up?
- keep length and tension the same but use different types of string samples to vary the mass per unit length
- find the first harmonic and record f against mass per unit length
71
What should increasing the length of a string do to the resonant frequency of a string?
the longer the string, the lower the resonant frequency because the half wavelength at the resonant frequency is longer
72
What should increasing the mass per unit length of a string do to the resonant frequency of a string?
the heavier the string, the lower the resonant frequency because it will travel more slowly down the string
-for a given length a lower wave speed makes a lower frequency
73
What should decreasing the tension of a string do to the resonant frequency of a string?
the looser the string the lower the resonant frequency because waves travel more slowly down a loose string
74
What is the equation for the frequency of the first harmonic?
frequency = (1 ÷ [2 x length]) x √(tension ÷ mass per unit length)
75
What is diffraction?
the way a wave spreads out as it comes through a narrow gap or go round obstacles
-all waves diffract but it isn't always easy to observe
76
What affects the amount of diffraction?
the wavelength of the wave compared with the size of the gap
77
What happens to diffraction when the gap is a lot bigger than the wavelength?
diffraction is unnoticeable
78
what happens to diffraction when a gap is several wavelengths wide?
diffraction is noticeable
79
What happens to diffraction when the gap is the same size as the wavelength?
diffraction is at its highest
80
What happens to diffraction when the gap is smaller than the wavelength?
the waves are mostly just reflected back
81
How do you get a clear diffraction pattern?
you need to use a monochromatic, coherent light source
82
What is a monochromatic light source?
all the light has the same wavelength (and frequency) and so it is the same colour
-lasers are all monochromatic and coherent light sources
83
How do you demonstrate light diffraction patterns with a laser? What happens in single-slit diffraction
- if the wavelength of light is the same as the aperture you get a diffraction pattern
- the pattern on the screen which you are shining the laser onto through the slit will have a central bright fringe (central maximum) with dark and bright fringes alternating on either side (caused by different interference)
84
What happens when you single slit diffract white light?
- white light is a mix of colours of different wavelengths
- all of the wavelengths diffract by different amounts
- meaning you get spectra of colour (rather than clear fringes which you'd achieve with monochromatic light)
85
What does intensity of light mean?
the number of photons
| -it is the power per unit area
86
What is the brightest part in single slit diffraction pattern?
the central maximum is the brightest part of the pattern
87
Why is the central maximum the brightest part of single slit diffraction pattern?
because the intensity of light is the highest in the centre
| -so there are more photons per unit area hitting the central maximum per second than other bright fringes
88
What does an increase in intensity for monochromatic light mean?
all photons have the same energy in monochromatic light so an increase in intensity means an increase in the number of photons per second
89
What two things affect the width of the central maximum in single slit diffraction?
wavelength
| slit size
90
what does increasing the slit size in single slit diffraction do to the central maximum?
increasing the slit width decreases the amount of diffraction, so the central maximum is narrower and the intensity of the central maximum is higher
91
what does increasing the wavelength in single slit diffraction do to the central maximum?
increasing the wavelength increases the amount of diffraction, so the central maximum is wider and the intensity of the central maximum is lower
92
Why is demonstrating two-source interference in water and sound easy?
they have wavelengths of a handy size that you can measure
93
How would you set up a two-source interference experiment?
- need two coherent sources (same wavelength and frequency)
- use the same oscillator to drive both sources
- for water one vibrator drives two dippers, for sound one oscillator is connected to two loudspeakers
94
Describe an experiment to demonstrate two-source interference for light:
- use two separate coherent light sources or you can shine a laser through two slits (as laser light is coherent and monochromatic)
- Young's double slit experiment shines a laser through two slits onto a screen
- slits have to be about the same wavelength as the laser light so it is diffracted (for the two slit one the light from each slit acts like two coherent point sources)
- you get a pattern of light and dark fringes (depending on the interference)
95
What did Thomas Young come up with and who is he?
Young was the first person to do a two source interference experiment (with a lamp rather than a laser)
He cam up with an equation to work out the wavelength of the light from this experiment
96
How can you reduce the danger of working with lasers and not damage your retina permanently?
- never shine the laser towards a person
- wear laser safety goggles
- avoid shining the laser beam at a reflective surface
- have a warning sign on display
- turn the laser off when it isn't needed
97
Describe an experiment demonstrating two-source interference with microwaves:
- you can replace the laser and slits with two microwave transmitter cones attached to the same signal generator
- also need to replace the screen with a microwave receiver probe
- if you move the probe along perpendicular to the microwave signal emitter, you will get an alternating pattern of strong and weak signals (just like the light and dark fringes on the screen)
98
What is Young's double-slit formula?
fringe spacing [w] = (wavelength x distance from slits to screen [D]) ÷ spacing between slits [s]
99
What do you need to do to work out the wavelength of light?
as the wavelength of light is so small you can see from the formula that a high ration of D (distance from slits to screen) ÷ s (spacing between slits) is needed to make the fringe spacing big enough to see
100
What theories of light were published 17th century?
TWO IMPORTANT THEORIES OF LIGHT
- Isaac Newton suggested that light was made up of tiny particles he called 'corpuscles'
- Huygens put forward a theory using waves
101
What was the problem with Newton's theory for light?
Corpuscular theory could explain reflection and refraction but diffraction and interference are both uniquely wave properties
102
What settled the debate between what the wave nature is of EM radiation?
Young's double-slit experiment provided the necessary evidence, it showed that light could both diffract and interfere
103
What happens to interference patterns when you diffract through more slits?
You gets the same pattern at for two slits but the bright bands are brighter and narrower and the dark areas between are darker
-this occurs because there are many beams reinforcing the pattern
104
Why are sharper fringes good?
sharper fringes make for more accurate measurements
105
What are the maxima for monochromatic light?
all the maxima are sharp lines
106
What is the line of maximum brightness at the centre of a diffraction pattern?
it is called the zero order line
107
What are the lines just either side of the central one? And what is the next pair out?
-first order lines
-second order lines
and so on
108
What is the equation for the monochromatic light on a diffraction grating?
distance of slits apart [d] x sin(angle between the incidence beam and the nth order maximum) = order number [n] x wavelength
109
How do you derive the equation for monochromatic light on a diffraction grating?
- at each slit the incoming wave is diffracted, these create an interference pattern
- consider the first order, this happens at the angle when the waves from one slit line up with waves from the next slit that are exactly one wavelength behind
- call the angle between the first order maximum and the incoming light angle
- this creates a triangle so using trig you can get the equation for the monochromatic light of d x sin(angle) = wavelength
- the other maxima occur when the path difference is multiples of wavelengths (so you can replace this with n)
110
What conclusions can you draw from d x sin(angle) = n x wavelength?
- if the wavelength is bigger so the angle is bigger, the pattern will spread out more
- if d is bigger and angle is smaller, the coarser the grating and the less the pattern will spread
- values of sin(angle) greater than 1 are impossible, so you know the orders past that don't exist
111
What happens when white light is sent through a diffraction grating?
- each order of the pattern becomes a spectrum
- with red on the outside and violet on the inside
- the zero order maximum stays white because all the wavelengths just pass straight through
112
Who study diffraction gratings rather than prisms?
astronomers and chemists often to identify elements
| -because they are more accurate than prisms
113
What is x-ray crystallography?
they are of a similar scale to the spacing between atoms in crystalline solids
-means that x-rays will form a diffraction pattern when directed at a thin crystal (the crystal acts like a diffraction grating and the spacing between atoms can be found from the diffraction pattern
114
What did x-ray crystallography discover?
it discovered the structure of DNA
115
When does light go the fastest?
Light goes the fastest in a vacuum
| -it slows down in other materials because particles interact in them
116
What does a material being more optically dense mean?
A more optically dense material will have light travel through it slower
117
What is the absolute refractive index?
the absolute refractive index, n, is a measure of optical density
-it is found from the ratio between speed of light in a vacuum and the speed of light in a material
refractive index [n] = speed of light in vacuum ÷ speed of light in that material
118
what is the relative refractive index between two materials?
the ratio between the speed of light in material 1 to material 2
refractive index [n] = speed of light in material 1 ÷ speed of light in material 2
119
What is an equation for refractive index which you can use when you have to different refractive indexes?
refractive index between material 1 and 2 = refractive index in material 2 ÷ refractive index in material 1
120
What is the absolute refractive index of a material a property of?
The absolute refractive index of a material is a property of that material only
121
What is a relative refractive index a property of?
it is a property of the interference between two materials (it is different for every possible pair)
122
What is the refractive index of air and therefore what is the refractive index of an air to glass boundary?
- refractive index of air is 1
| - therefore the refractive index of air to glass is just the refractive index of glass
123
What is the angle of incidence?
The angle the incoming light makes with the normal
124
What is the angle of refraction?
The angle the refracted ray makes with the normal
125
What happens to a light beam when it enters a more optically dense medium?
it refracts towards the normal
126
What is Snell's law?
sin(angle of incidence) ÷ sin(angle of refraction) = refractive index of material entered ÷ refractive index of initial material
127
What is the critical angle?
The critical angle is when the refracted angle is 90 degrees so the light is refracted along the boundary
128
What is the equation for the critical angle?
sin(critical angle) = refractive index of material entered ÷ refractive index of initial material
129
What happens when a beam of light enters a material at a greater value than its critical angle?
- refraction is impossible
- all the light is reflected back into the material
- this is called TOTAL INTERNAL REFLECTION
130
What is an example of a use of total internal reflection?
fibre optic cables (step-index cables specifically)
- have a high refractive index are surrounded by a cladding of a lower refractive index to allow total internal reflection (cladding also protects the fibre from scratches which could let light escape)
- light is shone in one end the fibre is so narrow so it hits the boundary at an angle bigger than the critical angle
- so all light is totally internally reflected for the length of the cable
131
What is an optical fibre?
a very thin flexible tube of glass or plastic fibre that can carry light signals over long distances and round corners
132
what can degrade a signal? and what can it cause
absorption
dispersion
-signal degradation can cause information to be lost
133
What can absorption of a signal cause?
some of its energy is lost through absorption by the material the fibre is made from
-resulting in energy loss causing the amplitude of the signal being reduced
134
What are the two types of dispersion?
modal dispersion
| material dispersion
135
What is modal dispersion?
light rays enter the fibre at different angles, so take different paths
the rays which take a longer path take longer time to travel down the middle of the fibre
136
What is a solution to modal dispersion?
a single-mode fibre only lets light take one path so it stops modal dispersion
137
What is 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
138
what is a solution to material dispersion?
using monochromatic light can stop material dispersion
139
What does dispersion of a light signal in a fibre optic lead to?
pulse broadening, so the signal sent down the fibre is broader at the other end, broadening pulses can overlap each other and confuse the signal
140
What can reduce signal degradation from both absorption and dispersion?
an optical fibre repeater can be used to boost and regenerate the signal every so often
