Waves Flashcards
1
Q
What is the definition of a progressive wave?
A
Oscillations that transfer energy without transferring matter
2
Q
What is the definition of a transverse wave?
A
Waves e.g visible light waves, where oscillations are perpendicular to the direction of energy travel
3
Q
What is the definition of a longitudinal wave?
A
Waves e.g sound waves, where oscillations are parallel to the direction of energy transfer
4
Q
What are mechanical waves?
A
Waves that have particles of matter that oscillate e.g seismic waves
5
Q
What are electromagnetic waves?
A
Waves in which the electric and magnetic fields oscillate e.g radio waves, visible light etc.
6
Q
What is the definition of frequency?
A
The number of complete cycles that pass a specific point per second
7
Q
What is the unit of frequency?
A
Hz
8
Q
What is the SI unit of frequency?
A
s (-1)
9
Q
What is the formula of frequency?
A
F = 1/T
10
Q
What is the definition of time period?
A
The time taken for a full oscillation cycle to pass through a fixed point
11
Q
What is the definition of amplitude?
A
A waves maximum displacement, positive or negative, from the equilibrium position
12
Q
What is the definition of wavelength?
A
The distance between two successive troughs or peaks, the length of one oscillation
13
Q
What is the equation that links velocity, frequency and wavelength?
A
Velocity = frequency x wavelength
14
Q
What is the experiment for calculating the velocity of a wave using a speaker and microphone?
A
- First, you would connect your speaker speaker and microphone to a signal generator and cathode ray oscilloscope
- You would observe the waves on the oscilloscope produced and received and move your microphone till the oscillations were instep with one another . You would record the position of the microphone at this point.
- Then you would move your microphone away from your speaker, in a straight line, until the two traces match up again. This means you would have moved your microphone exactly one wavelength away from your speaker.
- You would then record the position of your microphone and subtract it from the original position.
- Finally, you would calculate the velocity of your sound wave using the equation v = f(wavelength) reading off your frequency from your cathode ray oscilloscope.
15
Q
What is the definition of the phase of a wave at a certain time?
A
The fraction of a cycle it has completed since the start of the cycle
16
Q
What is the units for phase/ phase difference?
A
Radians or degrees (interchangeable)
17
Q
When will two waves have a phase difference?
A
When they are produced at 2 different times
18
Q
What is the equation for phase difference?
A
theta = 2(pi)d / wavelength
19
Q
How do you get to the equation for phase difference?
A
d/ wavelength = theta/ 2pi
rearrange for theta
20
Q
If 2 points on a wave/2 different waves have a phase difference of 2pi radians they are…
A
In phase
21
Q
If 2 points on a wave/2 different waves have a phase difference of pi radians they are…
A
In antiphase
22
Q
If 2 points on a wave/2 different waves have a phase difference of theta that does not equal 2pi they are…
A
out of phase
23
Q
What are longitudinal waves made up of?
A
compressions and rarefactions
24
Q
What are some differences between longitudinal waves and transverse waves?
A
TRANSVERSE:
Can travel through a vacuum
Oscillations perpendicular to the line of travel
Can be polarised
LONGITUDINAL:
Cant travel through a vacuum
Oscillations parallel to the line of travel
Can’t be polarised
25
What does it mean to describe a wave as plane polarised?
The wave has been passed through a polaroid so, therefore, its oscillations are only in one plane
26
Can longitudinal waves be polarised?
No
27
Can transverse waves be polarised?
Yes (EM waves)
28
Why can polarisation be used to distinguish between transverse and longitudinal waves?
As transverse waves travel on different planes and oscillate perpendicular to the direction of travel, whereas longitudinal waves only travel on 1 plane and oscillate perpendicular to the direction of travel
29
How do you polarise mechanical waves e.g rope that has been shaken?
You can create a physical gap to allow oscillations along one particular direction to go through
30
How do you polarise light waves?
By passing the light through a polaroid filter
31
In what direction does unpolarised light oscillate in?
All directions
32
If you pass unpolarised light through one polaroid filter that has a vertical axis of transmission, what will happen?
The light will be vertically polarised but will be 1/2 the intensity
33
If you pass vertically polarised light through another polaroid filter which has a horizontal axis of transmission what will happen?
No light will be let through
34
What are two applications of polarisation in everyday life?
Polaroid sunglasses:
The reduce the glare reflected by water and tarmac by blocking partially polarised light as they only allow oscillations in the plane of the filter
TV and Radio Signals:
These signals are usually plan polarised by the orientation of the rods on the transmitting aerial, therefore, the receiving aerial must be aligned in the same plane of polarisation to receive the signal at full strength
35
What is the definition of polarisation?
The way in which unpolarised, transverse waves that are oscillating along multiple planes are caused to only oscillate along one plane
36
What is the definition of superposition?
When the displacements of two waves are combined when they pass each other resulting in a vector sum of each waves displacement
37
What do rays represent on a wave diagram?
The direction of energy transfer (mainly used for light)
38
How are wavefronts represented on wave diagrams?
Lines that are perpendicular to the rays and that are a constant distance away from each other
39
What are wavefronts on a wave diagram?
Lines of constant phase that represent the peaks of the wave
40
What is the definition of reflection?
The phenomenon in which the wavefronts of a wave change direction at a boundary, returning to the medium where it originated
41
At a boundary the angle of incidence = ...
the angle of reflection
42
How do we see and image in a mirror even though the light rays are reflected and why is the image inverted?
- First you take 2 points from the object and draw 2 rays from both of them hitting the mirror boundary at points P and Q.
- When the rays hit the boundary's they reflect, however, this is at different angles as the angles of incident are different.
- if you continue your lines of reflection into your mirror they will eventually cross which is where you see your image, however, there has been a lateral inversion of the rays from the two different points which is why your objects appears flipped.
43
What is the definition of refraction?
A phenomenon where waves travel from one medium to another causing their speed to change and, therefore, direction
44
What is the definition of refractive index, n?
A property of a material that measures how much it slows down light passing through it
45
When going from a lower refractive index medium to a higher refractive index medium your ray bends...
towards the normal
46
When going from a higher refractive index medium to a lower refractive index medium your ray bends...
away from the normal
47
What is the formula for refractive index?
n = c (speed of light in vacuum) / cs (speed of light in substance)
48
A material with a higher refractive index is...
more optically dense - slows down light more
49
State Snells Law
n1sin(theta)1 = n2sin(theta)2
sini / sinr = wavelength(i) / wavelength (r)
sini / sinr = c (i) c (r)
50
When the angle of incidence is less than the critical angle what phenomenon occurs?
refraction
51
When the angle of incidence = the critical angle what phenomenon occurs?
refraction, where your refracted ray moves along the boundary with an angle of refraction which is exactly 90 degrees
52
What is the equation for the critical angle?
sin (theta) critical = n2 / n1 where n1 > n2
NB - when going from one medium into air, your value of n2 is 1
53
When does light undergo total internal reflection?
When the angle of incidence inside a more dense medium (greater refractive index than the one at the boundary) is greater than its critical angle
54
What two main sections make up optical fibres
- core
- cladding
55
What are optical fibres and application of?
Total internal reflection
56
What do optical fibres do?
carry information in the form of light signals
57
What are some physical properties of optical fibres?
- flexible
- thin
- made of plastic or glass
58
How does total internal reflection occur in optical fibres?
Your core is optically dense which is surrounded by cladding with a lower density, making your critical angle very small. This mean TIR is able to occur.
NB - you need to ensure that your light enters the optical fibre at a suitable angle so that refraction does not occur
59
What are 2 reasons for cladding around your core in optical fibres?
- Protects the core form damage
- Prevents signal degradation through light escaping via refraction from the core, which can cause information to be lost
60
What is the term for when a signal travelling through an optical fibre gets altered, therefore, the information received is not correct?
Signal degradation
61
What are the two different ways signal degradation occurs in optical fibres?
- Absorption
- Dispersion
62
What is Absorption in optical fibres?
A type of signal degradation where a signals energy is absorbed by the fibre, reducing the amplitude of the signal, resulting in the loss of information
63
What is dispersion in optical fibres?
This causes pulse broadening, which is where the received signal is broader than the original transmitted signal. Broadened signals can overlap and cause loss of information.
64
What are the two types of dispersion in optical fibres?
- Modal
- Material
65
What is modal dispersion?
Modal dispersion is caused by light rays entering the optical fibre from different angles, therefore they take different paths down the fibre. This leads to the rays taking different times to travel along your fibre causing pulse broadening.
66
What is material dispersion?
Material dispersion is caused by the light being consisted of different wavelengths, meaning your light rays will travel at different speeds along the fibre leading to pulse broadening.
67
How can modal dispersion in optical fibres be reduced?
By reducing the size of the core, making it very narrow, the possible difference in path lengths between the rays of light is much smaller
68
How can material dispersion in optical fibres be reduced?
By using monochromatic light
69
When does superposition of waves happen?
When two waves, of the same type, meet at a point
70
When does constructive interference occur?
When two waves have displacement in the same direction
71
When does destructive interference occur?
When one wave has a positive displacement and the other has a negative displacement, if the waves have equal but opposite displacements, total destructive interference occurs
72
What is the definition of coherent light source?
The waves all have the same frequency and wavelength and a constant phase difference
73
What is an example of a coherent light source?
A laser
74
What is the definition of path difference?
The difference in distance travelled by the two waves
75
How is a stationary wave formed?
From the superposition of 2 progressive waves, of the same type and frequency, travelling in opposite directions in the same plane. This happens when waves can reflect off of a boundary of an object.
NB - they have the same frequency, wavelength and amplitude
75
What is the definition of interference?
When the superpositions of coherent waves produces reinforcements and cancellations at fixed points, producing patterns
76
What is a stationary (standing) wave?
A wave that does not transfer energy between 2 points, instead stores it
77
What are antinodes?
Regions of maximum amplitude/ displacement from the equilibrium
78
What are nodes?
Regions of zero amplitude/ displacement form the equilibrium
79
When are antinodes formed in stationary waves?
When the two waves meet in-phase and constructive interference occurs
80
When are nodes formed in stationary waves?
When the two waves meet in antiphase and destructive interference occurs
81
In what two instances are stationary waves usually formed?
- On strings
- In tubes
82
Are all points between to consecutive nodes on a stationary wave in-phase with one another?
Yes
83
Do all points between two consecutive nodes on a stationary wave have the same max speed?
No, they all have different maximum speeds
84
In strings that have nodes at both ends what is your first harmonic? (method)
L (length of string) = wavelength /2
wavelength = 2L
fo(fundamental) = v / 2L
85
In strings that have nodes at both ends what is your second harmonic? (method)
L = wavelength
f2 = v / L
f2 = 2fo
86
In strings that have nodes at both ends what is your third harmonic? (method)
L = 3/2 wavelength
wavelength = 2/3 L
f3 = 3v /2L
f3 = 3fo
87
What is the general relationship between the fundamental harmonic and other harmonics?
fn = nfo (fundamental)
88
Why does refractive index not have a unit
As it is a ration of speeds, therefore, the units cancel
89
Are relationships between waves and harmonics with closed and open ends the same or different?
the same
90
In tubes what are the two end points?
nodes
91
On strings what are the two end points?
nodes
92
What is the first harmonic in tubes?
fo = v / 2L
93
What is the second harmonic in tubes?
f2 = v / L ( 2 x the first harmonic)
94
What is the third harmonic in tubes?
f3 = 3V /2L (3 times the first harmonic)
95
What is the first/fundamental harmonic when you have a string where one end is fixed (a node) and one end is not fixed ( an antinode)? (method)
L = wavelength / 4
wavelength = 4L
f = v/ wavelength
f = v/ 4L
96
What is the third harmonic when you have a string where one end is fixed (a node) and one end is not fixed ( an antinode)? (method)
L = 3/4 wavelength
wavelength = 4/3 L
f3 = 3V/4L
97
What is the fifth harmonic when you have a string where one end is fixed (a node) and one end is not fixed ( an antinode)? (method)
L = 5/4 wavelength
wavelength = 4/5 L
f5 = 5v/4L
98
What numbered harmonics can be formed when a string is fixed at one end and loose on the other?
ONLY ODD HARMONICS CAN BE FORMED
99
What is the equation for frequency in harmonics?
f1 = 1/2L x root (T/ mu)
T - tension (N)
mu - Mass per unit length
f1 = fundamental frequency (Hz)
100
What are the two factors that affect the frequency in harmonics?
- Tension of the string
- Length of the string
101
What does Youngs double slit experiment demonstrate?
Interference of light from two sources
102
How could you make a bulb be a suitable source for Youngs double slit experiment?
- Pass the light through single slit giving it a fixed path difference (acting like a point source)
- Pass the light through a filter to make the light monochromatic (have the same wavelength)
103
What type of light source do you need for Youngs double slit experiment?
a coherent source
104
What is produced on the screen in the Youngs double slit experiment?
an interference pattern
105
In the centre of your screen in Youngs Double slit experiment what is produced?
A central maximum:
There is a 0 path difference and phase difference between sources, therefore, constructive interference occurs.
106
What happens either side of your central maximum in Youngs Double slit experiment?
Dark fringes:
This is because there is a path difference of wavelength/2 and a phase difference of pi between the two rays, therefore, the waves are in antiphase so destructive interference occurs.
107
What happens after your first set of dark fringes in Youngs Double slit experiment?
First order maxima:
This is because although there is a path difference of 1 wavelength, the phase difference is 2pi meaning that the waves superpose in-phase. Therefore, constructive interference occurs.
108
What is the equation for fringe separation in Youngs Double Slit experiment?
W = wavelength x D / s
W = fringe separation (between two maxima or minima, however, usually you would take fringe separation between two minima as it is easier to see where the centre is)
D = distance between the screen and double slit
s = distance between the centres of the two slits
109
How would the appearance of the fringes differ if you used white light in the Youngs Double slit experiment in stead of monochromatic light?
- The bright fringes would be wider and less intense
- The central fringe would be white
- The rest of the fringes would be tinged with blue on the inner side (shorter wavelength) and red on the outside (longer wavelength)
110
What are three ways that you can improve the accuracy of the Youngs Double slit experiment?
- Repeat your experiment multiple times, increasing D each time
- Use an increased distance as this would make determining the centre of the maxima/minima easier
- Measure your fringe spacing across more than 2 maxima
all these would reduce your percentage error of your wavelength
111
When light is passed through a double slit, is the intensity of the light at each maximum roughly the same?
yes
112
What are some safety precautions when carrying out Youngs double slit experiment?
- Wear laser safety goggles
- Don't shine the laser on reflective surfaces
- Display a warning sign on the door of the room you are conducting the experiment in
- Don't shine the laser at a person
113
What is the definition of diffraction?
The spreading out of waves as they pass through or around a gap
114
Can light being shone through a single slit also produce a diffraction pattern?
Yes
115
Describe what the intensity of the light received on the screen when light is shone through a single slit looks like
The central maxima is the brightest of all the maxima - A LOT BRIGHTER - and double the width of any other maxima. Successive maxima have w/2 and becoming increasingly less intense depending on certain factors
116
What is the equation for the width of the central maxima when monochromatic light is shone through one slit?
W = 2piD / a
W = width of central fringe
D - distance between slits and screen
a - slit width
117
What is the equation for the width of the subsidiary slits when monochromatic light is shone through one slit?
W/2 = piD / a
W = width of central fringe
D - distance between slits and screen
a - slit width
118
What would you observe if you shone white light through a single slit instead of monochromatic light?
As white light is made up of all the colours, the different wavelengths of the visible light would be diffracted by different amounts so you would get a spectrum of colour in the diffraction pattern. There would be a central maximum that is white, the most intense, and twice the width of other maxima. The successive maxima would be the spectrum with violet on the inner side and red on the outer and decreasing in intensity.
119
What are the factors that you can vary to vary the width of the central maximum?
- slit width
- wavelength
120
What is a diffraction grating?
A slide containing many evenly spaced slits very close together
121
How does increasing your slit width and/or you wavelength effect your central maximum when light has been passed through one slit?
- Increasing the slit width decreases the amount of diffraction so the central maximum becomes narrower and the intensity increases
- Increasing the wavelength of your light increases the amount of diffraction as the slit is closer in size to the lights wavelength, therefore the central maximum becomes wider and the intensity decreases
122
Why is your diffraction grating interference pattern much sharper and brighter than when light is passed through a double slit like in Youngs experiment?
This is because there are more rays of light reinforcing the interference pattern
123
How do you observe an interference pattern using a diffraction grating?
You would use a ray box that emits white light and pass that through a filter to produce a monochromatic source which is then shone onto your diffraction grating.
Your interference pattern produced is very similar to the one produced from Youngs Double slit:
- Your central maxima is formed where the waves have a path difference n wavelength and are all in phase, therefore, constructive interference occurs.
- Your dark fringes are formed when the path difference between your rays is 1/2n wavelength and your rays are out of phase, therefore, destructive interference occurs
124
What is the equation that calculates the angle at which your nth order maxima appears in diffraction gratings?
dsin(theta) = n x wavelength
d - distance between slits (normally given in slits per metre)
n - order
theta - angle where nth maxima appears
125
How do you find the total number of orders that can be seen when using diffraction grating equipment?
you use the equation: dsin(theta) = n x wavelength and set theta = 90
126
Why will your intensity increase if you decrease your wavelength?
The central peak would be narrower but have a higher amplitude as energy is concentrated in a smaller area
127
Describe the motion of a particle through a full cycle of a wave from equilibrium
For the next 1/2 wavelength the particle would oscillate perpendicular to the direction of travel from a negative maximum and back to 0 displacement. Then for the next 1/2 wavelength the particle would oscillate to a positive maximum displacement before returning to 0 displacement once the full wavelength had been completed
128
What is the definition of phase difference between two particles vibrating at the same frequency?
The fraction of a cycle between the vibrations
129
Why are there slopes on the sides of ripple tanks?
To prevent waves reflecting off of the sides which would make it difficult to see the waves being produced
130
Are waves more or less slow in shallow water?
More slow
131
Compare the properties of particles in a stationary wave versus a progressive wave
STATIONARY WAVE
Frequency
- All particles except those at the nodes vibrate with the same frequency
Amplitude
- The amplitude varies from 0 at the nodes to a maximum at the antinodes
Phase Difference
- Equal to m(pi) where m is the number of nodes between the to particles
PROGRESSIVE WAVE
Frequency
- All particles vibrate with the same frequency
Amplitude
- The amplitude is the same for all particles
Phase Difference
- Equal to 2(pi)d/lambda, where d is the distance apart and lambda is the wavelength
132
What does the pitch of a note correspond to?
Its frequency
133
What is the key condition for a stationary wave to be produced using a frequency generator?
The time taken for the wave to travel along the string and back should be equal to the time taken for a whole number of cycles of the generator:
2L/c = m/f
134
What is the equation for the first harmonic frequency of a wire with tension, T, and mass per unit length, mu?
f = 1/2L x root(T/mu)
135
What can be used to test whether a wire is tuned or not?
Tuning fork
136
What is the x-axis called on an oscilloscope?
time base
137
What is the y-axis called on an oscilloscope?
y - sensitivity
138
When refraction occurs does the frequency of the wave change?
NO
139
Why do diamonds sparkle when white light is shone on them?
When white light enters the diamond it is split into the colours of the spectrum. Diamond has a very high refractive index so the individual light rays undergo TIR many times before emerging from the diamond causing the colours to spread out more and more. This causes the diamond to sparkle with different colours.
140
Definition of monochromatic light?
Light of a singular wavelength
141
What pieces of apparatus can be used to demonstrate reflection, refraction, diffraction, interference and polarisation of microwaves?
Microwave transmitter and a receiver connected to a meter to measure the intensity of the microwaves received
142
Describe briefly how reflection, diffraction, interference and polarisation of microwaves can be shown by using a microwave transmitter and receiver?
REFLECTION - A generator creates a direct wave and a wave that is angled towards a reflecting surface. A detector faces the direct wave. You change the distance of the reflecting surface from the generator which will cause the intensity of the readings from the detector to go through maxima and minima as interference occurs.
DIFFRACTION - Use a metal single slit and change how big the slit is in order to change the intensity of the signal received by the receiver
INTERFERENCE - Create two slits with metal plates and move the detector around to find points of cancellation and reinforcement (microwaves and chocolate bar)
POLARISATION - Microwave transmitters are naturally polarising with a wavelength of 3cm. Therefore, rotate your receiver in the vertical plane. When vertical the reading is a maximum, when horizontal the reading will be a minimum. The maximum value occurs when the receiver is aligned with the plane of polarisation of the transmitter
143
When is the Youngs Double slit equation valid to use?
If the slit-screen distance is a lot greater than the slit separation - this is because interference can only occur if the light from the two slits overlaps
144
Definition of critical angle
The angle of incidence beyond which waves passing from a denser medium to a less dense medium will no longer be refracted but undergo TIR
145
For an LDR as light intensity decreases , resistance...?
Increases
146
For an LDR as light intensity increases, resistance...?
Decreases
147
For a thermistor as temperature decreases, resistance?
Increases
148
For a thermistor as temperature increases, resistance?
Decreases
149
Without the single slit effect, what is the intensity of fringes produced on the screen?
All the fringes have the same intensity and same width
150
Derive diffraction grating equation
For an nth order maximum:
- Imagine 2 slits P and Q on the diffraction grating
- When light travels through the slit the waves are diffracted in the same way in both slits P and Q
- Imagine a wavefront that is diffracted by P and is now its direction is an angle theta from the direct line of travel
- The wavefront produced by P reinforces a wavefront emitted n cycles earlier from the adjacent slit Q (this is where the wavefront line of P and the direction of travel line from Q intersect -> called Y)
- The earlier wavefront from Q therefore must have travelled a distance of n wavelengths from the slit in order for constructive interference to have occurred
- Therefore, QY = n(wavlengths)
- Since the angle of diffraction of the beam, theta, is the same as the angle between the wavefront and the plane of the slits it follows that sin(theta) = QY/QP, where QP is the grating space, d.
- Substituting d for QP and n(wavelengths) for QY gives you:
dsin(theta) = n(wavelengths)
151
How do you find the maximum number of orders produced from a diffraction grating?
Sub in theta = 90 degrees so that sin(theta) = 1
Therefore, maximum number of orders:
n = d/ wavelength
NB - round down for the maximum number of orders
