Unit 2.5 - The Properties of Waves Flashcards
1
Q
Diffraction
A
The effect observed when waves pass an object…
When waves strike an object, part of the wave is slowed down, and so the wave front curves
When a wave strikes a space in a barrier, both sides curve
2
Q
Which waves undergo diffraction?
A
ALL waves
3
Q
What’s the word for the effect observed when waves pass an object?
A
Diffraction
4
Q
What happens when waves strike an object?
A
Part of the wave is slowed down, and so the wave front curves
5
Q
What happens wen a wave strikes a space in a barrier?
A
Both sides curve
6
Q
How can we show the diffraction of waves in water?
A
Ripple tank
7
Q
Draw and label a ripple tank
A
(See notes)
8
Q
Draw a wavefront and label its wavelength
A
(See notes)
9
Q
What’s the angle between the direction of a wave and the wavefront?
A
90 degree angle
10
Q
Describe the waves from a narrow gap between barriers in a ripple tank
A
A lot of spread
Waves become more and more curved
11
Q
Describe the waves from a wide gap between barriers in a ripple tank
A
A little spread
Waves flat in the middle and curve on sides
12
Q
How come you can hear sounds around corners?
A
A doorway is a good diffraction of sound waves, and sound waves can bend
13
Q
What type of waves bend the most?
A
The ones with the longest wavelengths
14
Q
What do waves with a longer wavelength do more?
A
Bend more
15
Q
What types of sounds are loudest and why?
A
The ones with the longest wavelengths
Curves more
16
Q
What does the wavelength of a sound wave depend on?
A
The frequency
17
Q
What does a longer wavelength mean in a sound wave and why?
A
It’s louder as the wave is curved more
18
Q
What is the wavelength of sound usually, and what is this similar to?
A
1m, similar to the doorway in a house
19
Q
Why is it not possible to see a light around corners in the same as as we can hear sound around corners?
A
The wavelength of light is a lot shorter - the waves bend less
20
Q
In which situation would you get the most diffraction?
A
If the gap or the size of the barrier is roughly the same as the wavelength of the diffracting waves
21
Q
What does a ‘lot of diffraction’ basically mean?
A
A lot of spread of the wave
22
Q
What do you get if the gap or size of the barrier in the pass of a wave is roughly the same as the wavelength of the diffracting waves?
A
A lot of diffraction
23
Q
Draw a diagram to explain how the diffraction of microwaves would be shown
A
(See notes)
24
Q
What do waves do when they interact?
A
“Pass through” each other
25
What happens in the region where waves overlap?
They superpose
26
What does it mean that waves superpose?
They “add up”
27
Principle of superposition
When two waves interact, their combined displacement at any point is the vector sum of the individual waves at that point
28
What does the ‘vector sum’ part of the principle of superposition show us?
The directions of the individual displacements need to be taken into account
29
Who was the first to demonstrate interference effects using light and when?
Thomas Young in the 19th century
30
What was Thomas Young the first to do?
Demonstrate interference effects using light
31
What first suggested wave-light properties of light?
Interference effects
32
What did interference effects suggest in Young’s experiment?
Wave-like properties associated with light
33
Where the ideas of Thomas Young’s experiment immediately accepted?
No, not until much later on
34
Draw the layout of Thomas Young’s experiment
(See notes)
35
Draw how Thomas Young’s experiment can be adapted to using sound
(See notes)
36
How would the sound version of Thomas Young’s experiment wok?
As a person walked from A to B, they would notice that the sound intensity (volume) wold rise and fall as the constructive interferences alternate
37
Why would the volume rise and fall whilst walked in a straight line by loudspeakers?
The constructive and destructive interferences alternate
38
What do we need to think about when considering interference?
The phase of the waves arriving at the screen
39
What is interference caused by?
Having 2 parallel slits for the wave to pass through
40
What are bright fringes in interference caused by?
Constructive interference
41
Constructive interference
The waves superimpose in phase, so the difference in distance both waves have travelled must be a whole number of wavelengths
42
Describe the interference of waves if they meet in phase
Constructive
43
How must waves meet for it to be constructive interference?
In phase
44
Which part of a wave overlaps with what for constructive interference?
Crest with crest
45
What are dark fringes in interference caused by?
Destructive interference
46
Destructive interference
Thaw waves must have travelled a distance that is different by a number of half wavelengths
47
Which part of a wave interacts with what for destructive interference?
Crest with trough
48
Is it still constructive interference if a point is 4 wavelengths apart from one slit and 3 from the other?
Yes, because they’re still in phase as its a WHOLE wavelength
49
Waves travelling by a distance hat is different by a number of half wavelengths
Destructive interference
50
Difference both waves have travelled is a whole number of wavelengths
Constructive interference
51
What does constructive interference cause?
Bright fringes
52
What does constructive interference cause?
Dark fringes
53
Do wavelengths change during diffraction?
No
54
What should you NOT show with the wave that comes out of a gap in an exam question?
Gaps between the ends of the waves and the barrier
55
How does the frequency of resultant waves from superimposing compare to the incoming waves?
The same
56
Which two conditions must be true for a fringe pattern to be observed with waves passing through slits?
The two sources of waves must have a constant phase difference
The vibrations must be in the same line (parallel)
57
Phase difference
The difference in time between 2 similar events
58
Path difference
The difference in distance travelled by 2 similar waves
59
Coherent
Two waves are coherent if thy have a constant phase difference
60
Give an example of a light which is coherent
Laser light
61
Give an example of a light that ISN’T coherent
Light from a filament lamp
62
What’s the phase difference is the path difference is zero?
Zero
63
What’s the phase difference if the path difference is half a wavelength?
Pi
64
Definition of interference
The superposition of two (or more) coherent waves. When this happens, it causes reinforcement at some points and cancellation at other points.
The resulting effect is called a “system of fringes” or an “interference pattern”
65
What was the first experiment to show the wave nature of light and as opposed to what?
Young’s double slit experiment
As opposed to particle nature
66
What did Young’s double slit experiment show us?
That light has a wave nature, not particle nature
67
Who first completed the double slit experiment and when?
Thomas Young
1801
68
Draw how the double slit experiment is completed
(See notes)
69
What can be used as the light source in the double slit experiment?
Either…
Monochromatic light source and a single slit
A laser source
70
In which direction do waves diffract when maximum diffraction occurs and from where?
90 degrees from the ‘straight through’ direction
71
What would change about the diffraction of a wave if the frequency increased?
Wavelength decreases (becomes smaller than the gap)
Less spreading
Side beams
72
When do you get the most diffraction from a wave travelling through a slit?
If the wavelength is equal to or greater than the width of the slit
73
What must wave sources have for two source interference to be observed clearly?
Zero/Constant phase difference (coherent)
The same frequency
The same or similar amplitudes
Oscillations in the same direction
The same wavelength
2 slits
74
Who believed that waves have particle nature around the time of Young’s experiment?
Newton
75
Path difference
The difference in the distance travelled by 2 waves in arriving at a particular point on the screen
76
What is the maximum amplitude of a wave caused by?
Constructive interference
77
What is the minimum amplitude of a wave caused by?
Destructive interference
78
What does constructive interference show up as on the interference pattern?
A light fringe
79
What does maximum amplitude of a wave show up as on the interference pattern?
Light fringe
80
What does destructive interference show up as on the interference pattern?
Dark fringe
81
What does minimum amplitude/zero amplitude show up as on the interference pattern of a wave?
A dark fringe
82
What is the path difference in waves that undergo constructive interference?
A whole number of wavelengths
83
How do waves arrive if their path difference is a whole number of wavelength?
In phase
84
What is the path difference of waves that undergo destructive interference?
An odd number of wavelengths
85
How do waves arrive that have a path difference that’s an odd number of wavelengths?
In anti phase
86
What does “a” represent in the equation for Young’s experiment?
Slit separation
87
What does “y” represent in the equation for Youngs experiment?
Fringe separation
88
What does “D” represent in the equation for Young’s experiment?
Distance from the double slits to the screen
89
What do we count on a diagram to work out fringe separation?
The gaps between bright fringes, not the bright fringes themselves
90
Diffraction grating
A piece of glass with a large number of parallel and equidistant lines scratched on it
91
What do the gratings of a diffraction grating do to light?
Split it up into its constituent colours
92
What is white light diffracted according to with a diffraction grating?
Wavelength
93
What happens when white light is passed through a diffraction grating?
Light is “split up” into its constituent colours, and diffracted according to wavelength
So, all the colours of the spectrum can be observed
94
How can we observe all of the colours of the spectrum using white light?
Diffract it through a diffraction grating, where its diffracted according to wavelength
95
When will diffraction occur on a diffraction grating?
When light is reflected onto the grating or if its projected through the grating
96
What effect does wavelength have on the amount of diffraction of light?
Longer wavelength = greater diffraction
97
Which light is deviated most - either or blue light - using the same diffraction grating and why?
Red light gets deviated more
Longer wavelength = greater diffraction
98
Draw and label a diagram of white light hitting a grating to diffract light
(See notes)
99
What can be observed when using a grating with more lines per mm (reducing d) to diffract light?
∆y is larger = beams are further apart (due to more constructive interference)
Bright fringes are sharper = bright maxima are very arrow = the angle can be measured accurately
100
Under which conditions is ∆y larger for diffracted light and why?
When using a grating with more lines per mm (reducing d)
More destructive interference
101
What is using a grating with more lines per mm reducing?
d
102
Under which conditions are the bright fringes sharper through a diffraction grating and why is this advantageous?
Using a grating with more lines per mm (reducing d)
The angle can be measured accurately
103
What does it mean if bright fringes are sharper after passing them through a grating with a lower value for d?
Bright maxima are very narrow
104
When are diffraction orders produced?
When monochromatic light is incident normally on a diffraction grating
105
What are diffraction orders?
A pattern of narrow, bright fringes
106
Monochromatic
Single wavelength
107
Normally
At right angles
108
Draw and label a diagram to represent diffraction orders
(See notes)
109
What do we label the different diffraction orders produced by a diffraction grating?
(X)th order maximum
110
What do we call the beam of light that travels in a straight line through the diffraction grating?
Straight through beam
111
What are diffraction orders caused by?
Constructive interference from light waves arriving in phase
112
How much light waves arrive for constructive interference to occur?
In phase
113
What number of maxima is there always for diffraction orders and why?
An odd number
Middle maximum, and an equal amount on both sides
114
What do wavelets do as they’re circular?
Overlap
115
Why would waves be circular?
By the diffraction process
116
What does nearly every part of every wavelet do to their wavelets when passed through 2 slits?
Nearly every part of every wavelet will overlap with another wavelet that is in anti phase, causing destructive interference
117
What causes destructive interference?
Waves overlapping in antiphase
118
What’s the only direction in which constructive interference can occur?
The direction that the pass difference from each slit is a whole number of wavelengths
119
What’s the distance between slits for the first order maximum?
A single wavelength
120
What unit to we work in using the diffraction grating equation?
Metres
121
Derive the diffraction grating equation
(Lovely triangle drawing drawn I’m sure)
d = slit separation
BC = path difference
θ = angle to bright spot
Sin θ = BC
——
AB
path difference
———————-
slit separation (d)
For constructive interference, path difference is nλ (a whole number of wavelengths)
Sin n λ
——
d
dsin(θ) = n λ
122
What’s the diffraction grating equation?
dsin(θ) = n λ
123
What is d in dsin(θ) = n λ?
Slit separation
124
Symbol and unit for slit separation
d
Metres
125
What’s the method for finding the total number of maxima? (diffraction orders)
The limit of θ = 90 degrees
So, there’s a limit to the number of orders that can be observed (has a maximum)
n λ = dsinθ
Sin 90 = 1
n= d
—
λ
nmax ≤ d
—
λ
126
Why is there a limit to the diffraction orders that can be observed?
The limit of θ = 90 degrees
127
What is sin 90 equal to?
1
128
Equation for finding the total number of maximum diffraction orders
nmax ≤ d
—
λ
129
What do we NOT do to the value of the maximum order for diffraction orders and why?
Round the answer up
It would surpass 90 degrees
130
How do we work out the total maxima if we have the maximum order?
Think - 1 on the middle, the value for maximum order on both sides
131
How do we calculate d for the diffraction grating equation?
1/lines per mm
Answer divided by 1000 to be in metres
132
How would we re-arrange the diffraction grating equation to find theta for example?
Use SHIFT sin
133
coherent light
waves or wave sources have a constant phase difference between them and therefore must have the same frequency
134
examples of coherent wave sources
lasers, speakers emitting the same frequency sound wave, two dippers on a beam in a ripple tank
135
incoherent light
emit waves that have no phase relationship
136
examples of incoherent wave sources
lightbulb, LED, person splashing wildly in water
137
what must it follow that a coherent light source is?
monochromatic
138
monochromatic
single wavelength
139
stationary waves
caused by the superposition of identical progressive waves that are travelling in opposite directions
140
what do we mean by 'identical waves' in the definition for stationary waves?
same amplitude and period
141
how can stationary waves be produced?
by reflecting progressive waves from a source back towards the source
142
nodes
points of zero amplitude on a stationary wave
143
antinodes
points of maximum amplitude on a stationary wave
144
what are nodes caused by in a stationary waves?
destructive interference
145
what are antinodes caused by in stationary waves?
contructive interference
146
draw and label the nodes and antinodes on a stationary wave
(see notes)
147
what's the distance between the nodes (or antinodes) in a stationary wave?
half a wavelength
148
what results in the stationary wave being produced when reflecting a progressive wave from a source back to the source?
the interference between the incident wave and the reflective wave
149
what's the relationship between the wavelength of a stationary wave and the progressive wave that causes it?
the same
150
what does a stationary wave have the same wavelength as?
the progressive wave that caused it
151
compare progressive and stationary waves in terms of energy
progressive - energy is transferred at the wave-front
stationary - energy is captive (i.e - not transferred)
152
compare progressive and stationary waves in terms of amplitude
progressive - every particle along the path is vibrated to the amplitude
stationary - varies from zero at the nodes to a maximum at the antinodes
153
compare progressive and stationary waves in terms of frequency
progressive - all particles vibrate with the frequency of the wave
stationary - all particles vibrate with the frequency of the wave except those at the nodes (which don't oscillate at all)
154
what do some of the particles in only a stationary wave have at all times?
a displacement of zero
155
compare progressive and stationary waves in terms of wavelength
progressive - the distance between adjacent particles having the same phase
stationary - twice the distance between adjacent nodes or adjacent antinodes
156
compare progressive and stationary waves in terms of phase
progressive - all particles within one wavelength have a different phase
stationary - all particles between 2 adjacent nodes have the same phase but are in antiphase with those in the next loop
157
Equation for the speed of sound
c = f λ
158
What’s c = fλ the equation for?
The speed of sound
159
Why does every particle between two adjacent nodes in a stationary wave have the same phase?
The wave moves up and down together
160
Fundamental stationary wave
The lowest frequency stationary wave
161
What does a fundamental stationary wave look like?
1 loop
162
Wavelength of a fundamental stationary wave
2L
(Think - node to node is half a wavelength, and this type of wave literally just has 2 nodes - 1 on each end)
163
Harmonic
The amount of “loops” in a stationary wave
164
What would a stationary wave with 2 “loops” be known as?
The 2nd harmonic
165
Harmonic of a fundamental stationary wave
1st harmonic
166
In the equation V^2 = T x L
——
m
If T is reduced by a factor of 4 (i.e - a quarter of the previous value), what factor does V change by?
1/4 of its value
√1/4
1/2
Decreases by a factor of 2
167
What do we mean when we say that waves diffract?
Spread out
168
What’s the same between 2 coherent waves?
Frequency
169
Under which situation would we get lots of diffraction orders?
If the skit separation is large compared to the wavelength
170
What’s the difference between the interference pattern with Young’s experiment and using a diffraction grating? Explain
(Using a diffraction grating…)
Beams are further apart and there’s fewer of them (more slits and so more destructive interference)
Bright fringes are sharper as the bright maxima are very narrow
171
What happens to the intensity of light a the distance from the “straight through” of diffracted light increases? Why?
The intensity decreases
Most intensity “straight through”
172
What will happen to fringe separation as wavelength increases? Why?
Increasing wavelength will increase fringe separation
(Directly proportional)
173
How can we increase fringe separation?
Increase distance from screen
174
Why must constructive interference occur in deriving the diffraction grating equation?
For waves to arrive in phase to cause a bright spot
175
Which word should we always use when describing a stationary wave?
Reflecting
176
Do path differences need to be inserted into the equation?
Yes because it IS the path difference equation
