Section 3 - Waves Flashcards
1
Q
What is a wave?
A
The oscillation of particles or fields.
2
Q
What is a progressive wave?
A
A wave that carries energy from place to place without transferring any material.
3
Q
What is a wave cycle?
A
One complete vibration of a wave.
4
Q
What is the displacement of a wave and what is the unit?
A
How far a point on the wave has moved from its undisturbed position. Unit: metres
5
Q
What is the amplitude of a wave and what is the unit?
A
The maximum magnitude of displacement.
/ distance from the undisturbed position to the crest or trough
Unit: metres
6
Q
What is the period of wave?
A
The time taken for a whole cycle (vibration) to pass a given point. Unit: seconds
7
Q
What is the wavelength of a wave and what is the unit?
A
The length of one whole wave cycle, from crest to crest or trough to trough. Unit: metres
8
Q
What is the frequency of a wave and what is the unit?
A
The number of cycles (vibrations) per second passing a given point. Unit: hertz
9
Q
What is the phase of a wave?
A
A measurement of the position a certain point along the wave cycle.
10
Q
What is the phase difference of a wave?
A
The amount one wave lags behind another.
11
Q
What are the units for phase and phase difference?
A
Angles (degrees or radians) or as fractions of a cycle.
12
Q
What are the symbols for displacement, amplitude, wavelength, period and frequency?
A
- Displacement - x
- Amplitude - A
- Wavelength - Lambda
- Period - T
- Frequency - f
13
Q
What is reflection?
A
When a wave is bounced back when it hits a boundary.
14
Q
What is refraction?
A
When a wave changes direction as it enters a different medium.
15
Q
What equation relates frequency and time period?
A
Frequency = 1 / Time period
f = 1 / T
16
Q
What is the wave equation?
A
Wave speed = Frequency x Wavelength
c = f x lambda
17
Q
What is c?
A
The speed of light in a vacuum - 3.0 x 10^8 m/s
18
Q
What is the equation for wave speed?
A
Wave speed = Distance travelled / Time taken
c = d / t
19
Q
What type of wave are EM waves?
A
Transverse
20
Q
Give some examples of transverse waves.
A
- EM Waves
- Water waves
21
Q
How do you measure the speed of sound with this setup?
A
Microphones = separate inputs so signals can be recorded separately.
22
Q
How can you measure the wave speed in water?
A
23
Q
What are the two types of graphs that can be drawn to show a transverse wave?
A
1) Displacement against distance along the path of a wave
2) Displacement against time for a POINT as the wave passes
(Note: 1 is just a standard graph of what a wave looks like. 2 is what happens to a specific point as a wave passes through it.)
24
Q
What does the distance between two crests/troughs represent on a displacement - distance graph?
A
Displacement - distance: Wavelength
25
What does the distance between two crests/troughs represent on a displacement - time graph?
Displacement - time: Time period
26
Electromagnetic waves travel as vibrations through...
... magnetic and electric fields.
27
When looking at a graph representing a transverse wave, what must you look out for?
The label on the x axis. This may be distance or time, depending on what the graph is showing.
28
Describe the vibrations on a transverse wave.
At right angles to the direction of energy transfer.
29
Give some examples of a longitudinal wave.
* Sound
* Pressure
30
What are the parts of a longitudinal wave?
* Compressions
* Rarefactions
31
What are the anti-compressions in a longitudinal wave called?
Rarefactions
32
33
Do transverse and longitudinal waves require a medium?
* Transverse - Usually no
* Longitudinal - Usually yes
34
How are longitudinal waves represented on a graph?
* Displacement against time.
* This can it look like a transverse wave!
35
Describe the vibrations in a longitudinal wave.
Parallel to the direction of energy transfer.
36
What is a polarised wave?
A wave that only oscillates in one direction (e.g. only up and down).
37
Can transverse and longitudinal waves be polarised?
* Transverse - Yes
* Longitudinal - No
38
Compare the vibrations in transverse and longitudinal waves.
* Transverse - Perpendicular to the direction of energy transfer.
* Longitudinal - Parallel to the direction of energy transfer.
39
What is polarisation?
Causing a transverse to only vibrate in one direction (e.g. up and down) usually by passing it through a polarisation filter.
40
What is some evidence for light being a transverse wave?
It can be polarised by reflection. A longitudinal wave could not do this, so light must be a transverse wave.
41
What is polarisation evidence for?
Which waves are transverse. For example, light can be polarised, so it must be transverse.
42
Why can light waves be polarised?
They are a mixture of different directions of vibration. This means that they can be polarised by allowing only some of these directions to pass through a filter.
43
What is a polarising filter?
A panel that polarised waves by only allowing a specific direction of vibration to pass through.
44
What happens in terms of polarisation when light is reflected off some surfaces?
It becomes partially polarised. This means some of it vibrates in the same direction.
45
What happens when two polarising filters are arranged at right angles to each other?
No light will get through.
46
What happens if the two filters aren't quite at right angles?
What is done to the filter to proves this?
It instead reduced the intensity of the light passing through it (but still allows some light through).
By rotating the filter we can see the change in intensity:
47
Is most light we see polarised?
No - most light we see is unpolarised
48
How does glare work?
Light reflected off some surfaces is partially polarised - some of it is made to vibrate in the same direction (Figure 9).
When light reflected off surfaces like water, glass or tarmac enters the eye, it can cause glare.
49
How does glare reduction work?
The fact that reflected light is PARTIALLY-POLARISED allows us to filter some of it out with polarising filters.
If you view PARTIALLY-POLARISED reflected light through a polarising filter at the right angle, you can block out some of the reflected light, while still letting through light which VIBRATES at the angle of the filter.
This reduces the intensity of light entering your eye.
50
What is the effect of reducing glare used for?
Reducing unwanted reflections in photography, and in polaroid sunglasses to reduce glare.
51
What does the amount of polarisation depend on?
The angle of the incident light
52
How do polaroid sunglasses work?
* Partially polarised light is reflected into a polarising filter at the correct angle.
* This blocks out unwanted glare.
53
How do TV and radio signals make use of wave polarisation?
* Broadcasting aerial has rods, which emit polarised waves
* TV aerials on homes have horizontal rods
* These rods must be lined up in order to get maximum signal strength
* The same thing happens with radio aerials
54
Give two examples of when wave polarisation is used.
* Polaroid sunglasses
* TV and radio signals
55
What is superposition?
When two or more waves pass through each other and their displacements combine.
56
What does the principle of superposition state?
When two or more waves cross, the resultant displacement equals the vector sum of the individual displacements.
57
Graphically, how do you superimpose waves?
Add the individual displacements at each point along the x-axis and then plot these.
58
What happens when a crest meets a crest (or a trough meets a trough) and what is this called?
* Constructive interference
* The amplitude of the wave is increased (i.e. the crest or trough gets bigger).
59
What happens when a crest meets a trough of the same size and what is this called?
* Destructive interference
* The displacements cancel themselves out.
60
What happens to these waves?
How do you work out the displacement of the combined wave?
Add the displacements of the two waves (=resultant)
61
What does it mean when two points on a wave are "in phase"?
They are both at the same point in the wave cycle. They are likely to be 360\*, 720\*, etc. out of phase. They are the same wavelength and velocity
62
What quantities are the same about points on a wave which are in phase?
* Same velocity
* Same displacement
63
How many degrees is one complete wave cycle said to be?
360\*
64
How many radians is one complete wave cycle?
2π radians
65
How many degrees is a radian?
180/π
66
What is the SI unit for angle?
Radian -\> 1 radian is equal to 180/π.
67
How do you convert from degrees to radians?
Multiply by π/180.
68
How do you convert from radians to degrees?
Multiply by 180/π.
69
What is half a wavelength in degrees and radians?
* 180\*
* π radians.
70
What is 1/4 of a wavelength in degrees and radians?
* 90\*
* 1/2 π radians
71
What is 3/4 of a wavelength in degrees and radians?
* 270\*
* 3/2 π radians
72
What is a whole wavelength in degrees and radians?
* 360\*
* 2π radians
73
What is the phase difference of a vibrating particle?
The fraction of a cycle it has completed since the start of a cycle.
74
What a the phase difference between two particles?
Thee fraction of a cycle between the vibrations of the particles, measured in either degrees or radians.
/ Difference in their positions in a wave's cycle
75
What is the phase difference between two maximums in a diffraction pattern (for double slit in this case)
Distance between two maximums = phases difference of 1 wavelength
76
What is the unit for phase difference?
Degrees or radians.
77
Waves with a phase difference of 0\* or a multiple of 360\* are said to be...
... in phase.
78
Waves with a phase difference of an odd number multiple of 180\* are said to be...
... exactly out of phase.
79
When are two sources said to be coherent?
When they have the same:
• Wavelength
• Frequency
And have a fixed phase difference between them.
80
When are interference patterns most clear?
When the two sources are coherent (have the same wavelength and frequency and have a fixed phase difference between them).
81
What is path difference and when is it relevant?
* How much further a wave has travelled compared to another
* This is used when looking at the type of interference between two waves that will occur at a certain point (see diagram pg 27 of revision guide).
82
Assuming that two sources are coherent and in phase, at what path difference will constructive interference occur?
At a whole number of wavelengths.
Path difference = nλ
83
Assuming that two sources are coherent and in phase, at what path difference will destructive interference occur?
At a whole number of wavelengths and a half.
Path difference = nλ + 0.5λ
84
When are superposed waves easier to 'see'?
* the waves are of similar amplitude (↑ contrast between maxima and minima)
* the waves have similar frequencies - otherwise the interference patterns create change so fast that they are difficult to detect
* the waves have a constant phase difference i.e. they are phase linked
85
Examples of coherent sources?
* light produced by a laser
* sound from two loudspeakers connected in parallel
* light emerging from two apertures illuminated by the same source
86
What is a stationary wave?
The superposition of two progressive waves with the same frequency (wavelength) moving in opposite directions.
87
What type of wave forms a stationary wave?
A progressive wave.
88
Do stationary waves transmit energy?
No
89
Describe how stationary waves in a string can be demonstrated.
* Vibration generator is attached to a piece of string at one end, while the string is fixed at the other end.
* The frequency of the generator is varied until a resonant frequency is found.
90
Describe how the wave on a fixed piece of string (so it reflects at the end) changes with frequency.
* At most frequencies, the pattern on the string is a jumble
* If the vibration generator produces an exact number of waves in the time it takes a wave to get to the end and back, the original and reflected waves reinforce each other. This produces a stationary wave. - The overall pattern doesn't move along, it just vibrates up and down.
91
When do stationasrty waves only occur?
Waves intefere with it's reflection.
Only happens at specific frequencies.
Nodes must occur at the point of return.
92
What is a node on a stationary wave?
Where the amplitude of the vibration is zero.
Total destructive inteference
93
What is an antinode on a stationary wave?
Where the maximum amplitude of the wave is.
constructive interence
94
What are the sections of stationary wave on a string called?
Oscillating loops
95
What is resonant frequency for stationary wave?
When an exact number of half wavelengths fit onto the string.
96
What is it called when one, two and three loops of stationary wave are found on a string?
1 Loop = 1/2 wavelength = First harmonic
2 Loops = 1 wavelength = Second harmonic
3 Loops = 1.5 wavelengths = Third harmonic
97
What is the first harmonic?
* When the stationary wave is vibrating at the lowest possible resonant frequency.
* One loop is on the string, with a node at each end.
98
At the first harmonic, what is the length of the section of string?
1/2 a wavelength of the wave
99
At the second harmonic, what is the length of the section of string?
1 wavelength
| (when 2 half wavelengths (loops) fit on a string, the wavelength is the length of the string)
100
At the third harmonic, what is the length of the section of string?
1.5 wavelengths
101
What is added each time you have another harmonic?
How do you work this out?
an extra loop and an extra node, the number of wavelengths goes up by 1/2.
At the a^th harmonic the number of antinodes = a. number of nodes is a + 1.
At the a^th harmonic, a/2 wavelengths fit on the string
102
How can you work out the frequency of the harmonic?
a x first harmonic frequnecy.
a = number of antinodes or the amount of harmonics or just the amount of bumps
103
How many wavelengths are in the first harmonic for a closed tube?
Closed tube: the first harmonic has 1/4 wavelength.
(still increases by 1/2 a wavelength but the first harmonic starts at 1/4 wavlenghs)
104
Closed tube: how many wavelengths are in the second harmonic
3/4 wavelengths (0.75)
(still increases by 1/2 a wavelngth but the first harmonic starts at 1/4 wavlenghs)
105
Closed tube: how many wavelengths are in the third harmonic
5/4 wavelngths (1.25)
(still increases by 1/2 a wavelngth but the first harmonic starts at 1/4 wavlenghs)
106
Closed tubes: what sound does a long wavelength create?
deep sound
107
Open tubes: What does an open tube look like?
Antinode at entrance and exit, normal number of wavelngths in a harmonic (1st = 1/2, 2nd = 1, 3rd = 1.5)
108
How do open and closed wavelengths compare?
Open tubes have more wavelength in the tube (in order to have an antinode at both ends).
This means they have a smaller wave in the same size tube.
When you hit a tube on a table with its 1 lid off, you get a deeper pitch than if it had both sides open and being hollow.
Watch TLPhysics video about closed and open tube harmonics.
109
Remember to revise harmonic diagrams
Pg 28 of revision guide.
110
How are stationary microwaves found?
* Microwaves beam reflected of a metal plate - the superposition of the wave and it's reflection produces a stationary wave.
* The nodes and antinodes are found by moving the probe between the transmitter and reflecting plate.
* The meter or loudspeaker receievs no signal at the nodes and maximum signal at the antinodes.
111
Describe how sound can be used to demonstrate stationary waves.
How do you find the speed of sound from this?
* A loudspeaker produces sound waves in a glass tube
* Glass tube with a speaker at the end is set up
* Lycopodium powder is laid along the bottom of the tube
* The powder is shaken away from the antinodes and left undisturbed at the nodes
* Distance, d, between each pile of power (node) is **λ** /2. ( **λ** = 2d)
* The speed of sound = c = f**λ**.
* c=f**λ** is the same as c = f x 2d which is c = 2df
* You can find the speed of sound by measring d and knowing the frequnency of the signal generator
112
Compare the frequency of the first, second and third harmonic.
* First = f
* Second = 2f
* Third = 3f
113
Which equation can be used to find the frequency of the nth harmonic on a piece of string?
f = c / λ
Where:
f = Harmonic frequency
c = Speed of wave on string
λ = The wavelength of the wave given in terms of the length of the string (e.g. first harmonic: λ = 2L)
114
In terms of wave speed and sting length, at what frequency is the first harmonic achieved?
f = c / 2L.
(½ wavlength = 1 length of string.
Therefore, 1 wavelngth = 2 lengths (2L))
When substituting L for wavelength, always start with “1 length of string = x wavelengths” then rearange to find 1 wavelength in terms of “lengths of string (L)"
115
In terms of wave speed and sting length, at what frequency is the second harmonic achieved?
f = c / L
(1 wavlength = 1 length of string)
116
In terms of wave speed and sting length, at what frequency is the third harmonic achieved?
f = 3c / 2L
Because
1.5 wavelngths = 1 length of string
1 wavelength = 1/1.5 = ⅔ lengths of string
f = c / (2/3 L)
rearange to get f = 3c / 2L
117
What is the equation for phase difference in radians?
Phase difference (radians) = 2πd / λ
Where d = the distance apart of the particles in wavelengths (**λ)**.
(e.g. d might equal 1/4 λ if there is a quarter of a cycle difference)
Remember the period of a wave is 2π
118
Describe an experiment used to show how mass, length and tension change the resonant frequencies of a string.
1) Measure the mass and length of the string using a mass balance and ruler. Work out the mass per unit length (μ = M/L) in kg/m.
2) Set up the equipment as shown. This involves connecting a vibration generator (connected to a signal generator) to a piece of string attached to a pulley and some masses. Clamp the entire setup to the bench.
3) Measure the length (l) of the string between the vibration generator and the pulley. Work out the tension in the string using (T = mg) where m is the mass of the masses on the end of the string.
4) Turn on the signal generator and adjust the frequency until the first harmonic is found.
Depending on the experiment, you can chose to either change the mass (per unit length), the length or the tension; of the string
Chose one to change and keep the rest the same.
119
What are the first, second, third, etc harmonics known as collectively?
The resonant frequencies.
120
Which factors during the stationary wave experiment may affect the resonant frequencies?
* Length of the vibrating string - longer the string the lower the resonant frequency - because the half wavelength is longer (c = f**λ**, f increases for a fixed c)
* Tension in the string - waves travel more slowly if the string is loose and there is less tension (lower c = lower f)
* Type of string (different μ) - heavier string (more mass per unit length) - waves more slowly down the string (lower c = lower f)
121
In the stationary wave experiment, what do the letters μ, Μ, L, T, m and g represent?
* μ = Mass per unit length of string
* Μ = Mass of the string
* L = Length of vibrating string
* T = Tension in the string
* m = Mass of the masses in the end of the string
* g = Gravitational field strength
122
What is the unit for tension?
Newtons (N)
123
Remember to revise the stationary waves experiment.
Pg 29 of revision guide.
124
How can the length of the vibrating string in the stationary waves experiment be varied?
* Keep the type of string and tension the same
* Move the vibration transducer towards or away from the pulley
125
How can the tension in the string in the stationary waves experiment be varied?
* Keep the string type and length the same
* Add or remove masses to vary tension
126
How can the string type in the stationary waves experiment be varied?
* Keep the vibrating string length and tension the same
* Use different string samples to vary μ (different masses of string with the same length)
127
How does string length affect the resonant frequency in the stationary wave experiment?
* The longer the string, the lower the resonant frequency.
* Because the half wavelength at the resonant frequency is longer.
128
How does the type of string affect the the resonant frequency in the stationary wave experiment?
* The heavier (greater μ) the string, the lower the resonant frequency.
* Because waves travel more slowly down the string. A lower wave speed, c, makes a lower frequency, f.
129
How does tension affect the the resonant frequency in the stationary wave experiment?
* The higher the tension, the higher the resonant frequency.
* Because waves travel more quickly on a taut string. A higher wave speed, c, makes a higher frequency.
130
In the stationary wave experiment, what equation is used to give the FIRST harmonic frequency of a STRING?
f = (1 / 2l) x root(T / μ)
Where:
l = String length (m)
T = Tension in string
μ = Mass per unit length of string (kg/m)
See page 29 of revision guide.
131
Remember to revise the equation for the first harmonic frequency in the stationary wave experiment.
Pg 29 of revision guide
132
What is diffraction?
The spreading out of waves when passing through a gap (or going around an object).
133
What determines the amount of diffraction observed?
The wavelength of the wave compared to the size of the gap.
134
When is diffraction most noticeable?
When the gap is the same size as the wavelength.
135
How does a narrower gap affect diffraction?
It is increased.
136
How does a smaller wavelength affect diffraction?
It is decreased.
137
What happens in terms of diffraction when the gap is a lot bigger than the wavelength?
Diffraction is unnoticeable.
138
What happens in terms of diffraction when the gap is a lot smaller than the wavelength?
The waves are mostly just reflected back.
139
When both are not in direct line of sight, why can sound be heard around a doorway, but light cannot be seen.
The doorway is a gap of a similar size to the wavelength of sound, so it diffracts to the listener. However, the gap is much larger than the wavelength of light, so the diffraction is not noticeable.
140
In a single-slit white light diffraction pattern, what is the order of colours in each spectrum band and why?
* Blue is on the inner side, while red is on the outer side
* This is because red light has a longer wavelength, so it diffracts more
141
What happens to each fringe in a single-slit diffraction pattern as you move from the central maximum?
The fringes become less bright.
142
What is intensity of light?
The power per unit area.
143
In a single-slit diffraction pattern, how does the width of the central maximum compare to the outer fringes?
* It is twice as wide
* The outer fringes are all of the same width
144
Name two monochromatic light sources.
* Laser
* Vapour lamps and discharge tubes
145
Do two light sources have to be in phase to be coherent?
No, as long as they have a constant phase difference.
146
What is the single-slit equation?
W = 2Dλ/a
Where:
• W - Width of the central maximum
• D - Distance between the slit and screen
• λ - Wavelength
• a - Slit width
(All units in m)
147
Which of the wave equations is not given on the equation sheet?
Single-slit interference
| (W = 2Dλ/a)
148
What is needed to demonstrate two-source interference?
Two coherent sources.
149
Why are the 2 loudspeakers coherent sources of sound waves?
* They have the same frequency/wavelength AND constant phase difference.
* This is achieved by both speakers being connected to same signal (generator)
150
Point A is at equal distances from P and Q. He then moves to point B.
A student moves from A to B and the amplitude of the sound wave he hears decreases and then increases. The amplitude starts to decrease again as he moves beyond B. Explain why the variation in amplitude occurs as he moves from A to B.
* The sound waves from the two speakers superpose (at a point) (not interfere)
* At A (and B) the two waves are in phase (they have zero phase difference) and a maximum is produced.
* Moving away from A introduces a path difference/phase difference = waves are out of phase (and amplitude decreases)
* Minima's are formed when there is destructive interference (odd number of half wavelength = path difference or π/ 180 degrees = phase difference or antiphase)
* (Moving on towards B the waves move back in phase)
151
* separation of the two loudspeakers = 0.30m
* distance OA = 2.25 m
* distance from A to B = 0.95 m
* Show that the path difference for the sound waves from the two loudspeakers to point B is about 0.1 m.
152
The frequency of the sound wave is 2960 Hz. Calculate the speed of sound from the student’s data.
(remember path difference = 0.12 meters)
Path difference between two maxima's = 1 wavelength.
1 path difference = 0.12 m
1 wavelength = 0.12 m
0.12 x 2960 = 360m/s
153
* As frequency increases, so does wavelength.
* Therefore, the separation of maxima's (along the line AB) increases
* Maximum moves (from B) towards C so amplitude of sound gets larger/louder (then quieter). OR Maximum moves even further along path/beyond C so amplitude of sound gets quieter.
154
How can you ensure that two sources are coherent when demonstrating two-source interference with water or sound?
Connect both dippers/loudspeakers to the same vibrator/oscillator.
155
How can you show this (picture) with an experiment?
Use two microwave transmitter cones attached to the same signal generator.
Use a microwave receiver probe - you get an alternating pattern of strong and weak signals.
156
What is another name for the experiment to demonstrate two source interference?
Young's double-slit experiment
157
In Young's double slit experiment, what do the slits act as?
Two coherent sources of light
158
Can a white light source be a coherent source?
No, due to the various frequencies of light in it.
159
Remember to revise the set up for Young's double slit experiment.
Pg 32 of revision guide.
160
How can you demonstrate young's double slit experiment?
What are the safety precautions?
161
How can you investigate the formula of young's double slit experiment with this set up?
162
Why can the bands in the diffraction pattern only be called "fringes" in the double slit pattern, not the single slit pattern?
Because they are all of the same width, unlike in the single slit pattern.
163
What can be said about the phase difference at a bright fringe in the double slit interference pattern?
P.d. = nλ
Where n is an integer.
164
What can be said about the phase difference at a dark fringe in the double slit interference pattern?
P.d. = nλ + 0.5λ
Where n is an integer.
165
What is fringe separation?
The distance from the centre of a bright fringe to the centre of the next one.
166
What is the danger of a powerful laser?
If you looked at the beam directly, your eye would focus it onto your retina, which would be permanently damaged.
167
What are some safety precautions that must be taken when working with a laser?
1) Never shine the laser towards a person.
2) Wear laser safety goggles.
3) Avoid shining the beam at a reflective surface.
4) Have a warning sign on display.
5) Turn the laser off when it's not needed.
168
How can Young's double slit experiment be adapted for microwaves?
* Replace the laser and slits with 2 microwave transmitter cones attached to the same signal generator
* Replace the screen with a receiver probe
* Move the probe along where the screen was and you'll get an alternating pattern of strong and weak signals
169
What is the equation for Young's double-slit experiment?
w = λD/s
Where:
w = Fringe spacing
λ = Wavelength
D = Distance from slits to screen
s = Slit separation
170
In Young's double slit experiment, what is the easiest way to get an accurate reading for 'w'?
Measure several fringes and divide by the number of fringe widths between them.
171
In Young's double slit experiment, what must you be careful of when measuring several fringes?
* When dividing to find 'w', remember to divide by the number of fringe WIDTHS between them, not the number of fringes.
* e.g. 10 bright lines only have 9 fringe widths between them.
172
Compare single and double slit diffraction patterns in terms of fringe widths and intensities.
Single slit:
• Widest central maximum + equal outer fringes
• Brightest central maximum + decreasing intensity of outer fringes
Double slit:
• All fringes of equal width
• Decreasing intensity of outer fringes
173
In a double slit interference pattern, why does the intensity of the fringes decrease as you get further away from the central maximum?
Because it's multiplied by the single slit diffraction pattern for either of the slits separately.
174
Compare the double slit interference pattern for red and blue light.
The blue light creates a smaller fringe separation. This makes the pattern appear more compact.
175
Describe and explain what is observed with double slit interference of WHITE light.
* White central fringe - Every colour contributes at the centre
* Inner fringes tinged with blue on the inside and red on the outer side - Red fringes are more spaced out than blue fringes.
* After a few fringes, no clear fringe pattern - The different colour's fringe patterns have all blended
176
What was the importance of Young's double slit experiment?
* It was evidence for light interference and diffraction.
* This was important in the debate between Newton's particle (corpuscle) theory of light and Huygen's wave theory of light.
* It supported Huygen's theory (even though the debate was raging again 100 years later).
177
Explain what happens when Young's double slit experiment is repeated with more slits.
* The same shaped pattern is observed, except the bright bands are brighter and the dark bands are darker
* This gives a sharper pattern
178
What makes the pattern so sharp when monochromatic light is passed through a diffraction grating?
There are many beams reinforcing the pattern.
179
What is the advantage of observing sharper lines in interference patterns?
It allows for more accurate measurements.
180
In double slit interference, what conditions must be met in order for a pattern to be seen?
* Each slit must be sufficiently narrow to diffract the light enough
* The two slits must be close enough for the diffracted waves to overlap
181
Explain simply why single-slit diffraction patterns are observed.
The waves from different points across the slit interfere to reinforce or cancel each other.
| (See pg 84 of textbook for a good explanation)
182
Explain why a diffraction grating produces several sharp lines.
* Diffracted light waves from adjacent slits reinforce each other in certain directions only and cancel out in all other directions.
* It works just like with double slit interference, except with many more slits.
* More slits result in more sharp lines, so the are several distinct, sharp lines produced by a diffraction grating.
183
What is the equation for distance between slits in a diffraction grating?
d sin θ = nλ
184
How do you derive:
d sin θ = nλ?
185
What must you be careful of when putting 'd' into the diffraction grating equation?
d is the slit spacing, not the number of slits per metre, which is how the data may be given.
186
When given a grating with 300 slits per mm, what value of d is used?
* 300 slits/mm = 300,000 slits/m
* Therefore, d = 1/300,000
187
Remember to practise deriving the diffraction grating equation.
See pg 34 of revision guide + have a go.
188
What effect does increasing wavelength have on the diffraction grating pattern?
It is more spread out.
189
What effect does increasing slit separation have on the diffraction grating pattern?
It is more compact.
190
When calculating the maximum order for a certain diffraction grating and a certain wavelength, what must you do to the value of n obtained?
Round it down to the next integer.
191
How can you calculate the maximum order for a given diffraction grating and wavelength? Explain why this works.
* θ can never be greater than 90. Therefore, the greatest value that sinθ can have is sin(90), which is 1.
* So, replace sinθ with 1 in the equation.
* This leaves d = nλ. Solve for n.
* Round n DOWN to the nearest integer.
192
What is X-ray crystallography?
* The wavelength of x-rays is similar to the spacing between atoms in crystalline solids.
* So x-rays directed at a thin crystal form a diffraction pattern -\> The crystal acts like a diffraction grating.
* Looking at the diffraction pattern, the spacing of the atoms can be calculated.
193
What was x-ray crystallography used for?
To discover the structure of DNA.
194
What is the \*absolute\* refractive index of a material?
* A measure of optical density
* A ratio of the speed of light in a vacuum compared to the speed of light in the material.
195
When does light travel the fastest?
In a vacuum.
196
Why does light slow down in optically dense materials?
It interacts with the particles in the material.
197
The more optically dense a material is, the more that light…..
slows down when entering it
198
The optical density is measured by what?
its refractive index
199
What does high optical density mean for it's refractive index?
Higher optical density = higher refractive index
200
What symbol is used for the speed of light in a vacuum?
c
201
What is ABSOLUTE refractive index?
the absolute refractive index of a material, n, is the ratio between the speed of light in a vaccuum, c, and the speed of light in that material cs (subscript s)
202
What is the equation for absolute refractive index?
n = c/cs
| (NOTE: This is just a specific case of the equation for relative refractive index)
203
What symbol is used for the speed of light in a material?
cs (subscript s)
204
What is the symbol for absolute refractive index?
n
205
The speed of light in air is only a bit smaller than the speed of light, c, so you can assume that nair =?
nair = 1
206
What is \*relative\* refractive index?
The ratio of the speed of light in material 1 to the speed of light in material 2.
207
What is the symbol for relative refractive index of a boundary?
1n2.
(it means the relative refractive index of a boundary, going from material 1 to material 2)
208
What is the speed of light in a vacuum?
3.00 x 10^8 m/s
209
What is the difference between absolute and relative refractive index?
• Absolute refractive index is the ratio of the speed of light in a vacuum compared to the speed of light in the material.
• Relative refractive index is the ratio of the speed of light in material 1 to the speed of light in material 2.
(NOTE: Absolute refractive index is just a case of the relative refractive index)
210
What are the equations for relative refractive index?
1n2 = c1/c2
or
1n2 = n2/n1
211
In the exam, you are given n = c/cs. Practise deriving the two equations for relative refractive index from this.
* n = c/cs
* 1n2 = c1/c2 (It's logical from the definition!)
* 1n2 = n2/n1
212
How many materials do absolute refractive index and relative refractive index refer to?
* Absolute - Property of one material only.
* Relative - Property of the interface between two materials. It is different for every possible pair.
213
What is the refractive index of air?
1
214
In refractive calculations, how are air and vacuum perceived?
They are essentially the same since they both have a refractive index of about 1.
215
What is the angle of incidence and what is the symbol?
The angle that incoming light makes to the normal.
Symbol: θ1
216
What is the angle of refraction?
The angle that the refracted ray makes to the normal.
Symbol: θ2
217
What must you be careful of when dealing with the angle of incidence and the angle of refraction?
They are measured from the NORMAL, not the boundary.
218
Which was does light bend when it enters a more optically dense material?
From this, if n1 \< n2, what can we say about θ1 and θ2
Towards the normal.
219
Which was does light bend when it enters a less optically dense material?
From this, if n1 \> n2, what can we say about θ1 and θ2
Away from the normal.
220
What is Snell's law?
n1 x sinθ1 = n2 x sinθ2
221
What is the critical angle?
The angle of incidence at which the angle of refraction is 90\* so that the light is refracted along the boundary.
222
What is the equation for the critical angle?
sinθc = n2/n1 = 1n2
223
How do you get the critical angle?
224
Derive the equation for the critical angle.
* In this case the angle of incidence is the critical angle: sinθ1 = sinθc
* At the critical angle, the angle of refraction is 90\*
* So sinθ2 = sin90 = 1
* Use snells law : n1 x sinθ1 = n2 xsinθ2
* n1 x sinθc = n2 x 1
* sinθc = n2/n1
225
How can we find the refractive index of a material if the boundary is material to air and we have the critical angle?
226
When drawing diagrams in semi-circle blocks, what is it important to remember?
There is partial reflection observed always, even when there is no total internal reflection.
| (See pg 73 of textbook)
227
What is total internal reflection?
When light going from a more optically dense to a less optically dense material hits the boundary at an angle greater than the critical angle and is completely reflected.
228
What are the conditions for total internal reflection?
1) Incident substance has a larger refractive index than the other substance
2) Angle of incidence exceeds the critical angle
229
What is an optical fibre?
A very thin flexible tube of glass or plastic fibre that can carry light signals over long distances using TIR
230
Describe how an optical fibre works.
* Step index optical fibres used
* The fibre has a very high refractive index (optically dense), but is surrounded by a cladding with a lower refractive index (less optically dense) -\> This enables TIR + Protects the fibre.
* The fibre is narrow -\> Light always hits the boundary at an angle greater than the critical angle.
* Light that enters at one end is totally internally reflected to the other end
231
Name some design features of an optical fibre.
* Thin -\> Ensures light hits at angle above the critical angle + Prevents modal dispersion
* Cladding of lower refractive index -\> Protects the fibre from scratches + Ensures TIR happens
232
What are the 2 reasons for cladding on an optical fibre?
* Protects the fibre from scratches
* Ensures TIR happens (by having a lower refractive index)
233
What are the 2 reasons for making an optical fibre thin?
* Ensures light hits at angle above the critical angle
* Prevents modal dispersion
234
How does light get from 1 end of the fibre to the other?
235
Name 2 uses of optical fibres.
* Endoscopes
* Communications
236
What are the benefits of optical fibres being used to transmit phone and cable TV signals (5)
* Light has high frequency = signal can carry lots of information
* Light does'nt heat up the fibre = no energy is lost
* No electrical interference
* Fibre-optics are cheap to produce
* Signal can travel very far, very quickly, with minimal signal loss.
237
What is signal degradation?
The disruption and changing of a signal as it passes through an optical fibre.
238
What are the two ways in which a signal can be degraded?
* Absorption
* Dispersion
239
What is signal degradation by absorption and how does it affect the signal?
* Some of the signal's energy is lost through absorption by the material of the fibre.
* This reduces the amplitude.
240
What does dispertion cause?
Pulse broadening
241
What is pulse broadening?
The recieved signal is boader than the initial signal. Broadened pulses can overlap each other, leading to information loss
242
What are the two types of signal dispersion?
* Modal dispersion
* Material dispersion
243
What is modal dispersion and how does it affect the signal?
* Light rays enter the fibre at different angles and so take different paths -\> Some arrive later than others. Rays taking a path straight down the middle of the fibre arrive faster than rays taking a longer path.
* This causes pulse broadening.
244
What is material dispersion and how does it affect the signal?
* Caused by different amounts of refraction experienced by different wavelengths of light
* Different wavelengths slow down by different amounts in the material so they travel at different speeds in the fibre -\> Some arrive later than others.
* This causes some parts of the signal to take a longer time to travel down the fibre than others.
* This causes pulse broadening.
245
How can signal degradation by absorption be reduced?
* Use a highly transparent fibre to stop absorption.
* Use an optical fibre repeater
246
How can modal dispersion be reduced?
* Use a very narrow fibre -\> Path difference is small.
* Use a single-mode fibre -\> Only allows light to take one path.
* Use an optical fibre repeater
247
How can material dispersion be reduced?
* Use monochromatic light
* Use an optical fibre repeater
248
What is an optical fibre repeater and what does it prevent?
* A device that boosts and regenerates the signal every so often
* This reduces signal degradation by absorption and dispersion
249
When signal dispersion is large, what can happen?
Broadened pulses can overlap, causing confusion and information loss
250
How does a medical endoscope work?
* Has 2 bundles of fibres.
* One is used to illuminate the area of the body.
* A lens forms an image on the end of the other bundle
* The fibres then take this image back to the other end, where it can be viewed.
251
What is important about the bundle of fibre in an endoscope?
The bundle must be coherent, so that the image on the other end is not muddled up.
252
In what planes can the displacements of oscillations in transverse waves be?
In all planes
253
What is a coherent bundle of fibres in an endoscope?
When the fibre ends are in the same relative positions (i.e. the fibres arrange themselves in the same order to recreate the original image).
254
What are plane-polarised waves?
Have the oscillations in one plane only
255
How is light polarised?
* by absorbing all planes of oscillation except one
* by reflection
256
What is Polaroid plastic formed from?
Many tiny crystals, all lined up
257
Which planes of oscillation does Polaroid plastic absorb?
All of them except the vertical one
258
When light is polarised by reflection, which way is the plane of polarisation?
Horizontal
259
What happens if you wear sunglasses made from Polaroid plastic and look at water?
Reflection off the water surface is absorbed, because its plane of polarisation is perpendicular to that of the Polaroid
260
Why do you not get dazzled by the reflected glare from water when wearing Polaroid sunglasses?
The plane of polarisation of the water is perpendicular to that of the Polaroid
261
What happens if two pieces of Polaroid are 'crossed' so that their transmission planes are at right angles?
No light will get through
262
What is polarisation used in?
* sunglasses
* alignment of aerials for transmission and reception
263
What is the effect of reducing glare used for?
264
5/4 wavelngths (1.25)
(still increases by 1/2 a wavelngth but the first harmonic starts at 1/4 wavlenghs)
265
Open tubes:
266
What is ABSOLUTE refractive index?
the absolute refractive index of a material, n, is the ratio between the speed of light in a vaccuum, c, and the speed of light in that material cs (subscript s)
