3.3 Waves Flashcards
(102 cards)
1
Q
What is a progressive wave?
A
A wave that transfers energy without transferring material and is made up of particles of a medium oscillating
2
Q
What is amplitude?
A
A wave’s maximum displacement from its equilibrium position, in metres (m)
3
Q
What is frequency?
A
The number of complete oscillations passing through a specific point per second, in Hertz (Hz)
4
Q
What is wavelength?
A
The length of a complete oscillation (distance between successive peaks/troughs), in metres (m)
5
Q
What is wave speed?
A
The distance travelled by a wave per unit time, in m/s
6
Q
What is a phase?
A
The position of a certain point on a wave cycle, in radians, degrees or fractions of a cycle
7
Q
What is phase difference?
A
How much a particle/wave lags behind another particle/wave, in radians, degrees or fractions of a cycle
8
Q
What is a period?
A
The time taken for one full oscillation, in seconds (s)
9
Q
What is a transverse wave?
A
A sinusoidal wave where the oscillation of particles/fields is perpendicular to the direction of energy transfer
10
Q
What type of wave are EM waves?
A
Transverse
11
Q
What speed do EM waves travel in a vacuum?
A
3x10^8 m/s
12
Q
What are examples of demonstrating a transverse wave?
A
Shaking a slinky vertically or attaching string to a signal generator
13
Q
What is a longitudinal wave?
A
A wave where the oscillation of particles is parallel to the direction of energy transfer
14
Q
What do longitudinal waves consist of?
A
Compressions and rarefactions
15
Q
What type of wave cannot travel in a vacuum?
A
Longitudinal
16
Q
What type of wave is sound?
A
Longitudinal
17
Q
How can you demonstrate a longitudinal wave?
A
Pushing a slinky horizontally
18
Q
What is polarisation?
A
When the orientation of a transverse wave is specified so that waves of that orientation only are allowed though a medium
19
Q
What type of waves can be polarised?
A
Transverse
20
Q
What does polarising a wave do?
A
Make it so the wave can only oscillate in one plane
21
Q
How is polarisation evidence for the nature of transverse waves?
A
It can only occur if a wave’s oscillations are perpendicular to its direction of energy transfer, as they are in transverse waves
22
Q
What are examples of applications of polarisation?
A
- Polaroid sunglasses
- The alignment of aerials for transmission and reception
23
Q
How do polaroid sunglasses work?
A
They only allow oscillations in the plane of the filter, making it easier to see by reducing glare by blocking partially polarised light reflected from other surfaces
24
Q
How is polarisation used for transmitting and receiving TV and radio signals from aerials?
A
The signals are polarised by the orientation of the rods on the transmitting aerial, so the receiving aerial must be aligned in the same plane of polarisation to receive the signal at full strength
25
What is superposition?
Where the displacements of two waves are combined as they pass each other, with the resultant displacement being the sum of each wave’s displacement
26
What are the 2 types of interference that can occur during superposition?
Constructive interference and destructive interference
27
What is constructive interference?
Interference that occurs when two waves have displacement in the same direction
28
What is destructive interference?
Interference that occurs when one wave has positive displacement and the other has negative displacement
29
When does total destructive interference occur?
If two waves have equal but opposite displacements
30
How are stationary waves formed?
When two coherent waves of opposite directions with the same amplitude meet, they superpose, constructively interfering to form antinodes where the waves meet in phase, and destructively interfering to form nodes where the waves are completely of out phase
31
What are antinodes?
Regions of maximum amplitude
32
What are nodes?
Regions of no displacement
33
What is an example of a way stationary waves can be formed?
Using a string fixed at one end and fixed to a driving oscillator at the other end
34
How are stationary waves formed on a string which is fixed at one end and fixed to a driving oscillator at the other end?
A wave travelling down the string from the oscillator will be reflected at the fixed end and travel back along the string causing superposition, as the two waves are coherent and have the same amplitude, but opposite direction, making a stationary wave
35
What is the fundamental mode of vibration also known as?
The first harmonic
36
How is the first harmonic formed?
A stationary wave at the lowest possible frequency
37
What does the first harmonic consist of?
Two nodes and an anitnode
38
How is wavelength shown on a stationary wave?
The distance between adjacent nodes/antinodes is half a wavelength
39
What does μ represent in the first harmonic equation?
The mass per unit length
40
What is a harmonic?
The multiple of the fundamental modes of vibration
41
What are 2 examples of stationary waves?
- Stationary microwaves, formed by reflecting a microwave beam at a metal plate
- Stationary sound waves, formed by placing a speaker at one end of a closed glass tube and laying powder across bottom of it, which will be shaken at antinodes and settle at nodes
42
How can nodes and antinodes be found in stationary microwaves?
Using a microwave probe
43
How can the speed of sound in air be found using stationary sound waves?
- Place speaker at open end of closed glass tube
- Lay powder across bottom of tube
- Powder shakes at antinodes and settles at nodes
- Wavelength found by doubling distance between 2 adjacent nodes/antinodes
44
What is path difference?
The difference in distance travelled by two waves
45
What is coherence?
Fixed phase difference
46
What is an example of a coherent and monochromatic light source?
Laser
47
What does Young’s double slit experiment demonstrate?
Interference of light from two sources
48
Describe Young’s double slit experiment.
- Shine a coherent light source through 2 slits about the same size as the wavelength of the laser light so the light diffracts
- Each slit acts as a coherent point source making an interference pattern of bright and dark fringes
49
How are bright fringes formed?
Where the waves meet in phase and constructively interfere (path difference is whole number of wavelengths)
50
How are dark fringes formed?
Where the waves meet completely out of phase and destructively interfere (path difference is a whole number and a half wavelengths)
51
What is the formula for the phase difference for constructive interference?
(2n)π
52
What is the formula for the phase difference for destructive interference?
(2n+1)π
53
What is the formula for the path difference for constructive interference?
(n)λ
54
What is the formula for the path difference for destructive interference?
(n+1/2) λ
55
What does using a white light source with double slits show?
- Wider central white maxima
- Less intense diffraction pattern
- Alternating bright fringes which are spectra, with violet being closest to the central maximum and red furthest
56
What are safety precautions that must be followed when using lasers?
- Wear laser safety goggles
- Don’t shine the laser at reflective surfaces
- Display a warning sign
- Never shine the laser at a person
57
How can you demonstrate interference with sound waves?
Use two speakers connected to the same signal generator, and measure the intensity of the waves using a microphone to find the maxima and minima
58
How did Young’s double slit experiment provide evidence for the wave nature of light (and disprove previous theories)?
- Diffraction and interference are wave properties, so proved that EM radiation must act as a wave
- There were theories that suggested light was formed of tiny particles
- Knowledge and understanding of any scientific concept changes over time in accordance to the experimental evidence gathered by the scientific community
59
What is diffraction?
The spreading out of waves when they pass through or around a gap
60
At what size of the gap does the greatest diffraction occur?
The same size as the wavelength
61
What does a gap smaller than the wavelength do to the diffraction?
Less noticeable diffraction as most waves are reflected
62
What happens when a wave meets an obstacle?
There is diffraction around the edges, the wider the obstacle (compared to the wavelength) the less diffraction
63
Describe the appearance of the diffraction pattern from a single slit using monochromatic light.
- Interference pattern of bright and dark fringes
- Central bright fringe double the width of all other fringes
- Alternating dark and bright fringes on either side of centre
64
Describe the appearance of the diffraction pattern from a single slit using white light.
A central white maximum with alternating bright fringes which are spectra, with violet being the closest to the central maximum and red furthest away
65
Why do you get a spectrum of colour in the diffraction pattern of white light through a single slit?
The different wavelengths of light are all diffracted by different amounts
66
What is the effect of increasing the slit width?
The amount of diffraction decreases, so the central maximum narrows and its intensity increases
67
What is the effect of increasing the light wavelength through a single slit?
The amount of diffraction increases as the slit is closer in size to the light’s wavelength, so the central maximum widens and decreases in intensity
68
What is a diffraction grating?
A slide containing many equally spaced slits very close together
69
What is the difference between the interference pattern when using a diffraction grating instead of a double slit with monochromatic light and why?
Much sharper and brighter because there are many more rays of light reinforcing the pattern
70
Why is it more accurate to measure slit widths using monochromatic light through a diffraction grating instead of a double slit?
There are many more rays of light reinforcing the pattern, making the fringes brighter and sharper, meaning measuring fringe widths is much more accurate as they are easier to take
71
What is the zero order line?
The ray of light passing through the centre of a diffraction grating
72
What are the lines either side the zero order line?
The first order lines (follows this pattern next to these)
73
How do you derive the diffraction grating formula?
1. Consider the first order maximum, where the path difference between two adjacent rays of light is one wavelength, the angle between the normal to the grating and the ray of light is θ
2. A right angle triangle is formed, with side lengths d and λ, the upper angle in the triangle is θ
3. For the first maximum sinθ = λ/d, (as sinθ = O/H) which rearranges to dsinθ = λ, (for the first order)
4. Replace λ with nλ to get: dsinθ = nλ
74
What are examples of applications of diffraction gratings?
- Line absorption spectra from splitting up light from stars: can be used to show elements present in star
- X-ray crystallography: x-rays directed at a thin crystal sheet acting as a diffraction grating to form a diffraction pattern as the wavelength of the x-rays is similar in size to the gaps between the atoms - can be used to measure atomic spacing in certain materials
75
What is the refractive index of a material?
The property measuring how much it slows down light passing through it
76
What do ‘c’ and ‘cs’ represent in the refractive index equation?
c = speed of light in a vacuum = 3x10^8 m/s
cs = speed of light in the medium
77
What is the refractive index of air (approximately)?
1
78
What is an optically dense material?
A material with a high refractive index
79
What is refraction?
When a wave enters a different medium it changes direction, either towards or away from the normal depending on the material’s refractive index
80
What is Snell’s law of refraction for a boundary?
𝑛1sin𝜃1 = 𝑛2sin𝜃2
81
In what direction does a light ray bend when travelling from a less optically dense material to a more optically dense material?
Bends towards the normal as it slows down
82
In what direction does a light ray bend when travelling from a more optically dense material to a less optically dense material?
Bends away from the normal as it speeds up
83
What is the equation for total internal reflection (critical angle)?
sin 𝜃𝑐 = 𝑛2/𝑛1
84
What is the critical angle?
The angle of incidence when angle of refraction is 90 degrees
85
How does total internal reflection occur?
When the angle of incidence is greater than the critical angle and the incident refractive index is greater than the refractive index of the material at the boundary
86
What is an application of total internal reflection?
Fibre optics
87
What is the purpose of fibre optics?
Uses total internal reflection to send high speed light signals over large distances
88
What are fibre optics used in?
- Communication of Internet transmission
- Medical imaging (e.g. endoscopes)
89
What are the three main components that make up fibre optics?
1. An optically dense core (e.g. plastic/glass)
2. A lower optical density cladding surrounding the core
3. An outer sheath
90
What is the function of the cladding in fibre optic cable?
- Protects core from damage
- Prevents signal degradation by increasing the critical angle within the core fibre to prevent adjacent fibres from touching each other and light escaping
- However, light with low angles of incidence can escape the fibre
91
Why does the cladding refractive index need to be lower than the core’s refractive index in fibre optic cable?
So that total internal reflection can occur
92
What is modal dispersion?
Waves enter an optical fibre at slightly different angles, meaning the distance each beam has to travel is slightly different, leading to the beams reaching the end at different times causing pulse broadening, meaning that signal is lost to the cladding from the core
93
Why can signal be lost to the cladding from the core in fibre optic cable?
If waves in an optical signal enter an optical fibre at slightly different angles, the distance each beam has to travel is slightly different, leading to the beams reaching the end at different times causing pulse broadening
94
What is the principle of pulse absorption?
The more transparent the material that makes up the core, the less light is absorbed, so the farther a pulse with a given energy can travel
95
What is the consequence of pulse absorption?
As light is absorbed, less of the pulse remains, to a point where it decays completely or can’t be received
96
How is pulse absorption minimised in fibre optic cable?
By the use of a very pure transparent material as the core
97
What is the principle of material dispersion?
- Waves of different wavelengths travel at slightly different speeds through an optical fibre so reach the end of the fibre at slightly different times, causing pulse broadening
- Using monochromatic light fixes this
98
Why is monochromatic light used in fibre optic cable?
So all the pulse travels at the same speed, so there is no pulse broadening into a spectrum (like for white light)
99
What are electromagnetic waves formed of?
Alternating sinusoidal transverse magnetic and electric fields travelling in phase and perpendicularly
100
What field of an EM wave can be polarised?
The electric field
101
How do you increase the fringe spacing in a double slit experiment?
Decrease the separation of the double slits
102
What are the effects of using light with a range of wavelengths in a single slit experiment?
- Central maximum unchanged in width
- Broader maxima/ range of angles for each maximum/order
- Gradually getting broader/ more spread out for greater order maxima
