Waves (W) Flashcards
1
Q
progressive waves:
A
a wave that transfers energy from one point to another without transferring the medium itself
2
Q
displacement:
A
vector quantity, distance a wave is from its equilibrium point
3
Q
amplitude:
A
maximum vertical displacement of wave from equilibrium position
4
Q
wavelength:
A
distance between successive oscillations of a wave
5
Q
period:
A
time for one complete oscillation
6
Q
frequency:
A
number of oscillations per unit time
7
Q
equation relating frequency and time period:
A
F=1/T
8
Q
Phase difference:
A
how much a point on a wave is in front/behind another
9
Q
When are waves in phase?
A
when crests/troughs are aligned
10
Q
When are waves in anti phase?
A
when the crest of one aligns with the trough of another
11
Q
How can you express phase difference in degrees?
A
The fraction of the wavelength x 360
12
Q
How can you express phase difference in radians?
A
Fraction of wavelength x ∏/2
13
Q
How many degrees is in phase?
A
360
14
Q
How many radians is in phase?
A
2∏
15
Q
How many degrees is anti phase?
A
180
16
Q
How many radians is anti phase?
A
∏
17
Q
Transverse wave:
A
a wave in which the particles oscillate perpendicular to the direction of travel and energy transfer
18
Q
Longitudinal wave:
A
When particles oscillate parallel to direction of wave travel and energy transfer.
19
Q
example of transverse wave:
A
radio wave, visible light, UV
20
Q
example of longitudinal wave:
A
ultrasound, sound
21
Q
What are the areas of increases pressure in longitudinal waves called?
A
compressions
22
Q
What are the areas of decreased pressure called in longitudinal waves?
A
rarefactions
23
Q
Which type of wave cannot be polarised?
A
Longitudinal
24
Q
Polarisation:
A
When particle oscillations only occur in one of the directions perpendicular to the direction of wave propagation.
25
Through what can polarising occur?
polarising filter or polariser.
26
how are stationary waves produced?
By the superposition of two waves of the same frequency and amplitude travelling in opposite directions
27
How are stationary waves USUALLY produced?
By a travelling wave and its reflection.
28
Node:
regions of no vibrations
29
antinodes:
regions where vibrations are at their maximum amplitude
30
Points between nodes are......
in phase
31
points that have an even number of nodes between them are.....
in phase
32
points that have an odd number of nodes between them are .......
out of phase
33
What is the principle of superposition?
When two or more waves with the same frequency arrive at a point, the resultant displacement is the sum of displacements of each wave.
34
What causes constructive interference?
In phase points (peaks line up/troughs line up = 2x amplitude)
35
What causes destructive interference?
Antiphase points (peaks and troughs line up = no amplitude)
36
Stationary waves can have different patterns, know as.....
harmonics
37
What do harmonics depend on?
The frequency of vibrations and situation in which they are created.
38
Where can harmonics be observed?
On strings with fixed ends
39
Wavelength of first harmonic in terms of L:
2L
40
Wavelength of second harmonic in terms of L:
L
41
Wavelength of third harmonic in terms of L:
2/3 L
42
What is the equation for the speed a wave whilst travelling along a string with two fixed ends?
v = root of: tension in string (N) / mu, mass per unit length of string (Kgm-1)
43
When are waves coherent?
If they have the same frequency and constant phase difference.
44
Path difference:
Difference in distance travelled by waves from their sources to a point where they meet.
45
If the path difference is nλ...
constructive interference
46
If the path difference in (n+1/2)λ
Destructive interference
47
What two conditions must there be in order for two source interference to be observed?
Coherent waves, monochromatic
48
In Young's double slit experiment, where are dark fringes observed?
Where path difference is (n+1/2)λ, so destructive interference occurs
49
What does Young's double slit experiment demonstrate?
How light waves can produce an interference pattern
50
In Young's double slit experiment, where are light fringes produced?
Where path difference in nλ so constructive interference occurs
51
What is the first maxima labelled?
n=0
52
Why is it that in young's double slit experiment both light sources originate from the same primary source?
So that the light is coherent
53
What is the fringe spacing equation for Young's double slit experiment?
fringe width = (wavelength of source x distance from double slit to screen) / distance between centres of slits
54
What is diffraction?
The spreading out of waves when they pass an obstruction.
55
What does extent of diffraction depend on?
The width of the gap compared to the wavelength
56
What changes about a wave when it is diffracted?
The amplitude as some of the energy is dissipated
57
(diffraction) If gap size >> wavelength...
Then the effect becomes less pronounced
58
(diffraction) if slit was made narrower...
intensity of diffraction would increase and fringe spacing would be wider
59
If the gap size remained the same, which would diffract more and why: red light or blue light?
Red light as it has a larger wavelength making the width of the gap compared to the wavelength relatively smaller so the fringes become more spaced out.
60
What is a diffraction grating?
A plate on which there are a very large number or parallel identical close spaced slits.
61
What happens when light is put through a diffraction grating?
A pattern of narrow bright fringes are created.
62
What is the diffraction grating equation?
spacing between adjacent slits x sin x θ = order of maxima x wavelength
63
How can lines per mm be converted into d (as in dsinθ=nλ)
d = 1/ lines per metre
64
What is the angular separation?
The angle from the central maxima (n=0)
65
In diffraction, at what angle has the maximum angle to see orders of maxima been reached?
when theta = 90 or sin theta = 1
66
When does refraction occur?
When light passes a boundary between two different transparent media.
67
Why does refraction occur?
Light will bend towards the normal if it enters a more optically dense material as the wave will be slowed down (and vice versa)
68
How can you calculate refractive index?
n = speed of light in vacuum / speed of light in a substance
69
What is refractive index?
Property of light which measures how much light slows down
70
If an object is optically very dense it will have a ......
high refractive index
71
Refractive index will always be...
n < 1
72
What is refractive index of air said to approximately be?
1
73
What is snell's law?
n1sinθ1=n2sinθ2 (where 1 is first material and 2 is material it refracts in and theta is angle form normal)
74
What happens when angle of incidence is critical angle?
The angle of refraction will equal 90 and light will refract across the boundary
75
When does total internal reflection occur?
When the angle of incidence is greater than the critical angle
76
What does the angle of refraction equal in total internal reflection?
the angle of incidence
77
What do fibre optics utilise?
Total internal reflection
78
What do fibre optics do?
Send high speed light signals over large distances
79
Why can total internal reflection occur in fibre optics?
Because n cladding < n core
80
What are the three main components of fibre optics?
optically dense core, lower optical density cladding, an outer sheath
81
What is the purpose of the outer sheath?
Prevents physical damage and strengthens fibre
82
What is the purpose of the cladding in fibre optics?
Protects core from damage, keeps signal secure
83
What is pulse broadening?
When light pulses in optical fibres spread out due to different angles of incidence.
84
What are three advantages of a narrow core of a fibre optic?
less light lost by refraction out of core, less overlapping pulses, angle of incidence less likely to drop below critical angle as there is smaller change in angle between each reflection
85
What can be done to reduce absorption in optic fibres?
Using a transparent core and using optical fibre repeaters so that the pulse is regenerated before significant absorption occurs.
86
how can you calculate the critical angle?
n2/n1 where n1 is the more optically dense material
87
When does material dispersion occur?
The refractive index of the fibre varies with frequency. Different wavelengths of light in the signal travel at different speeds. This causes a sharp pulse to spread into a broader signal. So the duration of each pulse increases.
88
What is material dispersion? short
the spreading of a signal caused by the variation of refractive index with wavelength
89
What is modal dispersion?
the spreading fo a signal caused by rays taking a slightly different path in the fibre
90
What is absorption in terms of fibre optics?
when energy from a signal is absorbed by the optical fibre in which it travels
91
When does modal dispersion occur?
When rays inside and optical fibre take slightly different paths so rays taking longer paths take longer to travel through the fibre broadening the pulse
92
What is a photon?
A discrete packet of energy
93
how can you calculate energy of a photon?
E=hf
94
How can you calculate energy of a photon not using frequency?
E=hc/λ
95
What is light intensity determined by?
the number of photons
96
What is the frequency of light determined by?
the energy of photons
97
How many joules is an electron volt?
1.6 x 10 -19
98
How can you calculate power of a light source?
number of photons x hf
99
what is the photoelectric effect?
Where photoelectrons are emitted from the surface of a metal after light above a certain frequency is shone on it. this certain frequency is the threshold frequency and it differs for each metal.
100
Why couldn't the photoelectric effect be explained by wave theory?
Because it said that any frequency of light should be able to cause the emission of photoelectrons as the energy absorbed by each electron will gradually increase with each incoming wave.
101
What is the photon model of light?
1. Em waves travel in discrete packets of energy called photons which each have an energy directly proportional to the frequency
2. Each electron can absorb a single photon, therefore a photoelectron is only emitted if frequency is above threshold frequency
3. If the intensity of light is increased, if the frequency is above threshold, more photoelectrons are emitted per second
102
What is the work function of a metal?
Φ, is the minimum energy required for electrons to be emitted from the surface of the metal
103
What is the stopping potential?
The potential difference you would need to apply across the metal to stop photoelectrons with the maximum kinetic energy.
104
What is the equation for stopping potential?
Ekmax = charge of electron x stopping potetnial
105
What is the photoelectric equation?
KE(max) = hf - threshold frequency
106
Electrons in atoms can only exist in....
discrete energy levels
107
What happens when electrons gain energy?
If an electron gains energy form say colliding with another free electron, it will move up an energy level. This is known as excitation.
108
What can happen if an electron gains enough energy?
It can be removed from the atom entirely, which is called ionisation. This occurs if the energy of the free electron is greater than the ionisation energy.
109
What happens immediately if an electron becomes excited?
It will quickly return to its original energy level (ground state) and therefore release the energy it gained in the form of a photon.
110
What is an example of excitation?
Fluorescent tubes
111
How do fluorescent tubes work?
The voltage across it accelerates free electrons through the tube which collide with the mercury vapour atoms, causing them to become ionised, releasing more free electrons. The free electrons then collide further with the mercury atoms causing them to become excited. When they de-excite they will release photons, most of which are in the UV range. The fluorescent coating in the tube will absorb these UV photons and electrons in the atoms of the coating become excited and de-excite releasing photons in the form of visible light.
112
How do you convert from eV to joules?
x 1.6 x 10-19
113
How do you convert from joules to eV?
divide by 1.6 x 10-19
114
How can a fluorescent tube produce a line spectrum?
By looking at it through a diffraction grating or prism.
115
What does each line in a light spectrum represent?
Each line represents a different wavelength of light emitted by the tube.
116
Is a photon emission spectrum discrete or continuous?
Discrete. The only photon energies emitted will correspond to these wavelengths, therefore this is evidence to show that electrons in atoms can only transition between discrete energy levels.
117
What is wave particle duality?
Light can be shown as having both wave and particle properties.
118
What are examples of light acting as a wave?
diffraction and interference
119
What is an example of light acting as a particle?
photoelectric effect
120
What is an example of wave particle duality?
The wave nature of electrons can be observed through electron diffraction, as only waves can experience diffraction.
121
What is the de Broglie wavelength equation?
λ = h/mv
122
What does the de Broglie wavelength allow you to do?
how the amount of diffraction changes as a particle's momentum changes
123
What happens when the momentum of a particle is increased?
When momentum increases, the wavelength decreases and therefore the amount of diffraction decreases, so the concentric rings of interference come closer.
124
What happens when the momentum of a particle is decreased?
The wavelength will increase and the diffraction will increase so the concentric rings of interference will move further apart.
