Chapter 5 Waves Flashcards
1
Q
Define amplitude
A
A wave’s maximum displacement from the equilibrium position
2
Q
Define frequency
A
The number of complete oscillations passing through a point per second
3
Q
Define period
A
The time taken for one full oscillation
4
Q
Define wave speed
A
The distance travelled by the wave per unit time
5
Q
Define wavelength
A
The length of one whole oscillation - the distance between a point on the wave and the same point on the following one
6
Q
What is the wave equation?
A
- v = fλ
- wave speed = frequency x wavelength
7
Q
Describe a longitudinal wave and what they are made up of.
A
The oscillation of particles is parallel to the direction of energy transfer. They are made up of compressions (high pressure regions) and rarefactions (low pressure regions). They cant travel in a vacuum
8
Q
Which direction are particles moving in a longitudinal wave?
A
Particles are always trying to move to the nearest compression and so move away from rarefactions
9
Q
Describe a transverse wave.
A
- The oscillation of particles is perpendicular to the direction of energy transfer.
- All EM waves are transverse and travel at 3x10^8 ms-1 in a vacuum
10
Q
What can displacement-distance graphs of waves tell you?
A
You can calculate the wavelength
11
Q
What can you calculate from a displacement-time graph of a wave?
A
The time period
12
Q
Define a wave’s phase.
A
- The phase of a point on a wave relative to the origin can be defined as the fraction of the wave cycle that has elapsed since the origin
- The position of a certain point on a wave cycle. Can be measured in degrees, radians or fraction of a cycle
13
Q
Define phase difference.
A
How much a particle / wave lags behind another. Same units as phase
14
Q
Define path difference
A
The difference in the distance travelled by two waves
15
Q
Define superposition
A
Where the displacements of two waves are combined as they pass each other. The resultant displacement is the vector sum of each wave’s displacement
16
Q
Define coherence
A
Coherent waves must have the same frequency and wavelength and a fixed phase difference
17
Q
Define wavefront
A
A surface / line connecting points on a wave that have the same phase
18
Q
What are the two types of interference?
A
- Constructive interference: occurs when the waves are in phase and so their displacements are added
- Destructive interference: occurs when the waves are completely out of phase and so their displacements are subtracted
19
Q
When are two waves in phase?
A
Two waves are in phase if they are both at the same point of the wave cycle: the same frequency and wavelength.Their phase difference is a multiple of 360 degrees / 2pi
20
Q
What is a progressive wave?
A
One that transfers energy from one point to another without any transfer of matter
21
Q
What is a standing wave?
A
A wave that stores energy rather than trasnferring it from one place to another
22
Q
What is path difference a measure of?
A
Path difference is a measure of how far ahead one wave is compared to another
23
Q
What is a wave’s phase?
A
A wave’s phase at a given point is a measure of how far through its cycle the wave is. It is usually measured in radians, where a complete cycle is 2pi
24
Q
What happens when two waves meet in phase?
A
They will undergo constructive interference
25
What happens when two waves meet in antiphase?
They will undergo destructive interference
26
How is a standing wave formed on a string?
A wave reflects from a closed end meaning two identical waves are travelling in opposite directions down the same string
- At points where the waves meet in phase, constructive interference occurs and an antinode is formed
- At points where the waves meet in antiphase, destructive interference occurs and a node is formed
27
What is a node?
A point of minimum displacement - there is no movement from the equilibrium position
28
What is an antinode?
A point of maximum displacement
29
What two factors does the speed of a transverse waves on a string depend on?
- Tension
- Mass per unit length of the string
30
State the equation used to calculate the speed of a transverse wave on a string
v = sqrt(T/μ)
31
State the equation used to calculate intensity
I = P / A
Intensity = Power / Area
32
What is the refractive index of a material through which light travels a speed of 'v' ?
n = c/v = sin(i) / sin(r)
where c is the speed of light in a vacuum
i is the angle of incidence, r is the angle of refraction
33
State the equation linking the refractive indexes and angles at an interface between two mediums
n1 sin θ1= n2sinθ2
34
What is the critical angle?
The angle of incidence for which the angle of refraction is 90 degrees and all the light passes along the boundary between the mediums. Beyond this angle all light will be reflected
35
State the equation used to calculate a critical angle
sin C = 1/n
(if in air)
36
What is total internal reflection?
Total internal reflection is where all the light is reflectde back at the boundary between two mediums. It occurs when light is incident at an angle greater than the critical angle
37
What is the focal point of a converging lens?
The single point where all the parallel rays of light entering the lens converge to
38
What is the focal length of a lens?
The distance between the lens and the focal point
39
What is the equation used to calculate the power of a lens?
Power = 1 / focal length
40
How do you calculate the total power of a combination of thin lenses?
P = P1+P2+P3+...
41
What is a real image?
One that can be projected onto a screen and is always inverted. Real images are the consequence of light meeting at a focal point
42
What is a virtual image?
virtual images are the consequences of rays of light appearing to meet at a point. They cannot be projected onto a screen
43
State the equation used to calculate the magnification of an image?
Magnification = Image Height / Object Height
44
What is plane polarisation?
When the oscillations of a wave are restricted to a single plane
45
What is diffraction?
The spreading out of a wave as it passes through a gap
46
What criteria must be met for maximum diffraction to occur?
The size of the gap (the aperture) must be the same magnitude as the wavelength of the wave
47
What happens if the gap is much smaller than the wavelength of the wave?
The wave willl be reflected
48
State the diffraction grating equation.
nλ =d sinθ
49
What does electron diffraction provide evidence for?
The wave nature of electrons. It suggests that particles can demonstrate wavelike properties
50
Describe the diffraction pattern produced by electrons.
Concentric circles of bright and dark fringes from a central bright point
51
If electrons didnt have a wave nature, describe the pattern that would be produced when they pass through a slit.
The electrons would be unaffected by the gap and pass straight through. A single bright region would be formed
52
What is the name given to the wavelength of a particle?
De Broglie wavelength
53
What two factors does the de Broglie wavelength depend on?
Mass
Velocity
54
State the equation used to calculate the de Broglie wavelength.
λ = h/mv
lambda = plancks constant / mass x velocity
55
What is the basic process of a pulse-echo technique?
A wave pulse is emitted. It is transmitted and reflected at the boundary between two media. The returning wave (echo) is detected. The speed and time taken are used to calculate the distance to the object.
56
Suggest two things that may limit the amount of information that can be obtained by a pulse echo technique
The wavelength of the radiation
The duration of the pulse
57
What are the two models that can be used to describe electromagnetic radiation?
The wave model
the particle model
58
What model does the photoelectric effect provide evidence for?
The particle model
59
Outline the photoelectric effect.
Light is shone on a metal plate. If the light has a high enough frequency, electrons are emitted from the metal surface. If the frequency is too low, no electrons are emitted
60
What are the particles of light used to explain the photoelectric effect called?
Photons
61
How do you calculate the energy of a photon?
E = hf
Energy = plancks constant x frequency
62
Explain how a photon can liberate an electron.
One photon interacts with one electron and transfers all of its energy to it. If this energy is greater than the metal's work function, the electron will have sufficient energy to be released
63
What is threshold frequency?
A metal's threshold frequency is the minimum frequency that a photon requires to liberate an electron from its surface
64
If the intensity of light being shone on a metal increases, how does the enrgy of the photoelectrons change?
The energy remains unaffected. An increase in intensity means more photons per area and so more photoelectrons are emitted
65
Why are photoelectrons emitted with a range of kinetic energies?
The electrons are at different depths in the metal and so require different amounts of energy to be liberated. The excess energy from a photon once an electron has been liberated, is the kinetic energy of the photon
66
State the equation for the maximum kinetic energy of a photon.
hf = ϕ + 1/2mv^2
ϕ is the metal's work function
67
What is the conversion factor between eV and J?
1eV = 1.6x10^-19 J
68
What happens when electrons transition between energy levels?
If electrons move to a higher energy level, radiation must be absorbed
If electrons move to a lower energy level, radiation is emitted
69
Why can only certain frequencies of radiation be absorbed by an atom to cause an electron transition?
the electrons only exist in discrete energy levels. The energy of the photon absorbed must be the exact amount of energy required to cover the difference between two discrete energy levels
70
What are EM waves emitted and absorbed by and what is the exception?
Electrons, except gamma which is emitted by nuclei
71
What is the range of wavelengths of visible light?
400-750nm
Blue light has the lowest wavelength at 400nm and red light has the largest
72
For a sound wave, what is the impact of a higher frequency?
It leads to a higher pitch
73
For a sound wave, what is the impact of a greater amplitude?
The sound will be louder
74
What is the relationship between amplitude and intensity?
Intensity is proportional to Amplitude^2
(if amplitude doubles, intensity quadruples)
75
Define refraction
When waves change direction and speed as they enter a new medium
76
How can you remember which way a ray refracts when going into a new medium?
FAST
Faster Away Slower Towards (the normal)
77
Why does a prism disperse light?
in reality, different wavelengths of light have slightly different refractive indexes eg blue light has the highest n, so it refracts more than red
78
What conditions must be necessary for TIR?
- The light must be IN the medium with the HIGHER refractive index(n)
- The angle of incidence (i) must be greater than the CRITICAL ANGLE
79
What is an application of TIR?
Optic fibres
80
What is multiplexing?
Sending multiple signals using different wavelengths down same fibre
81
What are the two problems with optic fibres?
Material Dispersion - blue light travels slower than red light
Multipath Dispersion - light of the same wavelength taking slightly different routes and therefore time to reach the end, results in pulse broadening
82
How do fibre optics mitigate their problems?
- Thinner core
- Repeaters
- Graded fibre
83
How does graded fibre work?
n decreases closer to the edge, so rays taking the longer route travel faster and curve back, meaning they arrive at the same time
84
What is the method for drawing a ray diagram?
Draw a line straight through the centre of the lens
Draw a line parallel to the principle axis, until the lens and then through the principle focus
Draw the image where they meet
85
What are the two ways of calculating magnification?
M = i / o = v / u
= image height / object height
= image distance / object distance
86
What is the lens equation?
1/f = 1/u + 1/v
87
What is the image produced by a *convex* lens at a distance from the lens that is greater than 2f?
- real image
- inverted
- diminished
88
What is the image produced by a *convex* lens at a distance from the lens that is between f and 3f?
- real image
- inverted
- magnified
89
Describe the image produced by a convex lens closer than f to the lens?
virtual image
upright
magnified
90
Describe the image produced by a concave lens at any distance
virtual image
upright
diminished
91
Which type of waves cannot be polarised?
Longitudinal
92
How do you calculate phase difference?
Δd/λ x360 or 2 pi
Δt /T x 360 or 2pi
93
What can points on a stationary wave only be?
In phase or in antiphase
94
What is the length of the string when there is a first harmonic on a stationary wave?
L = 0.5λ
95
Describe how stationary waves can be produced in tubes?
A node must be made at a closed end of a tube and an antinode at an open end
96
What are the conditions for coherence?
Same frequency, same wavelength and kept at a constant phase difference
97
What is Young's double slit equation?
w = λD /s
where w is fringe spacing, D is slit-screen distance, s is slit separation
98
Why is Young's double slit equation limited?
It only applies if D >> s
99
What happens when white light passes through the double slit experiment?
It results in coloured fringers as red light diffracts more than blue light because it has a larger wavelength
100
How is the number of orders related to the wavelength from the diffraction grating equation?
They are inversley proportional
101
How does a flourescent tube work?
A free electron from a cathode excites a low pressure mercury gas aotm.
A UV photon is produced, this is absorbed by the coating which excites an electron in the coating
The electron de-excites and a visible light photon is emitted
102
What happens if you put a sheet of metal with light incident on it, in a circuit?
A small current will flow
103
What is 'stopping potential'
the pd required in the opposite direction to the sheet of metal, stopping the current
104
What piece of equipment is needed for both the photoelectric effect and electron diffraction?
An evacuated tube
105
How is momentum related to kinetic energy?
Ek = p^2 /2m
