6-Wave Behaviour Flashcards
1
Q
What is the principle of superposition?
A
When 2 or more waves overlap, the resultant displacement at a given instant and position is equal to the sum of the individual displacements at that position.
2
Q
What does the term ‘superpose’ mean?
A
Waves overlapping
3
Q
What is the wavelength?
A
The distance between any 2 points on the same part of a wave cycle
4
Q
What is the amplitude?
A
The maximum displacement from the equilibrium position
5
Q
What can you use to measure the time period of a wave?
A
An oscilloscope
6
Q
What equation relates time period and frequency?
A
Frequency = 1 / Time period
7
Q
What does phase describe?
A
The stage in the wave cycle
8
Q
What does the term ‘in phase’ mean?
A
When two points are at the same stage in the cycle
9
Q
If point P is at the amplitude of a wave, and point Q is at the same amplitude of the wave, 2 wavelengths along, are the points in phase?
A
Yes
10
Q
When 2 waves are ‘in phase’ what does this mean?
A
They are doing the same thing in the exact same moment
11
Q
When 2 waves are in phase, is there any phase difference?
A
No, there is 0 phase difference
12
Q
What does it mean when 2 waves are in antiphase?
A
They are doing the exact opposite things in the exact same moment
13
Q
When 2 waves are neither in phase, nor in antiphase, what are they said to be?
A
Out of phase
14
Q
How can phase and phase difference be measured?
A
By a phase angle
15
Q
What is a phasor?
A
A rotating arrow which shows where the wave is in its cycle
16
Q
How many radians does a phase arrow turn as the wave goes through 1 cycle?
A
2 pi radians
17
Q
What does the vertical displacement of the clock arrow represent?
A
The displacement of the wave in that given instant and time
18
Q
If 2 waves are in phase, what is the difference in phase angle?
A
0
19
Q
If 2 waves are in antiphase, what is the phase angle difference?
A
pi radians
20
Q
How can we calculate the displacement when 2 waves superpose?
A
We add the individual displacements together
21
Q
When we add phasor arrows together, tip-to-tail, what can we find?
A
The amplitude of the resultant phasor
22
Q
What is a ripple moving across the surface of water?
A
An example of a progressive wave-as you can see the crest of the wave moving
23
Q
When 2 progressive waves move in opposite directions(e.g. along a string) what appears to happen?
A
The waves appear to stop moving
24
Q
What is a standing wave?
A
When 2 progressive waves travel in opposite directions and it appears that the wave has stopped moving
25
When a string is plucked, what do waves do?
They move along the string in opposite direction, reflect at the end of the string, and superpose as they pass through one another
26
What is a node in a wave cycle?
A point where the waves travelling in a string meet in antiphase and there is 0 amplitude
27
What is the antinode in a wave cycle?
A point where the waves meet in phase causing a maximum displacement
28
What is the fundamental frequency?
The lowest frequency of vibration
29
What is the longest standing wave?
Twice the length of the string
30
What does wave velocity equal?
Wavelength X Frequency
31
Describe how standing waves can be formed in air:
Sound wave travels along a tube, where it reflects and waves travelling up and down the tube superpose with each other
32
Can sound waves be reflected from both the closed end and open end of a tube?
Yes
33
When is a node formed?
When waves meet in antiphase
34
With sound waves travelling along a tube, what is always formed at the open end of the tube?
An antinode
35
What is refraction?
When waves change speed when they change medium
36
What is a medium?
The material the wave is travelling through
37
When waves refract, what causes them to change directiomn?
The change in speed causes the light rays to bend and change direction
38
What is a vacuum?
A region of space which contains no matter
39
What is the speed of light in a vacuum?
3 X 10^8 ms-1
40
Why does light travel slightly slower in air than in a vacuum?
Because it interacts with the electrons in the atoms in the air
41
Why do electrons in atoms not cause light waves to slow down considerbly?
Because the air is not very dense, so interaction between light and the electrons are few and far between
42
What speed does light travel at through glass?
2 X 10^8 ms-1
43
Why does light travel much slower in glass than in air?
Because glass is much denser than air, so the number of interactions per metre between the light and the electrons is much higher
44
What is the refractive index?
The ratio of the speed of light in once medium compared to the speed of light in another medium
45
What is the first equation for the refractive index?
Refractive index = Speed of light in medium 1 / Speed of light in medium 2
46
If the first medium is a vacuum, what does the equation for the refractive index become?
Speed of light in vacuum / Speed of light in material
47
What is the normal?
An imaginary line at 90 degrees to the surface of the glass
48
When a ray of light travels from air into glass, what does the ray do?
It bends towards the normal
49
What is snell's law?
sin i / sin r = Cmedium1 / Cmedium2
50
For Snell's law, what is i and what is r?
i is the angle of incidence
51
When wave fronts enter glass from the air, what happens to the waves?
They slow down, as the wavelengths get shorter
52
What is the term interference used to describe?
The effect of the superposition of waves
53
What does the superposition of waves produce?
An interference pattern
54
When two waves meet in phase, will the sound be loud or quiet?
Loud
55
If the two waves from 2 different speakers are of the same amplitude when they meet in antiphase, what will ahppen?
They will cancel completely, producing silence
56
What is path difference?
The difference in distance travelled by two waves from their source to the pattern
57
When waves meet with zero path difference, are they in phase?
Yes
58
For what value of the path difference do waves meet in phase?
When the path difference = n X wavelength
59
For what value of the path difference do waves meet in antiphase?
(n+0.5) X wavelength
60
Two loudspeakers emit a wavelength of 10cm, suggest 2 values of path difference which will produce superposition maxima:
10cm, 20cm
61
Two loudspeakers emit a wavelength of 10cm, suggest 2 values of path difference which will produce superposition minima:
15cm, 25cm
62
What is a stable superposition pattern?
One which the position of the maxima and minima don't change over time
63
When will stable superposition patterns only occur?
When there is a constant phase difference
64
What are coherent waves?
Waves with a constant phase difference
65
What are incoherent waves?
Waves that don't have a constant phase difference
66
When does constructive interference occur?
When waves meet in phase, leading to large superposition amplitude
67
When does destructive interference occur?
When waves meet in antiphase, leading to lower superposition amplitude
68
When waves pass through a gap of roughly the same width as their wavelength, what do they do?
They spread out
69
What is diffraction?
Diffraction is when waves spread out after being passed through a narrow slit
70
What does the amount that wave spread out depend upon?
The width of the gap compared to the wavelength of the waves passing through
71
For a given wavelength passing through a gap, what happens as the gap gets narrower?
The greater the spreading caused by refraction
72
Does diffraction alter the wavelength, speed or frequency of waves?
No
73
Why do we not experience the diffraction of light in everyday life?
Because the wavelength is in the order of 10^-7, so is not diffracted often
74
What did Young's double Slit experiment support the theory of?
That light travels as a wave
75
In Young's double slit experiment, what is 'd'?
The distance between the slits
76
For waves with a path difference of 1 wavelength, what does wavelength equal?
Wavelength = distance between slits X sin thet
77
For path difference of n X wavelength, what equation can we derive?
n wavelength = d sin theta
78
What does the order of maximum show?
The number of wavelengths path difference from two adjacent slits
79
For small angles, what does tan theta equal?
sin theta
80
What does path difference equal?
d X sin theta
81
As sin theta = x / L, what does path difference equal?
dx / L
82
As tan theta = x / L , and sin theta = lander/d, what equation can be derived from this?
wavelength = xd / L
83
What is x?
The spacing between the bright fringes on the screen
84
What is L?
How far away the screen is from the slits
85
What is d?
The distance between the slits
86
What is a diffraction grating?
A multiple slit version of the two slit system
87
Why does using a grating of many slits increase the brightness of the image on the screen?
Because more light gets through
88
Why are diffraction gratings able to spread white light into its component colours?
Because each wavelength of light will produce a maxima at a different angle
89
What does the line separation equal?
1 / number of lines per metre
90
Describe how in a slit experiment we get a maximum:
Where all the phasor arrows are in the same direction we get a large resultant, so an area of high intensity/ a maximum
91
Describe how in a slit experiment we get a minimum value:
The phasors add to zero, creating an area of zero intensity. The waves have (n+0.5)wavelength path difference, creating destructive interference
92
What is the first order of maximum?
The first bright image to either side
93
What is the second order of maximum?
The second bright image to either side
94
What does the term 'coherent' mean?
Two or more waves are said to be coherent if the phase difference between them is constant. Note that you can infer from this that if the waves are coherent, then they must have the same frequency and wavelength.
95
Why are a pattern of nodes / antinodes seen along string of musical instrument or column of air in tube or water on surface of pond etc?
Travelling wave and reflected travelling wave pass through each other in opposite directions, which is caused by constructive superposition at specific locations leads to antinodes, destructive superposition at other locations leads to nodes
96
Why are bright fringes observed at specific angles on a screen in a double slit experiment? (in terms of path length)
Path difference for wave trains passing through adjacent slits is an integer number of wavelengths for this angle(n lander), so the waves SUPERPOSE in phase / constructively to give bright spot / fringe
97
Why are bright fringes observed at specific angles on a screen in a double slit experiment? (in terms of phasors)
Phasors add to give large resultant amplitude to give bright spot / fringe
98
Why are dark fringes observed on screen in double slit experiment or diffraction grating experiment at a particular angle? (in terms of path length)
Path difference for wave trains passing through adjacent slits is an ODD number of HALF wavelengths for this angle, so waves SUPERPOSE in antiphase / destructively to give dark spot / fringe
99
Why are bright fringes observed at specific angles on a screen in a double slit experiment? (phase explanation)
Waves in phase as they pass through the slits; in phase when they reach the screen as the phase difference between them is integer multiple of 2π radians, so waves SUPERPOSE in phase / constructively to give bright spot / fringe
100
Why are dark fringes observed on screen in a double slit experiment or diffraction grating at a particular angle? (in terms of phase)
Waves in phase as they pass through the slits; out of phase when they reach the screen as the phase difference between them is an ODD multiple of π radians (π, 3 π, 5 π etc.), so the waves SUPERPOSE in antiphase / destructively to give dark spot / fringe
101
Why do waves spread out when passing through a single slit?
At one particular angle, path lengths for set of n wave trains across the slit differ by λ/n, Waves SUPERPOSE in antiphase / destructively: this defines the edge of the beam
102
Why is there a spectrum of colours produced by light falling on oil film / soap bubbles?
Path difference for waves reflected from front and back surface of film / bubble is an integer number of wavelengths FOR ONE PARTICULAR COLOUR / ANGLE, so waves superpose in phase constructively for one colour and angle of light hitting film / bubble, leading to this colour being seen brightly in reflection
103
Why does narrower slit spacing / grating line spacing produces wider spread of fringes / spots?
nλ = dsinθ, so if d is reduced then sinθ must increase as nλ is unchanged, so angles at which bright / dark spots / fringes seen is increased.
104
Why is there a multiple spectra of colours produced when white light shone through diffraction grating, but straight-through beam remains white?
nλ = dsinθ, so for straight-through beam n=0, so sinθ = 0 for all colours, so see white as all colours superimposed. For n=1 and higher orders, red light is diffracted more than violet light, as red has longer wavelength, so red beam seen at larger angle than violet for each order.
105
What is the limitation of the wave model of light?
Cannot explain photoelectric effect. Wave model predicts that number of photoelectrons emitted per second should depend only on the light intensity, not its frequency.
Cannot explain random build-up of intensity in a photographic image. Wave model predicts that there should be the same variation of intensity initially as in the final image
106
What is the limitation of the photon model of light?
Cannot explain interference phenomena, as two photons, being particles, cannot add together and cancel each other out.
