Waves Flashcards
1
Q
What are mechanical waves?
A
Waves that require a medium to travel through
2
Q
Give examples of mechanical waves
A
Sound waves
Seismic waves
3
Q
What are electromagnetic waves?
A
Oscillating electric and magnetic fields that travel without the need of a medium
4
Q
Give an example of electromagnetic waves
A
All electromagnetic waves
5
Q
What are longitudinal waves?
A
Waves, whose direction of vibration is parallel to the direction of wave travel
6
Q
Give examples of longitudinal waves?
A
Sound
Primary seismic waves
7
Q
What are transverse waves?
A
Waves, whose direction of vibration is perpendicular to the direction of wave travel
8
Q
Give examples of transverse waves
A
All electromagnetic waves
Secondary seismic waves
9
Q
What is the amplitude?
A
The length from the trough/crest to the equilibrium, maximum displacement
10
Q
What is the wavelength?
A
The distance in metres between the same point on two successive waves
11
Q
What is frequency?
A
The number of cycles per second(Hz)
12
Q
What is displacement?
A
The distance of a vibrating particle from its equilibrium position
13
Q
What is a cycle?
A
The maximum displacement from one wave to the next
14
Q
What is the time period?
A
The time for one complete wave to pass a fixed point
15
Q
What is the equation for the time period?
A
T = 1/f
16
Q
What happens to the wavelength when the frequency of a wave increases?
A
The wavelength shortens when the frequency increases
17
Q
What is the equation for wave-speed?
A
Wave-speed = frequency * wavelength
18
Q
What is polarisation?
A
The production of waves oscillating in one plane from a source of randomly oscillating waves
19
Q
What does it mean when the waves are unpolarised?
A
Waves are unpolarised when the vibrations change from one plane to another
20
Q
What happens when a light goes through a polaroid?
A
The light travelling through the polaroid has a reduced intensity as it is polarised due to the arrangements of the molecules in the polaroid
21
Q
What is reflection?
A
The wave is bounced back when it hits a boundary
22
Q
What is refraction?
A
The wave changed direction as it enters a different medium
23
Q
What is the constant speed of all EM waves in a vacuum?
A
3.00*10^8m/s
24
Q
How do X-rays and Gamma rays cause ionisation?
A
They knock electrons out of orbits
25
What is a progressive wave?
A wave that caries energy from one place to another without transferring any material
26
What evidence shows that electromagnetic waves are transverse?
Polarisation
| In 1808, Etienne Louis Malus discovered that light was polarised by reflection
27
Why can light be polarised?
Light is a transverse wave consisting of vibrating electric and magnetic fields at right angles to the transfer of energy
28
When does superposition happen?
When two or more waves pass through each other
29
What is the principle of superposition?
When two or more waves cross and the resultant displacement equals the vector sum of the individual displacements
30
How can interference be described?
Constructive or destuctive
31
Give two examples of when constructive interference can occur
Crest + Crest = Supercrest
| Trough + Trough = Supertrough
32
Give an example of destructive interference
Crest + Trough = Zero
33
What does it mean when waves are in phase?
The wave meet at the same point in the wave cycle
| At the point, they have the same displacement and velocity
34
What is 360 in radians?
2Pi radians
35
How do you convert from degrees to radians and vice versa?
Degrees to radians, multiply by Pi/180
| Radians to degrees, multiply by 180/Pi
36
What does it mean if sources are coherent?
The coherent sources have the same wavelength and frequency and a fixed phase difference between them
37
What factor affects interference?
Path difference
38
What is the path difference?
The amount by which the path travelled by one wave is longer than the path travelled by the other wave
39
When does constructive interference occur?
When there is an equal distance at a point from two sources that are coherent and in phase
Also when the path difference is a whole number of wavelengths
Path difference = n(wavelengths)
40
When does destructive interference occur?
The path difference is half a wavelength or one and a half e.t.c wavelengths out of phase
path difference = (n+0.5)wavelengths
41
What is diffraction?
The spreading out of waves as they go through a narrow gap or go around obstacles
42
What factors affect diffraction?
Wavelength
| Size of the gap
43
When is the diffraction unnoticeable?
The size of the gap is bigger than the wavelength
44
When does most diffraction occur?
The size of the gap is the same as the wavelength
45
When are most of the waves reflected back?
The size of the gap is smaller than the wavelength
46
What is needed to observe a clear diffraction pattern?
A monochromatic, coherent light source
47
Give an example of a monochromatic and coherent light source
Lasers
48
What is the meaning of monochromatic?
All the light has the same wavelength, frequency, and therefore the same colour
49
What can be observed when a monochromatic, coherent laser light is shone through a narrow list?
A central bright fringe with dark and bright fringes alternating on either side
50
What type of interference causes the dark fringes?
Destructive interference
51
What type of interference causes the bright fringes?
Constructive interference
52
What is white light?
A mixture of difference colours, each with different wavelengths
53
What happens when white light is shone through a single narrow slit?
All the different wavelengths are diffracted by different amounts
You get a spectra of colours rather than clear fringes
54
What is the intensity of light?
The power per unit area
55
Why is the central maximum in a single slit light diffraction the brightest?
There are more photons per unit area hitting the central maximum per second than the other bright fringes
56
What factors affect the width of the central maximum?
Wavelength
| Slit size
57
What happens when the slit width is increased?
The amount of diffraction decreases
The central maximum is narrower
The intensity of the central maximum is higher
58
What happens when the wavelength is increased?
The amount of diffraction increases
The central maximum is wider
The intensity of the central maximum is lower
59
What can you do to prevent damage when using lasers?
```
Never shine the laser towards a person
Wear laser safety goggles
Avoid shining the laser beam at a reflective source
Have a warning sign on display
Turn the laser off when it's not needed
```
60
Describe the Young's Double Slit experiment?
A laser is shone through two slits onto a screen
The slits have to be about the same size as the wavelength so that the laser light is diffracted
You get a pattern of light and dark fringes
61
Describe a similar experiment to the Young's slit experiment with microwaves
Replace the laser and slits with two microwave transmitter cones attached to the same signal generator
Replace the screen with a microwave receiver probe
If you move the probe perpendicular to the microwave generator, you'll get an alternating pattern of strong and weak signals
62
What is the Young's Slit formula?
w=(wavelength*D)/s
63
What is the meaning of fringe spacing?`
The distance between the centre of the one minimum to the centre of the next minimum or the centre from one maximum to the centre of the next maximum
64
What is 's' in Young's slit formula?
Spacing between slits
65
What is 'D' in Young' slit formula?
The distance from the slits to the screen
66
What was Young's slit experiment evidence for?
It showed that light could both diffract and interfere
67
What happens when light diffracts through more slits?
Interference patterns become sharper
68
Which order is the maximum brightness?
Zero-order
69
How can the wavelength be calculated from diffraction grating?
wavelength = d*sin(angle)/n
70
How would you calculate the slit spacing(d), if you are given the number of slits per metre?
d = 1/slits per metre
71
What general conclusions can be drawn from d*sin(angle) = n*wavelength?
If the wavelength is bigger, the bigger the angle.
If d is bigger, the smaller the angle.
Values of sin(angle) greater than 1 means the order doesn't exist
72
How can diffraction grating be used?
It can be used to identify elements
| It can be used to calculate atomic spacing
73
What is white light?
A mixture of colours with different wavelengths
74
What will happen if you diffract white light?
The colours in the light will spread out by different amounts because of the different wavelengths
75
Which colour will be on the inside and on the outside?
| Why?
```
Inside = violet, has the shortest wavelength
Outside = red, has the longest wavelength
```
76
What is a stationary wave?
The superposition of two progressive waves with the same frequency and wavelength moving in opposite directions
77
What don't stationary waves transmit?
Stationary waves do not transmit energy
78
What is a node?
This is where the amplitude of the vibration is zero
79
What is an antinode?
This is where the point of maximum amplitude
80
Where do stationary waves occur?
They occur at certain special values of frequency and wavelength called harmonics
81
What causes progressive and stationary waves?
Fluctuation
| Disturbance
82
What is the difference in energy transfer between stationary and progressive waves?
Progressive waves
-transfer energy and momentum from the source to
surroundings
Stationary waves
-no energy or momentum transfer as they are confined
to a specific region
83
What is the difference in occurrences between stationary and progressive waves?
Progressive waves
-occur at all frequencies
Stationary waves
-occur at harmonics
84
What is the difference in the amplitude of particles between stationary and progressive waves?
Progressive waves
-all particles will reach a maximum and minimum
amplitude
Stationary waves
-some points have a maximum amplitude and others
do not. The amplitude of the particle depends on the
position of the particle
85
What is the difference in the phase difference of particles between stationary and progressive waves?
Progressive waves
-only particles exactly one wavelength apart move in
phase with each other
Stationary waves
-within a one-half wavelength, all particles move in
phase
86
What is the first harmonic on a string?
Length = wavelength/2
87
What is the second harmonic on a string?
Length = wavelength
88
What is the general formula for the length of the string?
Length = n*wavelength / 2
89
Wha is the first harmonic in a tube with one closed end?
Length = wavelength/4
90
What is the second harmonic in a tube with one closed end?
Length = 3/4 * wavelength
91
What is the first harmonic in a tube with both ends open?
Length = wavelength/2
92
What is the second harmonic in a tube with both ends open?
Length = wavelength
93
What is the fundamental mode frequency?
f = 1/2L√(T/µ)
94
What is the second harmonic frequency?
f = 1/L√(T/µ)
95
What is the third harmonic frequency?
f = 3/2L√(T/µ)
96
What is refraction?
The change in direction that occurs when light passes at an angle across a boundary between 2 transparent substances
97
What happens to the light when it travels from a less dense material to a more dense material?
The light bends towards the normal
98
What happens to the light when it travels from a more dense material to a less dense material?
The light bends away from the normal
99
What is the refractive index of a material?
It is a measure of how much the material slows down light
100
What is the absolute refractive index of a material?
A measure of optical density
| it is found from the ratio between the speed of light in a vacuum and the speed of light in that material
101
How do you calculate the refractive index of a substance?
n = C/Cs
102
What is the relative refractive index between two materials?
The ratio of the speed of light in material 1 to the speed of light in material 2
103
How do you calculate the relative refractive index?
n = C1/C2
104
What is the refractive index of air?
1
105
What is the angle of incidence?
The angle the incoming light makes to the normal
106
What is the angle of refraction?
The angle the refracted ray makes to the normal
107
What is Snell's law?
n1 sin angle 1 = n2 sin angle2
