Waves Flashcards
(72 cards)
1
Q
Give the wavelength range of gamma waves
A
10^-16 - 10^-9 m
2
Q
Give the wavelength range of X-rays
A
10^-12 - 10^-7 m
3
Q
Give the wavelength range of UV waves
A
10^-9 - 3.7x10^-7 m
4
Q
Give the wavelength range if visible light
A
3.7x10^-7 - 7.4x10^-7 m
5
Q
Give the wavelength range of infrared waves
A
7.4x10^-7 - 10^-3 m
6
Q
Give the wavelength range of microwaves
A
10^-4 - 10^-1 m
7
Q
Give the wavelength range of radio waves
A
10^-1 - 10^4 m
8
Q
Give some uses of radio waves
A
TV and radio communications
9
Q
Give some uses of microwaves
A
Microwave ovens, mobile phones
10
Q
Give some uses of infrared waves
A
Night-vision equipment, remote controls
11
Q
Give some uses of visible light
A
Lasers, sight
12
Q
Give some uses of UV waves
A
Disco lights, tanning studios
13
Q
Give some uses of X-rays
A
CT scans, X-ray photography
14
Q
Give some uses of gamma rays
A
Radiotherapy (cancer treatment)
15
Q
Define a stationary wave
A
A wave whose energy is confined to a fixed position
16
Q
What wave property allow stationary waves to form?
A
Interference, superposition
17
Q
Give 3 conditions that a stationary wave needs to be produced
A
The two superimposing waves must be travelling in opposite directions
The waves must be the same frequency
They must have approximately equal amplitudes
18
Q
Briefly describe how a stationary wave is set up
A
Initially there are two progressive waves travelling in opposite directions. The two waves are in antiphase, leading to a resultant wave of zero displacement.
A small time later, one wave has moved 1/2 π radians to the right, and the other 1/2 π radians to the left. This gives the vector sum of the individual waves. They are in phase hence greater resultant displacement.
Some time later the process repeats with the waves being in antiphase again
19
Q
Define node in the context of stationary waves
A
Any point along a stationary wave where the displacement is always zero
20
Q
What is an antinode?
A
Any point along a stationary wave where the magnitude of displacement is always maximum when the two waves are in phase.
21
Q
What is the path and phase difference between a node and the next antinode?
A
1/4 λ which is equal to 1/2 π radians
22
Q
What is the path and phase difference between two successive nodes or two successive antinodes?
A
1/2 λ which is equal to π radians.
23
Q
How does a microwave generator use stationary waves?
A
It transmits microwaves towards the metal sheet which are reflected back from the sheet along their initial path, forming stationary waves.
24
Q
How can you use a microwave generator, detector and ruler to measure the wavelength / speed of microwaves
A
Move the detector slowly along the path of the microwaves. You should pick up a varying signal strength. Measure the distance between two maxima using a ruler. This will be equal to 1/2 λ. Multiply by two to get the wavelength of the wave.
Multiply by the (known) frequency to get the speed of the wave. This should be 3 x10^8 (speed of light)
25
What 3 factors govern the frequency of vibration on a string ?
Its mass per unit length
Its tension
The length of the string
26
Define fundamental mode of vibration on a string
It is the simplest stationary wave that can be set up on a string. Length of string L = 1/2 λ
27
What is the fundamental frequency on a string?
It is the lowest frequency that can be produced on a string.
28
Notes of higher frequency that can be produced on a string are called what ?
harmonics
29
What is the relationship between the frequency of a harmonic and the fundamental frequency?
The frequency of a harmonic is always an integer multiple of the fundamental frequency
30
For each harmonic, give the length of the string L in terms of λ for a string.
```
fundamental frequency: L= 1/2 λ
first harmonic: L= λ
second harmonic: L= 3/2 λ
third harmonic: L= 2 λ
Increases by 1/2 λ each time
```
31
Where are stationary longitudinal waves produced?
In air columns for example in wind instruments
32
How are stationary waves produced in half-closed tubes?
When air is blown down the pipe, a progressive wave travels down the tube and is reflected at the closed end. These two waves interfere constructively and destructively producing a stationary wave`
33
How does the length of a half-closed tube relate to its nodes and antinodes of a stationary wave?
The length of the tube must be such that a node is formed at the closed end and an antinode at the open end.
34
For each harmonic, give the length of the string L in terms of λ for a half closed tube
```
fundamental frequency: L = 1/4 λ
first harmonic: L = 3/4 λ
second harmonic: L = 5/4 λ
third harmonic: L= 7/4 λ
Increases by 1/2 λ each time
```
35
What is the relationship between pressure and air displacement in longitudinal waves?
At a node, displacement is always zero but pressure varies.
Node for displacement = antinode for pressure
Antinode for displacement = node for pressure.
36
How are stationary waves produced in open tubes?
The sound wave can also be reflected at the open ends. The length of the tube must be such that there is an antinode at both ends
37
For each harmonic, give the length of the string L in terms of λ for an open tube
```
fundamental frequency: L = 1/2 λ
first harmonic: L = λ
second harmonic: L = 3/2 λ
third harmonic: L = 2 λ
increases by 1/2 λ each time
```
38
What equipment might you need to measure the speed of sound using stationary waves? Describe the set-up of the the experiment.
Tuning fork of known frequency, clamp and stand, ruler, transparent glass tube, measuring cylinder, water.
The tube is held by a clamp above and partially into the water and is movable so its length can be adjusted. Water will be poured into the measuring cylinder, creating a half-closed tube.
39
Outline the method and analysis of results of the measuring speed of sound in air experiment
Sound the tuning fork and hold it at the top of the tube. Slowly increase the length of the tube while listening carefully. When the sound reaches a maximum at first, measure the distance from the tuning fork to the water. This should be when the length of the tube is 1/4 λ. Extend the tube further until you hear another maximum volume. This should be when the length of the tube = 3/4 λ. Measure this new distance from the tuning fork to the water. Subtract the two distances and this should be equal to 1/2 λ. Multiply by 2 to get λ. Multiply by the known frequency of the tuning fork to get the speed of sound.
40
State the principle of superposition.
When two or more waves of the same type meet, the resultant wave can be found by the vector sum of the displacements of the individual waves.
41
Describe constructive interference
If two waves A and B of the same amplitude exist at the same point and are travelling in phase, the amplitude of the resultant wave will be twice that of the individual waves.
42
Describe destructive interference
If two waves are in antiphase then destructive interference will occur. This means the resultant wave will have zero amplitude
43
Define coherence and explain the need for it
In order to calculate a meaningful resultant using the principle of superposition, the two waves must have a constant phase difference.
44
Define path difference
Path difference is the difference in the distance travelled by two waves arriving at the same point. This is referred to in terms of wavelength λ
45
Define phase difference
Phase difference in the difference in the phases of two waves of the same frequency. Measured in radians.
46
Given that two waves are coherent, what decides whether constructive or destructive interference occurs?
phase / path difference
47
What path and phase differences cause constructive interference?
Path difference: 0, 1λ, 2λ 3λ -----> nλ, where n in an integer
Phase difference: 0, 2π, 4π, 6π ---> 2nπ, where n in an integer.
48
What path and phase differences cause destructive interference?
Path difference: 1/2 λ, 3/2 λ, 5/2 λ --->(2n+1)/2 λ, where n is an integer
Phase difference: π, 3π, 5π ----> (2n+1)π, where n is an integer
49
How can interference with sound waves be demonstrated?
You can use 2 loudspeakers connected to the same signal generator. Set them up next to each other. As a person walks along in front of the speakers, they will hear a loud sound in areas of constructive interference, and quiet sounds in areas of destructive interference. The distance between loud and quiet sounds in longer for lower frequencies
50
How can interference using microwaves be demonstrated?
In radar systems, microwaves travel in tubes called waveguides. By adjusting the lengths of the two paths, you can create either constructive or destructive interference when the wave rejoins.
51
What are the two conditions needed for measuring the wavelength of light?
The light must be monochromatic
There must be an accurate method of producing a very small path difference
52
What was the significance of the Young double-slit experiment with regards to the nature of light?
Before the experiment many physicists had thought of light being a stream of particles called corpuscles (incl. Newton)- when Thomas Young conducted his experiment, he established the wave theory of light.
53
Outline the procedure for the double-slit experiment.
A monochromatic light source is placed behind a single slit. Light spreads out from the slit by diffraction until it reaches two narrow slits where further diffraction occurs. Waves from both slits that are in phase will interfere constructively; waves in antiphase will cancel out by destructive interference.
Alternate bright and dark "fringes" are seen on the screen.
54
In the double-slit experiment, how can the percentage uncertainty for the wavelength λ be reduced?
Measure across all fringes then divide by the number of fringes.
Increase slit-to-screen distance D. This will increase D (obvs) and x which will reduce their respective % uncertainties. This will reduce % uncertainty for λ. However this will reduce the light intensity on the screen.
55
What is the equation for working out the wavelength of light using the double-slit experiment.
λ = ax/nD
56
How can you use the double-slit method to determine the wavelength of microwaves?
A microwave transmitter is placed in front of a metal sheet which has two gaps where the microwave diffracts and interferes behind the sheet.
A microwave detector can be used to determine areas of maximum intensity
57
What is a diffraction grating?
a piece of optical equipment used to measure the wavelength of light
58
Why is the diffraction grating method preferred over the double slit method when determining the wavelength of light?
In the double slit method, the fringes are quite blurred making measurement difficult.
The gratings have more slits which improves brightness and makes maxima sharper.
The maxima are also further apart, which reduces percentage uncertainty for the angle θ
59
What is the method for determining the wavelength of light using a diffraction grating?
A monochromatic light source is shone through a diffraction grating. This diffracts the light by an angle θ . This angle can be determined using trigonometry, where tanθ = x/D , where x is the distance from the central maximum to the desired fringe and D is the distance from the grating to the screen. Use the equation
nλ = dsinθ to determine the wavelength λ of the light.
60
How can percentage error be reduced for the wavelength of light using a diffraction grating?
taking lots of measurements from different order maxima
61
When does refraction happen?
When a wave passes between different media
62
What two things about the wave change when refracted?
Its speed and its direction
63
What is the definition of the refractive index of a material n
the refractive index is the ratio of the speed of light in the material to the speed of light in a vacuum.
n = v/c
64
State Snell's law
nsinθ = constant for any medium
65
How are angles of incidence and refraction always measured?
With respect to the normal to the surface
66
How can you measure the refractive index of a glass block
Trace around the semi-circular glass block using a pencil on the paper. Draw a normal to the block using a protractor. Measure angles of incidence every 10 degrees and draw a line to the block. Shine the light ray along these angles and measure the corresponding angle of refraction using a protractor and a ruler to extend the lines. Once data has been collected, draw a graph of sinθ (incidence) against sinθ (refracted). Calculate the gradient- this will be equal to n of the block.
67
Total internal refection is a result of which two wave phenomena?
Reflection and refraction.
68
If a light ray is travelling from a material of a higher refractive index to one of lower refractive index at an angle lower than the critical angle, what will happen?
refraction will occur where θ (2) is greater than θ (1) and there will also be a weak reflected ray.
69
What happens when we increase the angle of incidence θ (1) when going from higher to lower refractive index?
The angle of refraction will increase and the partially reflected ray will get stronger
70
What happens when the angle of incidence is equal to (and above) the critical angle C?
The angle of refraction will be 90 degrees. Above this angle C all light is totally internally reflected
71
What is the equation for determining the critical angle C for a medium?
Sin C = n (2) / n (1),
```
where n(2) < n (1)
Usually when using air as the smaller n, this works out to be sinC = 1 / n
```
72
How can you determine the critical angle of a glass block?
Draw around the block using a pencil. Use the protractor to draw a noral to the block. increase the angle of incidence until the ray emerges along the surface of the boundary between the block and the air. The angle that the incident ray makes will the normal is C the critical angle. This could be used to calculate the refractive index of the glass block.
