A-Level Physics > 3: Waves > Flashcards

3: Waves Flashcards

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1
Q

What is a progressive wave?

A

A wave that transfers energy without transferring material

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2
Q

What is amplitude?

A

Maximum displacement from equilibrium (m)

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3
Q

What is frequency?

A

The number of complete oscillations passing a point per second (Hz)

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4
Q

What is wavelength?

A

The length of one whole oscillation (m)

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5
Q

What is wave speed?

A

Distance travelled by the wave per unit time (m/s)

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6
Q

What is the phase of a wave?

A

the position of a certain point on a wave cycle (rad, degrees, or fractions of a cycle)

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7
Q

What is phase difference?

A

How much a particle/wave lags behind another particle/wave (s)

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8
Q

What does it mean if two points are in phase?

A

-They are both at the same point of a wave cycle
- They have the same displacement and velocity
- their phase difference will be a multiple of 360

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9
Q

What does it mean if two waves are completely out of phase?

A

They’re an odd integer of half cycles apart

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10
Q

What is the equation for wave speed?

A

c = fλ

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11
Q

What equation links frequency and time period?

A

f = 1/T

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12
Q

What is a transverse wave?

A

oscillations of particles at right angles to the direction of energy transfer

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13
Q

What is a longitudinal wave?

A

Oscillations of particles parallel to the direction of energy transfer

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14
Q

What is a polarised wave?

A

A transverse wave that oscillates in only one plane

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15
Q

How does polarisation provide evidence for the nature of transverse waves?

A

Polarisation only occurs if a wave’s oscillations are perpendicular to its direction of travel

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16
Q

What are 2 applications of polarisation?

A
  • Polaroid sunglasses: reduce glare by blocking partially polarised light reflected from water and tarmac, as they only allow oscillations in the plane of the filter, making it easier to see
  • TV and radio signals: they are plane-polarised by the orientation of the rods on the transmitting aerial, so the receiving aerial must be aligned in the same plane of polarisation to receive the signal at full strength
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17
Q

What is 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

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18
Q

What is constructive interference?

A

Interference when 2 waves have displacement in the same direction

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19
Q

What is destructive interference?

A

Interference that occurs when one wave has positive displacement and the other has negative displacement. If the waves have equal but opposite displacements, total destructive interference occurs.

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20
Q

How is a stationary wave formed?

A

From the superposition of 2 progressive waves, travelling in opposite directions in the same plane, with the same frequency, wavelength and amplitude

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21
Q

What happens when waves meet in phase when making a stationary wave?

A

Constructive interference occurs so antinodes are formed, which are regions of maximum amplitude

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22
Q

What happens when waves meet completely out of phase?

A

Destructive interference occurs and nodes are formed, which are regions of no displacement

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23
Q

How could you form a stationary wave?

A
  • A string fixed at one end, and fixed to a driving oscillator at the other end
  • A wave travelling down the string from the oscillator will be reflected at the fixed end of the string, and travel back along the string causing superposition of the two waves
  • because the waves have the same wavelength, frequency and amplitude, a stationary wave is formed
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24
Q

What is the first harmonic?

A

The lowest frequency at which a stationary wave is formed, formed with two nodes and one antinode

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25
What is the distance between adjacent nodes (or antinodes) on a stationary wave (of any harmonic)
half a wavelength
26
How do you find the resonant frequency of a stationary wave?
f = 1/2L (T/μ)^1/2
27
Examples of stationary waves?
- Stationary microwaves - Stationary sound waves
28
What is path difference?
The difference in the distance travelled by two waves
29
What is meant by a coherent light source?
It has the same frequency and wavelength and a fixed phase difference
30
What is used as an example of coherent and monochromatic light?
Lasers
31
What is meant by monochromatic?
emits a single wavelength (and therefore frequency) of light and so is a single colour
32
Why are lasers used in diffraction experiments?
they form clear interference patterns
33
What does Young's double slit experiment demonstrate?
Interference of light from two sources
34
What could you do to create coherent, monochromatic light using a light bulb in the double slit experiment?
Place a single slit before the double slit to make the light have a fixed path difference, and a filter to make the light monochromatic
35
Describe Young's double slit experiment
- Shine a coherent light source through 2 slits about the same size as the wavelength of the laser light so the light diffracts - Each slit acts as a coherent point source, making a pattern of light and dark fringes
36
When are light fringes formed (double slit experiment)?
- When the light meets in phase and interferes constructively - This occurs where the path difference between waves is a whole number of wavelengths
37
When are dark fringes formed?
- when light meets completely out of phase and interferes destructively - this occurs when the path difference is a whole number and a half wavelengths
38
What is the formula associated with Young's double slit experiment?
w = λD/s w = fringe spacing λ = wavelength D = distance between the screen and the slits s = slit separation
39
What happens if you use white light instead of monochromatic laser light?
- it gives wider maxima - less intense diffraction pattern - central white fringe with alternating bright fringes which are a spectra, violet closest to central maximum and red furthest
40
What safety precautions should you follow when using lasers?
- Wear laser safety goggles - Don't shine the laser at reflective surfaces - Display a warning sign - Never shine the laser at a person
41
How can you demonstrate the interference pattern formed in the double slit experiment using sound waves?
- use two speakers instead of a double slit connected to the same signal generator - the intensity of a wave can be measured using a microphone to find the maxima and minima
42
How does Young's double slit experiment provide evidence for the wave nature of light?
Diffraction and interference are wave properties, so EM radiation must act as a wave
43
What is diffraction?
The spreading out of waves when they pass through or around a gap
44
When does the greatest diffraction occur?
When the gap is the same size as the wavelength
45
What happens when gaps are smaller than the wavelength of a wave?
Most waves are reflected
46
What happens when gaps are larger than the wavelength?
There is less noticeable diffraction
47
What happens when you diffract monochromatic light through a single slit onto a screen
- It forms an interference pattern of light and dark fringes - The pattern has a bright central fringe, double the width of all other fringes - alternating light and dark fringes either side of the central fringe - intensity of the fringes decreases from the central fringe
48
What would happen if you used white light instead of monochromatic light when creating a diffraction pattern?
- all different wavelengths of light are all diffracted by different amounts so you get a spectrum of colour in the diffraction pattern - white central maximum with alternating bright fringes which are spectra, with violet closest to central maximum
49
How do you vary the width of the central maximum?
Vary slit width and wavelength
50
What happens to the central maximum if you increase the slit width?
Increasing slit width decreases the amount of diffraction so the central maximum becomes narrower and its intensity increases
51
What happens to the central maximum if you increase the wavelength?
Increasing the light wavelength increases the amount of diffraction as the slit is closer in size to the light's wavelength, so the central maximum becomes wider and its intensity decreases
52
What is a diffraction grating?
- A slide containing many equally spaced slits very close together
53
What happens when you shine monochromatic light through a diffraction grating?
- the interference pattern is much sharper and brighter, as there are more rays of light reinforcing the pattern - measurements of slit widths are much more accurate as they are easier to take
54
What is the zero order line?
The ray of light passing through the centre of the diffraction grating
55
What is the formula associated with diffraction gratings
dsinθ = nλ d = distance between slits λ = wavelength n = the order θ = the angle to the normal made by the maximum
56
What happens if you increase the wavelength of light passing through a diffraction grating?
- the distance between the orders will increase because θ is larger due to the increase of diffraction as the slit spacing is closer in size to the wavelength - this means the pattern will spread out
57
Why is the value of sinθ important?
the maximum value of sinθ is 1, so any values of n that give sinθ to be greater than 1 are impossible
58
What are the applications of diffraction gratings?
- splitting up light from stars to get line absorption spectra, used to show elements present in the star - X-ray crystallography: to measure atomic spacing in certain materials
59
What is a refractive index?
A property of a material which measures how much it slows down light passing through it
60
How do you work out refractive index (n)
n = c/c(s) c= speed of light c(s) = speed of light in the substance
61
How else can you describe materials with higher refractive indexes?
More optically dense
62
What is the refractive index of air?
Approximately 1
63
When does refraction occur?
when a wave enters a different medium, causing it to change direction, either towards or away from the normal depending on the materials refractive index
64
What is Snell's law?
n1sinθ1 = n2sinθ2
65
When does light bend towards the normal in refraction?
when n2 is more optically dense than n1
66
When will light bend away from the normal in refraction?
when n2 is less optically dense than n1
67
What happens when the angle of refraction is exactly 90°?
The light is refracted along the boundary, and the angle of incidence has reached the critical angle
68
What formula can you use to find the critical angle?
sinθc = n2/n1 WHERE n1 > n2
69
When can total internal reflection (TIR) occur?
When the angle of incidence is greater than the critical angle and the n1 > n2
70
What is an application of TIR?
optical fibres
71
What are optical fibres?
- flexible, thin tubes of plastic or glass which carry information in terms of light signals - optically dense core surrounded by cladding with a lower optical density allowing TIR to occur
72
What are the benefits of the cladding around optical fibres?
- protects the core form damage - prevents signal degradation through light escaping the core, causing information to be lost
73
What 2 things can cause signal degradation?
- Absorption - Dispersion
74
What is absorption?
where part of the signals energy is absorbed by the fibre, reducing the amplitude of the signal, which could lead to a loss of information
75
What is pulse broadening?
- where the received signal is broader than the original transmitted signal - these can overlap causing a loss of information
76
How does disperison cause signal degradation?
It causes pulse broadening
77
What are the two types of dispersion?
Modal Material
78
what is modal dispersion and how does it cause pulse broadening?
- light rays enter the fibre at different angles, and take different paths along the fibre - rays then take a different amount of time to travel along the fibre, causing pulse broadening
79
what is material dispersion and how does it cause pulse broadening?
- caused by using light of different wavelengths - light rays will travel at different speeds, which leads to pulse broadening
80
How can you prevent material dispersion?
Using monochromatic light
81
How can you reduce signal degradation?
using an optical fibre repeater, which regenerates the signal during its travel to its destination
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