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A-Level Physics > Waves > Flashcards

Waves Flashcards

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1
Q

What is a progressive wave

A

A moving wave that transfers energy from one point to another without moving matter

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2
Q

What is the displacement of a wave

A

The distance a point on a wave has moved from equilibrium position

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3
Q

What is amplitude

A

The maximum magnitude of displacement of a point on a wave from equilibrium position

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4
Q

What is wavelength

A

The distance between 2 adjacent points on a wave that are moving in phase

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5
Q

What is a time period

A

The time taken for a complete wave cycle to pass

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6
Q

What is frequency

A

The number of cycles per second

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7
Q

How are freuency and time period linked

A

f = 1/T

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8
Q

How do you calculate wavespeed

A

c = fλ

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9
Q

Transverse waves features

A

Oscillations of the particles/ field is perpendicular to the direction of wave travel/ energy transfer
e.g. electromagnetic waves, water waves, seismic S waves

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10
Q

Longitudinal waves featrues

A

Oscillations of the particles/ field is parallel to the direction of wave travel/ energy transfer
e.g. sound waves (ultrasound, infrasound), seismic P waves

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11
Q

What is an unpolarised wave

A

A wave that has oscillations in all planes that are perpendicular to the direction of wave travel

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12
Q

What is a polarised wave

A

A wave that has oscillations in only one plane that is perpendicular to the direction of wave travel

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13
Q

How can waves become polarised

A

Pass the waves through a polarising filter
Only oscillations in a certain plane (the transmission axis) are transmitted
Oscillations in other planes are absorbed
Intensity is reduced

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14
Q

What is partial plane polarisation

A

It happens when waves are reflected from a reflective surface
e.g. if the surface is horizontal, a proportion of the reflected light will oscillate more in the horizontal plane than the vertical plane

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15
Q

What are some uses of polarisers

A

Polaroid sunglasses and photography - to reduce glare
Radio and microwave signals - aerial orientation

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16
Q

What is the principle of superposition

A

When 2 (or more) waves arrive at a point, they interfere and the resultant displacement is the vector sum of the displacements of each wave.

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17
Q

What are coherent waves

A

Waves with the same frequency (or wavelength)
Waves with a fixed phase difference between them

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18
Q

What occurs when 2 coherent waves are in phase

A

Constructive interference

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19
Q

What occurs when 2 coherent waves are in anti-phase

A

Destructive interference

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20
Q

What are stationary waves

A

Waves that store energy

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21
Q

How do you form stationary waves

A

The superposition of 2 progessive waves travelling along the same line
The waves must have the same speed, frequency (or wavelength), similar amplitude and be travelling in opposite directions

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22
Q

Stationary wave features

A

Nodes have zero amplitude, anti-nodes have maximum amplitude
The wavelength is twice the distance between 2 adjacent nodes
Between 2 nodes: the points are in phase and in anti-phase with the points bewteen the next set of nodes

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23
Q

What occurs at resonant frequencies

A

An exact number of half wavelengths fit into the string

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24
Q

Features of the nth harmonic (string of length L, with wavespeed v)

A

n + 1 nodes and n anti-nodes
Number of wavelengths is n/2
λ = 2L/n
fn = vn/2L = nf1

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25
Q

How does the lenght of the string affect the resonant frequency

A

f = v/2L, f ∝ 1/L
As length increases, frequency decreases

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26
Q

How does mass per unit length affect the resonant frequency

A

μ = m/L
As μ increases, a heavier string causes waves to travel slower, as f = c/λ, f decreases as c decreases
f ∝ 1/√μ

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27
Q

How does the tension in the string affect the resonant frequency

A

T = mg
As T increases, waves travel faster
As f = c/λ, f increases
f ∝ √T

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28
Q

What is the equation for the resonant frequency of the first harmonic

A

f1 = 1/2L x √T/√μ

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29
Q

What is path difference

A

The difference in the distance travelled by 2 waves from their sources to the point where they meet

30
Q

What does a path difference of nλ mean

A

Constructive interference occurs
The waves are in phase

31
Q

What does a path difference of (n + 1/2)λ mean

A

Destructive interference occurs
The waves are in anti-phase

32
Q

What happens during Young’s double slit experiment

A

Light diffracts through 2 slits
Light superposes and interferes
Produces pattern on a screen

33
Q

How are bright fringes formed in Young’s double slit

A

The maxima are formed by constuctive interference
The path difference is nλ (waves are in phase)

34
Q

How are dark fringes formed in Young’s double slit

A

Minima are formed by destructive interference
Path differernce is (n + 1/2)λ (waves are in anti-phase)

35
Q

Features of the pattern on the screen in Young’s double slit

A

Bright and dark fringes with even spacing parallel to the slits
The intensity of bright fringes decreases as you move away from the centre

36
Q

Define fringe spacing

A

The distance between the centre of 2 adjacent bright (or dark) fringes

37
Q

What is Young’s double slit equation

A

w = λD/s
w is fringe spacing, D is distance from the slit to the screen, s is slit seperation

38
Q

What does the Young’s double slit screen pattern look like for white light

A

White central bright fringe
Outer bright fringes are a spectrum (violet closest to centre and red furthest out)
Outer bright fringes are wider (may merge)
Outer dark fringes become harder to see

39
Q

Ways to be safe around lasers

A

Never look directly at laser
Don’t shine laser at people
Wear safety goggles
Stand behind the lases

40
Q

What is diffraction

A

The spreading out of waves as they pass through a gap or go around an obstacle

41
Q

How does reducing the size of the gap affect the amount of diffraction

A

The amount of diffraction increases

42
Q

How does decreasing the wavelength for the same sized gap affect the amount of diffraction

A

Diffraction amount increases

43
Q

Features of the single slit diffraction pattern

A

Central bright fringe is 2x width of other fringes
Central bright fringe has musch higher intensity
Intensity of other fringes decreases as you go further out

44
Q

How are the bright fringes formed in single slit diffraction formes

A

Constructive interference
Waves arrive in phase
Path difference is nλ

45
Q

How are the dark fringes formed in single slit diffraction

A

Destructive interference
Waves arrive in anti-phase
Pathe difference is (n + 1/2)λ

46
Q

What is the pattern formed by white light for single slit diffraction

A

Central bright fringe is white, has the highest intensity and 2x width of other fringes
Outer bright fringes are a spectrum (violect closest to centre and red furthest out)
Wider outer fringes than with monochromatic light

47
Q

How does increasing the slit width affect the intensity of the central maximum

A

Amount of diffraction decreaases
Central maximum is narrower
Intensity of the central maximum is higher
As the same amount of energy is concentrated over a smaller area

48
Q

How does increasing the wavelength have an effect on the intensity and width of the central maximum

A

The amount of diffraction increases
The central maximum is wider
The intensity of the central maximum is lower
As the same amount of energy is spread over a larger area

49
Q

What is a diffraction grating

A

Plate with hundreds of parallel slits per mm

50
Q

Explain the pattern created by using a diffraction grating on monochromatic light in terms of path difference

A

Central maximum is the 0 order where path difference = 0λ
Outer maxima are the nth order lines where path difference = nλ
Minima are formed by destructive interference where path difference = (n + 1/2)λ

51
Q

How do you calculate the maximum number of orders

A

d/λ rounded down to nearest integer

52
Q

What is the diffraction grating equation for monochromatic light

A

dsinθ = nλ
d is distance between slits
n is order number
θ is angle between the incident beam and the nth order

53
Q

What happends to the angle of diffraction if the wavelength of light is increases

A

sinθ = nλ/d
sinθ increases, θ increases
Pattern is more spread out

54
Q

What happens to the angle if the distance between the slits is increases

A

sinθ = nλ/d
sinθ decreases, θ decreases
Pattern is less spread out

55
Q

Features of the diffraction grating pattern for white light

A

The zero order maximum is white
All other orders are a spectrum (violet closest to centre and red furthest out, as sinθ ∝ λ)

56
Q

Applications of diffraction gratings

A

Identifying elements
X-ray crystallography (discovering structure of DNA)

57
Q

What is refraction

A

When a wave crosses a boundry between 2 medium at an angle which makes is change direction

58
Q

What happens when light enters a more optically dense material

A

Wave goes towards normal as its wavespeed decreases
Frequency does not change so wavelength also decreases

59
Q

What is refractive index and how do you calculate it

A

Refractive index is a measure of optical density (how much light slows down when it enters a material)
n = c/cs
The refractive index of air ≈ 1

60
Q

What is Snell’s law

A

n1sinθ1 = n2sinθ2

61
Q

What happens when in Snell’s law, θ2 = 90°

A

sinθ2 = 1
Light is refracted along the boundary
This is when θ1 = θc (the critical angle)

62
Q

Under what conditions does total internal reflection occur

A

If n1 > n2
If the angle of incidence is greater than the critical angle
Then all light is reflected back into the material

63
Q

How do you calculate the critical angle

A

sinθc = n2/n1
(where n1 > n2)

64
Q

Uses of optical fibres

A

Communications - high speed internet transmission
Medical imaging - endoscopy for digestive system problem diagnosis

65
Q

Features of the core in step index optical fibres

A

Medium the light travels through (made of plastic/ glass)
Very narrow
Has a greater refractive index than the cladding

66
Q

Features of the cladding in step index optical firbes

A

Lower refractive index than the core
Protects core from scratches or breakages
Prevents signal crossover of adjacent cores

67
Q

What is signal degredation by absorption and how can you fix it

A

Material of the core absorbs signal’s energy
Amplitude of signal is reduces
Solution: make core out of a low absorption material

68
Q

What is signal degredation by modal dispersion and how can you fix it

A

Light enters the core at different angles so they take different paths
Longer paths take more time which causes pulse broadening (signals overlap and mix up)
Solution: single-mode fibre (very narrow and has small difference between refractive indeces of core and cladding)

69
Q

What is signal degradation by material dispersion and how can you fix it

A

When light of different wavelengths is used, some travel slower (violest slowest, red fastest)
Some light reaches the other end faster causing pulse broadening
Solution: use monochromatic light

70
Q

What can solve all 3 types of signal degradation

A

Optical fibre repeater which boosts and regenerates the signal
This reduces signal degradation

A-Level Physics (6 decks)

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