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AQA A-level Physics > Waves > Flashcards

Waves Flashcards

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1
Q

What is a progressive wave?

A

a moving wave that carries energy from one place to another without transferring material

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2
Q

What is a transverse wave?

A

a wave where the oscillations are perpendicular to the direction of energy travel

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3
Q

Label a transverse wave (7)

A

should label:
-crest
-wavelength
-amplitude
-equilibrium position
-trough
-direction of energy transfer
-time period

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4
Q

What are some examples of transverse waves? (4)

A

-surface water waves
-electromagnetic waves
-seismic s-waves
-guitar strings

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5
Q

What are longitudinal waves?

A

a wave where the oscillations are parallel to the direction of energy transfer

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6
Q

Label a longitudinal wave (5)

A

should label:
-amplitude
-wavelength (from the middle of one rarefaction to another)
-rarefaction
-compression
-direction of energy transfer

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7
Q

What are rarefactions in longitudinal waves?

A

regions of low pressure due to particles being spread further apart

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8
Q

What are compressions in longitudinal waves?

A

regions of high pressure due to particles being close together

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9
Q

What are some examples of longitudinal waves? (4)

A

-sound waves
-ultrasound waves
-seismic p-waves
-waves through a slinky coil

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10
Q

What do we mean by frequency?

A

the number of cycles a wave undertakes per unit time

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11
Q

What do we mean by wavelength?

A

the minimum distance between two parts of a wave oscillating in phase e.g. crest to crest

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12
Q

What do we mean by amplitude?

A

the maximum displacement from equilibrium

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13
Q

What do we mean by wave speed?

A

the distance a part of the wave form moves divided by the time taken to get there

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14
Q

What do we mean by period/time period?

A

the time taken for a point on a wave to pass through an entire cycle

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15
Q

What is the formula for wave speed? Give units

A

c=fλ
wave speed (ms^-1)= frequency (Hz) x wavelength (m)

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16
Q

What is the formula for frequency? Give units

A

f= 1/T
frequency (Hz)= 1/time period (s)

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17
Q

What do we mean by phase?

A

the stage of a particles’ motion in its sinusoidal cycle

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18
Q

What do we mean by phase difference?

A

the difference in the fraction of a cycle completed between two oscillating particles

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19
Q

What are the 2 formulas for phase difference? Give units and provide both formulas explaining when they should be used.

A

Δ ϕ = 2π x (d/λ) —> for points on a single wave
phase difference (rad or degrees)= 2π(OR 360) x (distance (m)/wavelength (m) )

Δ ϕ = 2π x (t/T) —> for two waves meeting w/ same frequency
phase difference (rad or degrees)= 2π(OR 360) x (time lag for wave 1 to get into phase with wave 2/time period (s) )

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20
Q

2π rad in degrees

A

360 degrees

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21
Q

1/2π rad in degrees

A

90 degrees

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22
Q

1π rad in degrees

A

180 degrees

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23
Q

3/2π rad in degrees

A

270 degrees

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24
Q

What are E-M (electromagnetic) waves?

A

consists of electric and magnetic waves travelling together oscillating perpendicular to the direction of energy transfer
in phase w/ one another

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25
Q

What are all E-M waves? What other qualities do they have?

A

transversal waves

-all have speed of 3 x 10^8 ms^-1 in a vacuum

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26
Q

Write down the E-M spectrum in order from lowest frequency. What else changes as you go down? (7)

A

radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, gamma rays

Remember
Me
I
Very
Useful
Xylophone
Great

lowest–>highest

wavelength decreases as you go down

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27
Q

List the colour spectrum in order of decreasing wavelength(7)

A

red, orange, yellow, green, blue, indigo, violet
highest–>lowest

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28
Q

What is polarisation?

A

where particle oscillations occur in only one of the directions which is perpendicular to the direction of energy transfer

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29
Q

What wave can polarisation occur through? Why?

A

can occur only in transverse waves as they oscillate in any plane perpendicular to the direction of energy transfer

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30
Q

What stops polarisation?

A

when oscillations change from one plane to another, the waves become unpolarised (oscillations go in all directions)

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31
Q

Why can’t longitudinal waves be polarised?

A

they oscillate parallel to the direction of travel

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32
Q

How can waves become polarised?

A

-through a polariser or polarising filter
-this only allows oscillations in a certain plane to be transmitted

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33
Q

When can waves become polarised?

A

only when reflected, refracted or scattered

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34
Q

Describe what happens to light intensity being polarised through a polarising filter as the filter rotates

A

-a maximum (high intensity) occurs when the transmission axis of the filters are parallel (at 180 or 360/0 degrees rotated)

-a minimum (no light) occurs when the transmission axis of the filters are perpendicular (at 90 or 270 degrees rotated)

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35
Q

Draw the graph showing how the light intensity varies with the angle of a polarising filter

A

-x axis showing angle of polarising filter
-y axis showing intensity of light
-high intensity at 0, 180 and 360 degrees
-no intensity at 90 and 270 degrees

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36
Q

What are some applications of polarisers?

A

-polarised sunglasses
-polaroid photography
-aerials

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37
Q

Explain how polarised sunglasses work and what their uses could be

A

-contain lenses with polarising filters with transmission axes that are vertically/horizontally oriented, so blocks partially polarised light to pass through

-useful in reducing the glare on the surface of the water and objects under the surface of the water can be viewed more clearly

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38
Q

Explain how polaroid photography works

A

-work in the same way as polaroid sunglasses
-enables photographers to take photos of objects underwater
-glare is eliminated by the polarising lens

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39
Q

Explain how aerials work

A

-radio/television services are broadcast horizontally/vertically-polarised
-therefore, the reception aerial needs to be mounted horizontally/vertically for signals to be transmitted

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40
Q

How do stationary (standing) waves form?

A

produced by the superposition of two progressive waves of the same frequency, wavelength and amplitude travelling in opposite directions (usually achieved from the reflection of a travelling wave)

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41
Q

What are some examples of stationary waves?

A

-stationary microwaves which are formed by reflecting a microwave beam at a metal plate (to find antinodes/node use a microwave probe)

-stationary sound waves which are formed by placing a speaker at one end of a closed glass tube, lay powder across the bottom of the tube, it will be shaken at the antinodes and settle
at the nodes.

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42
Q

Describe the energy for a stationary wave compared to a progressive wave

A

stationary: store energy

progressive: transfer energy

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43
Q

Describe the amplitude for a stationary wave compared to a progressive wave

A

stationary: each point has a different amplitude depending on the amount of superposition

progressive: all points have same amplitude

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44
Q

Describe the phase difference for a stationary wave compared to a progressive wave

A

stationary: points between nodes are in phase, points on either sides of nodes are out of phase

progressive: points exactly a wavelength apart are in phase

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45
Q

Describe the wave speed for a stationary wave compared to a progressive wave

A

stationary: each point on the wave oscillates at a different speed, overall the wave doesn’t move

progressive: the wave speed is the speed at which a wave moves at through a medium

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46
Q

What are nodes?

A

points on a stationary wave which have no amplitude

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47
Q

What are antinodes?

A

points on a stationary wave which have maximum amplitude

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48
Q

What is the principle of superposition?

A

when two waves meet, the total displacement at a point is equal to the sum of the individual displacements at that point

49
Q

What does constructive interference/reinforcement mean?

A

when waves meet and their displacement is at the same direction, they superpose (combine) to make a bigger displacement
e.g. a crest + a crest superpose to make a supercrest, same with a trough

50
Q

What does destructive interference/cancellation mean?

A

when a wave with positive displacement meets one with a negative displacement, they cancel out

51
Q

What are harmonics? Give an example

A

harmonics are the different wave patterns that stationary waves create

e.g. When a stationary wave is fixed at both ends, the simplest wave pattern is a single loop made up of two nodes and an antinode (called the first harmonic)

52
Q

How would you find the frequency from a harmonic? Give the equation for the first harmonic

A

depends on the length of the string L (m) and the wave speed (ms^-1)

for a string of length L, the wavelength of the lowest harmonic is 2L (there is only one loop of the stationary wave, which is a half wavelength) so the equation for frequency:

f =c/λ
and the wavelength= 2L so can be written as f=c/2L

53
Q

Draw the first three harmonics and their frequency formulas

A

1st: string on fixed end, two nodes and one antinode, 1/2 wavelength
f= c/2L

2nd: string on fixed end, three nodes, two antinodes, 1 wavelength
f= c/L

3rd: string on fixed end, four nodes, three antinodes, 1 1/2 (3/2) wavelengths
f= 3c/2L

54
Q

How can you distinguish wavelengths on a harmonic/stationary wave?

A

the distance between adjacent nodes is always 1/2 a wavelength

55
Q

Give another way to find the wave speed from a stationary wave

A

c=√ T/μ

56
Q

Give another way to find the frequency for the first harmonic and give units

A

f= 1/2L x √ T/μ
√ T/μ –> tension (N) /mu (kgm^-1)

57
Q

How can you work out μ?

A

divide mass by length of string

58
Q

Harmonics on a string

RP1- describe the graph and find the equation y=mx

A

-value of 1/frequency on y-axis and length on x-axis
-gradient= 2/c

equation:
1/f= 2/c x L

59
Q

What are the three interactions waves do?

A

-refract
-reflect
-diffract

60
Q

What is refraction?

A

waves change direction crossing an interface between media when they have different wave speeds (e.g. different optical density/refraction index for light)

note: frequency is constant, wavelength changes after diffraction

61
Q

What is something to know about reflection?

A

angle of incidence= angle of reflection

62
Q

What is diffraction?

A

the spreading of a wave front around an obstacle or through a gap

63
Q

What is needed for diffraction to occur?

A

the wavelength must be approx. the width of the gap

64
Q

What happens when the gap is smaller than the wavelength in diffraction?

A

most of the wave is reflected

65
Q

What happens when an obstacle is wider compared to the wavelength in diffraction?

A

the wider the obstacle, the less diffraction occurs

66
Q

What is path difference

A

the difference in the distance travelled by two waves

67
Q

How do you work out path difference?

A

path difference= distance travelled from source 1- distance travelled from source 2

68
Q

What does a coherent light source have?

A

same: frequency, wavelength
fixed phase difference

69
Q

What does monochromatic mean?

A

single frequency/small range of wavelengths

70
Q

Why is a laser useful in showing interference and diffraction?

A

produces monochromatic light so diffraction and interference patterns are more defined

71
Q

RP2: Describe young’s double slit experiment

A

2 coherent sources (2 slits) interfere with each other to produce interference patterns (both constructive and destructive)

72
Q

RP2: What can be seen from the young’s double slit experiment?

A

dark fringes (points of destructive interference) and light fringes (points of constructive interference)

73
Q

Young’s double slit

RP2: When do light and dark fringes occur?

A

-light fringes occur when the path difference is a whole number (2λ, 3λ, 4λ etc.) , both sources are in phase

-dark fringes occur when the path difference is a whole number+1/2 wavelength (2 1/2λ, 3 1/2λ etc.) ,both sources are in anti-phase

74
Q

Young’s double slit

RP2: How can we find the slit spacing/fringe separation/distance between screen and slits/wavelength used?

A

W= λD/s

where:
w= fringe spacing/m
D= distance between screen and slits/m
λ= wavelength used/m
s= slit separation/m

75
Q

Describe the interference pattern created when white light is diffracted through a single slit

A

a white central maximum surrounded by a spectrum of outer fringes with decreasing intensity from violet to the zero order and red at the furthest

76
Q

Compare the width of a central maximum to the outer fringes

A

CM is twice the width of the outer fringes

77
Q

Describe the effect on the CM when increasing the slit width or incident wavelength

A

increasing slit width: CM is narrower and more intense

increasing λ: CM is wider and less intense

78
Q

How can we find the width of the CM for a single slit?

A

W= 2λD/a

where:
W= width of CM
D= distance between slit and screen/m
λ= wavelength used/m
a= slit width

79
Q

What are some safety precautions when lasers are used?(6)

A

-wear safety goggles
-don’t shine the laser at reflective surfaces
-display a warning sign
-never shine the laser at a person
-never look the laser at eye level,
-use your hand to see where it is

80
Q

TRUE/FALSE: Only light can produce interference patterns

A

FALSE
both sound and E-M waves can produce interference patterns

81
Q

How can interference patterns be produced with sound waves?

A

two speakers connected to the same signal generator, the intensity of the wave can be measured using a microphone to find the maxima, and minima

82
Q

What is a diffraction grating?

A

a slide containing many equally spaced slits which are very close together

83
Q

What happens when monochromatic light is passed through a diffraction grating compared to a double slit? Why?

A

-the interference pattern is much sharper and brighter than it would be after being passed through a double slit

-because there are many more rays of light reinforcing the pattern

84
Q

What is the formula associated with diffraction gratings?

A

d sinθ= nλ

where:
d= distance between the slits
θ= the angle to the normal made by the maximum (zero order)
n= the order
λ= the wavelength

85
Q

How do you find out the distance for a diffraction grating?

A

1/n
where n= lines per mm

86
Q

What happens when λ increases in a diffraction grating?

A

the distance between the orders will increase because θ is larger due to the increase in diffraction as the slit spacing is closer in size to the wavelength, this means the pattern will spread out

87
Q

How can we find the distance of the fringes/orders from the zeroth order?

A

tan θ= x/d

where:
θ= the angle made by the zeroth order to the nth order
x= the order
d= distance between the slits

88
Q

What is the max. possible value for sin θ? (for a diffraction grating)

A

the max. value is1, therefore any values of n, which give sin θ as greater than 1 are impossible

89
Q

Explain how we can get the values for d sinθ= nλ

A

https://upload.wikimedia.org/wikipedia/commons/a/ab/Youngs_slits.png

-the path difference between two adjacent rays of light is one wavelength
-a right angle triangle is formed, with side lengths d and λ
-you can see the upper angle in the triangle is θ (because the lower angle is 90-θ°)

90
Q

What is a refractive index?

A

a property of media which measures how much it slows down light passing through it/how optically dense it is

91
Q

How can we calculate the refractive index of a media?

A

refractive index= dividing the speed of light in a vacuum (C) by the speed of
light in that substance (Cs)

n = C/Cs

remember: speed of light in a vacuum= 3 x10^8 ms^-1

92
Q

Define surface normal

A

an imaginary line perpendicular to a surface between two materials at a point of incidence

93
Q

What does it mean when a material has a higher refractive index?

A

known as more optically dense

94
Q

What is the refractive index of air?

A

1

95
Q

When does refraction occur?

A

when a wave enters a different medium, causing it to change direction, either towards or away from the normal depending on the material’s refractive index

96
Q

How can we remember which way refraction occurs from air to glass?

A

TAG
towards normal air to glass
(as light in air passes through to glass, the direction of light changed direction towards the normal)

97
Q

When light enters a more optically dense medium does it bend towards or away to the normal? What about if it enters a less optically dense medium?

A

more optically dense- towards the normal
less optically dense- away from the normal

98
Q

State Snell’s law and draw a diagram showing this

A

n1 sinθ1 = n2 sinθ2

where:
n1= the refractive index of material 1
n2= the refractive index of material 2
θ1= the angle of incidence of the ray in material 1
θ2= the angle of refraction of the ray in material 2

https://upload.wikimedia.org/wikipedia/commons/thumb/b/b5/Snell%27s_law3.svg/300px-Snell%27s_law3.svg.png?20140526191916

99
Q

What do we mean by the critical angle?

A

when the angle of refraction is exactly 90° (equal to the angle of incidence), and the light is refracted along the boundary, the angle of incidence has reached the critical angle (θc)

100
Q

How can we work out the critical angle?

A

sin θc = n2/n1
where n1 > n2

101
Q

What is total internal reflection?

A

complete reflection of a ray of light within a medium

102
Q

What are the two conditions required for total internal reflection (TIR) to occur?

A

-the ray must start in a more optically dense medium than it is travelling towards (n1>n2)

-the angle of incidence must be greater than the critical angle (θ1>θc)

103
Q

Draw three diagrams showing the different paths of light rays that can occur at different angles of incidence when n1>n2. Label the angles, refractive indexes and what the diagram shows

A

https://upload.wikimedia.org/wikipedia/commons/thumb/a/a5/Total_Internal_Reflection.png/799px-Total_Internal_Reflection.png?20150821043859

104
Q

Describe a useful application of TIR

A

step index optical fibres
-these are flexible, thin tubes of plastic or glass which carry information in the form of light signals

-they have an optically dense core surrounded by cladding with a lower optical density allowing TIR to occur

105
Q

What is the purpose of cladding in a step index optical fibre? (5)

A

-protects the core from damage
-prevents signal degradation through light escaping the
core, which can cause information to be lost
-allows TIR as n,core > n,cladding (n1 > n2)
-prevent crossover of signal in a bundle of fibres
-increases θc to reduce pulse broadening and modal dispersion

106
Q

Draw a diagram showing the use of TIR in real-life applications

A

https://upload.wikimedia.org/wikipedia/commons/thumb/b/b3/Optic_fibre-numerical_aperture_diagram.svg/580px-Optic_fibre-numerical_aperture_diagram.svg.png?20101126090923

107
Q

What are the reasons why signal degradation occurs in optical fibres? (2)

A

-absorption
-dispersion

108
Q

What is absorption and why does it cause signal degradation?

A

where part of the signal’s energy is absorbed by the fibre, reducing the amplitude of the signal, which could lead to a loss of information

109
Q

What is pulse broadening?

A

when the received signal is wider than the original which can cause an overlap of signals leading to information loss

110
Q

What is dispersion and why does it cause signal degradation?

A

this causes pulse broadening therefore can overlap signals causing loss of information

111
Q

What are the types of dispersion?

A

-modal
-material

112
Q

What is modal dispersion?

A

light rays enter the fibre at different angles (all at same speed) and therefore take different paths. this leads to the rays taking a different amount of time to travel along the fibre, causing pulse broadening.

113
Q

How can we reduce modal dispersion?

A

can be reduced by making the core very narrow, therefore making the possible difference in path lengths smaller

114
Q

What is material dispersion?

A

caused by using light consisting of different wavelengths, meaning light rays will travel at different speeds along the fibre, which leads to pulse broadening

115
Q

How can we reduce material dispersion?

A

use monochromatic light

116
Q

How can we reduce both absorption and dispersion?

A

using an optical fibre repeater, which regenerates the signal during its travel to its destination

117
Q

Draw a diagram showing an optical fibre pulse once absorption and dispersion has occurred

A

https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2021/04/Absorption-Pulse-Broadening.png

118
Q

State the advantages of optical fibres over traditional copper wires(5)

A

-signal can carry more information as light has a high frequency
-no energy lost as heat
-no electrical interference
-cheaper
-very fast

AQA A-level Physics (11 decks)

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