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aqa a level physics > waves > Flashcards

waves Flashcards

1
Q

state a property of microwaves [1]

A

microwaves are polarised

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2
Q

Distinguish between longitudinal and transverse waves [2]

A

For transverse waves oscillations are at right angles to direction of propagation while in
longitudinal waves they are in the same direction

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3
Q

Explain how a stationary wave is produced when a stretched string is plucked. [3]

A

Waves travel to the boundaries and are reflected
two waves, with the same frequency and amplitude, travelling in opposite directions interfere/superpose
Fixed boundaries (cannot move so) are nodes where destructive interference occurs

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4
Q

Suggest how stationary waves are formed in a microwave oven [2]

A

waves are reflected off of the oven wall
they superpose with wave travelling in opposite direction

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5
Q

Explain what is meant by a progressive wave. [2]

A

A wave which transfers energy from one point to another without transferring material

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6
Q

Explain how light from the diffraction grating forms a maximum on the screen [3]

A

Light from slits diffract
Path difference is a whole number of wavelengths so they arrive at the screen in phase
they superimpose (interfere constructively) to form maxima

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7
Q

Explain what is meant by modal dispersion in an optical fibre [2]

A

Spreading of pulse / parts of a pulse take different times to travel through the
fibre
Due to different paths through the optical fibre / due to entering the optical fibre
at different angles

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8
Q

Explain what would make two loudspeakers coherent sources of sound waves. [2]

A

they have the same frequency/wavelength AND
constant phase difference
And that this achieved by both speakers being connected to same signal (generator).

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9
Q

Discuss the properties of a step-index optical fibre [6]
. the names of parts
* a description of the functions of said parts
* a discussion of the problems caused by material dispersion and modal dispersion and how these problems can be overcome.

A

inner part is the core
outer part is cladding

the core propagates the wave
by total internal reflection
with low attenuation (loss of energy)
refractive index of the core > cladding

cladding protects core from damage

both cause pulse broadening - limited bandwidth
material - different wavelengths have different speeds - use monochromatic source
modal - different paths take different amounts of time - use a narrow core

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10
Q

Suggest why the amount of reflected light changes as the fibre bends [2]

A

Angle of incidence may become less than critical angle
Light may encounter impurities at different positions/angles

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11
Q

apart from wearing eye protection
Describe one other safety measure to minimise the risk of eye damage when using a laser in the laboratory. [1]

A

switch laser off when not in use
or
don’t look directly into the laser (wow, geniuses taking a-level physics huh)

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12
Q

define frequency

A

number of complete waves passing a point per second

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13
Q

frequency {}
waves

A

v / λ
Hz (hertz)

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14
Q

define wavelength

A

distance between two adjacent crests on a wave

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15
Q

define amplitude

A

maximum displacement of a wave from it’s rest(equilibrium) position

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16
Q

time period {}

A

1 / f
seconds (s)

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17
Q

what is a longitudinal wave

A

a wave in which the oscillation of the particles is parallel to energy transfer
e.g sound

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18
Q

rarefactions and compressions

A

areas of low and high pressure
in a longitudinal wave

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19
Q

what is a transverse wave

A

a wave in which the oscillation of the particles is perpendicular to energy transfer
e.g EM waves

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20
Q

speed of Electromagnetic (EM) waves in a vacuum

A

3.00 * 10^8 ms^-1

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21
Q

what is an electromagnetic wave

A

vibrating electric and magnetic fields perpendicular to the direction of energy transfer

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22
Q

what does a polarising filter do

A

only allows oscillations in one plane
(plane polarises a wave)

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23
Q

what is evidence for the nature of transverse waves

A

polarisation
Transverse waves usually have multiple planes of oscillation perpendicular to motion

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24
Q

how is polarisation used in antennas

A

Tv and radio signals are usually plane polarised by the orientation of the transmitting aerials
so receiving aerials must be aligned in the same plane so signal can be received at full strength

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25
Q

what is a stationary wave

A

a wave which transfers no energy
positions of max and min amplitude are constant

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26
Q

what is a node

A

a point on a stationary wave where displacement from equilibrium position is 0

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27
Q

what is an antinode

A

a point on a stationary wave where displacement from equilibrium position is at its maximum

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28
Q

conditions for a stationary wave to be produced

A

waves must be of same frequency, wavelength and amplitude
must be travelling in opposite directions
(often when a wave is reflected upon itself)

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29
Q

how are stationary waves produced

A

superposition of two progressive waves travelling in opposite directions with the same frequency, wavelength and amplitude
where waves meet in phase - constructive interference - antinodes
where waves meet completely out of phase - destructive interference - nodes

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30
Q

describe the first harmonic for a stationary wave with two closed ends

A

2 nodes
1 antinode in the middle
L = 1/2 λ

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31
Q

describe the second harmonic for a stationary wave with one closed end and one open

A

2 nodes
1 at closed end
2 antinodes
1 at open end

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32
Q

define coherence

A

potential for two waves to interfere, fixed phase difference
same frequency and wavelength

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33
Q

why is a laser often used to model diffraction and interference

A

laser uses monochromatic light
more defined patterns produced

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34
Q

describe young’s double slit experiment
(bro these a-levels are making me want to do a double slit experiment too man(i’m joking))

A

single light source directed at a double slit
acts as two coherent light sources
diffracts through slit
interferes constructively and destructively to form pattern

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35
Q

describe the interference pattern produced by doing young’s double slit experiment with white light

A

bright white central maximum
fringes of decreasing intensity with violet closest to order zero and red furthest

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36
Q

why is an interference pattern produced when white light is shone through a single slit

A

all the different wavelengths are diffracted by different amounts
instead of clear fringes - spectra of colours

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37
Q

how does the size of the slit relate to the amount of diffraction

A

slit should be closer to wavelength for more diffraction

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38
Q

what does increasing the slit width do to the central maximum

what does increasing wavelength do

A

increasing slit width decreases diffraction
so central maximum becomes narrower and more intense

more wavelength
more diffraction
central maximum wider and less intense

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39
Q

approximate refractive index of air

A

1

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40
Q

when light enter a more optically dense material does it bend towards or away from the normal

A

towards

41
Q

when does total internal reflection occur

A

when light is at a boundary to a less optically dense medium
angle of incidence > critical angle

42
Q

purpose of cladding in step index optical fibre

A

protects core from scratches
allows total internal reflection as it has a lower refractive index than the core

43
Q

what is attenuation

A

part of the signal’s energy is absorbed by the fibre
so the amplitude is reduced

44
Q

what is pulse broadening

A

received signal is wider than the original
this can cause overlap of signals and loss of information

45
Q

how does modal dispersion cause pulse broadening

A

light enters fibre at different angles (different angles of incidence)
so takes different paths
so rays take different times to travel down fibre

46
Q

what is material dispersion

A

when light with different wavelengths is used
different wavelengths travel at different speeds
arrive at different times
pulse broadening

47
Q

how can you reduce modal dispersion

A

use a narrow core
so possible difference in path lengths is smaller

48
Q

how can you reduce material dispersion

A

use monochromatic light

49
Q

how can both absorption and dispersion be reduced

A

use a optical fibre repeater to regenerate signal

50
Q

why optical fibre cables > traditional copper cables

A

signals can carry more information as light has a high frequency
no energy lost as heat
cheaper
very fast

51
Q

what happens when angle of incidence = critical angle

A

goes along boundary
angle of refraction is 90 degrees

52
Q

you know the refractive indices of two materials, how do you work out the critical angle

A

sin Critical = n2 / n1
where n1 > n2

53
Q

finding the angle or refraction {}

A

n1sini = n2sinr

54
Q

what does a higher refractive index mean

A

more optically dense
The higher the refractive index the slower the light travels
More refraction

55
Q

refractive index {}

A

n = c / v
refractive index = speed of light / speed in substance

56
Q

2 applications of diffraction gratings

A

splitting light up received from stars
to form line absorption spectra
to identify elements present

x-ray crystallography
crystal sheet acts as a diffraction grating
x-rays pass through
to find spacing between atoms

57
Q

what is a first order maximum

A

path difference between two adjacent rays of light is 1λ

58
Q

nλ = {}

A

nλ = dsinθ
where n is the order
λ is the wavelength
d is the separation (in m)
θ is the angle between the normal to the grating and the ray

59
Q

when light passing through a diffraction grating is changed from blue to red what happens to the orders

A

red has a greater wavelength than blue light
so will diffract more
so orders will become more further apart

60
Q

what is diffraction

A

the spreading out of waves when they pass through (or around) a gap

61
Q

what are wave properties shows by young’s double slit experiment

A

diffraction and interference

62
Q

fringe separation {}

A

w = (λD) / s
fringe spacing = ( wavelength * distance between source and screen) / slit separation

63
Q

what is path difference

A

the difference in path travelled by 2 waves

64
Q

frequency {}
including tension

A

f = (1/2L) * √(T/u)
where
T = tension (N)
u = unit per mass length (kgm^-1)
L = length of harmonic (2L = λ)

65
Q

wave speed {}

A

v = fλ
wave speed = frequency * wavelength

66
Q

what is phase

A

the position of a wave point on a wave cycle
measured in
radians
fractions of a cycle nλ
degrees

67
Q

what is reflection

A

when the wave is bounced back when it hits a boundary

68
Q

what is refraction

A

when the wave changes direction as it enter a different medium
result of the wave slowing down or speeding up

69
Q

what is time period

A

time taken for one whole wave cycle

70
Q

what do you get with two polarising filters at right angles to one another

A

no light passes through

71
Q

what are some real life applications of polaristion

A

glare reduction
light reflected off of some surfaces is partially polarised
when light is reflected by surfaces they can cause glare
since it is partially polarised some of it can be filtered out using polarising filters

improving TV and radio signals
signals are polarised by orientation of the rods on the transmitting aerial
receiving aerials should be lined up similarily so the signal can be received at full strength

72
Q

what is superposition

A

When two or more waves cross at a point, the displacement at that point is equal to the sum of the displacements of the individual waves.

73
Q

describe constructive interference

A

when two waves pass through each other and their displacements combine to make a displacement with greater magnitude

74
Q

total destructive interference

A

when two waves of equal and opposite displacements meet and cancel each other out completely

75
Q

what do points in phase share in common

A

displacement and velocity

76
Q

1 complete cycle of a wave in radians

A

2 π (360 degrees)

77
Q

when are two points on a wave completely out of phase

A

when their phase difference is an odd multiple of 180 degrees
odd multiple of π
odd multiple of 1/2 λ

78
Q

when are two points are in phase

A

phase difference of 0
multiple of 360
even multiple of half wavelength (π)

79
Q

what is resonant frequency

A

when an exact number of half wavelengths fit on the string

Natural frequency at which the medium oscillates with highest amplitude

80
Q

explain the first harmonic

A

a stationary wave vibrating at its lowest possible resonant frequency

81
Q

how does frequency vary with the harmonics

A

second harmonic has twice the frequency of the first harmonic
third harmonic, thrice the frequency of the first harmonic

82
Q

investigating resonant frequency

A

signal generator
string
masses for tension
vary tension and move prism along string to find the harmonic
record length of the string

83
Q

how is resonant frequency affected by length of the string

A

the longer the string the lower the resonant frequency
(since half wavelength is longer, v = fλ, greater lamda -> f decreases for a fixed v)

84
Q

how is resonant frequency affected by material of the string

A

heavier ( more unit per mass length ) strings give a lower resonant frequency
waves travel more slowly down the string
( λ remains the same , v= fλ , if v decreases so must f )

85
Q

how is resonant frequency affected by tension on the string

A

the greater the tension on the string the higher the resonant frequency
this is because waves travel faster on a tight string

86
Q

diffraction around an obstacle

A

diffraction about the edges
leaves a “shadow”
the greater the obstacle in comparison to the wavelength, the less diffraction
so a longer shadow

87
Q

what is intensity

A

power per unit area

88
Q

where does constructive interference occur

A

path difference is a whole number of wavelengths
in phase
Constructive interference occurs when the phase difference between the waves is an even multiple of π (180°)

89
Q

double slit experiment

A

two source interference
use monochromatic light source - laser
shine through slits of about the same size of the wavelength so diffraction occurs
pattern of light and dark fringes
w = (λD)/s
fringe separation

90
Q

what is a diffraction grating

A

lots of equally spaced slits
creates a sharp interference pattern since there are many beam reinforcing the pattern

91
Q

when do maxima occur (diffraction grating experiment)

A

nth order maxima occur when path difference = nλ

92
Q

what are conclusions that can be drawn from the diffraction grating formula about

order

A

values of sinθ greater than 1 are impossible
so if for a certain n you get sinθ > 1
that order doesn’t exist

93
Q

what are conclusions that can be drawn from the diffraction grating formula about

wavelength

A

if wavelength is bigger
sinθ is bigger therefore θ is bigger
larger the wavelength - the more spread out the pattern is (the more diffraction)

94
Q

what are conclusions that can be drawn from the diffraction grating formula about

d

A

if d is small ( number of slits per metre is large )
the greater the angle of diffraction
more spread out pattern

95
Q

what is critical angle

A

where angle of refraction is 90
light is refracted along the boundary
when light is passing from a more optically dense material to a less optically dense one

96
Q

how does total internal reflection occur in step-index optical fibres

A

light is shone in
the fibre is so narrow that the light always hits the boundary between the fibre and the cladding at an angle greater than the critical angle
so all light is totally internally reflected

97
Q

total internal reflection

A

at angles greater than the critical angle refraction doesn’t occur
all the light is reflected back into the material

98
Q

how do you calculate the highest order of maxima visible

A

n = d/λ
nλ = dsinθ
max angle to see orders of maxima is 90
sin 90 = 1
nλ = d

aqa a level physics (10 decks)

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